6. ch-image
¶
Build and manage images; completely unprivileged.
6.1. Synopsis¶
$ ch-image [...] build [-t TAG] [-f DOCKERFILE] [...] CONTEXT
$ ch-image [...] build-cache [...]
$ ch-image [...] delete IMAGE_REF
$ ch-image [...] gestalt [SELECTOR]
$ ch-image [...] import PATH IMAGE_REF
$ ch-image [...] list [-l] [IMAGE_REF]
$ ch-image [...] pull [...] IMAGE_REF [DEST_REF]
$ ch-image [...] push [--image DIR] IMAGE_REF [DEST_REF]
$ ch-image [...] reset
$ ch-image [...] undelete IMAGE_REF
$ ch-image { --help | --version | --dependencies }
6.2. Description¶
ch-image
is a tool for building and manipulating container images, but
not running them (for that you want ch-run
). It is completely
unprivileged, with no setuid/setgid/setcap helpers. Many operations can use
caching for speed. The action to take is specified by a sub-command.
Options that print brief information and then exit:
-h
,--help
Print help and exit successfully. If specified before the sub-command, print general help and list of sub-commands; if after the sub-command, print help specific to that sub-command.
--dependencies
Report dependency problems on standard output, if any, and exit. If all is well, there is no output and the exit is successful; in case of problems, the exit is unsuccessful.
--version
Print version number and exit successfully.
Common options placed before or after the sub-command:
-a
,--arch ARCH
Use
ARCH
for architecture-aware registry operations. (See section “Architecture” below for details.)--always-download
Download all files when pulling, even if they are already in builder storage. Note that
ch-image pull
will always retrieve the most up-to-date image; this option is mostly for debugging.--auth
Authenticate with the remote repository, then (if successful) make all subsequent requests in authenticated mode. For most subcommands, the default is to never authenticate, i.e., make all requests anonymously. The exception is
push
, which implies--auth
.--cache
Enable build cache. Default if a sufficiently new Git is available.
--cache-large SIZE
Set the cache’s large file threshold to
SIZE
MiB, or0
for no large files, which is the default. This can speed up some builds. Experimental. See section “Build cache” for details.--no-cache
Disable build cache. Default if a sufficiently new Git is not available. This option turns off the cache completely; if you want to re-execute a Dockerfile and store the new results in cache, use
--rebuild
instead.--no-lock
Disable storage directory locking. This lets you run as many concurrent
ch-image
instances as you want against the same storage directory, which risks corruption but may be OK for some workloads.--profile
Dump profile to files
/tmp/chofile.p
(cProfile
dump format) and/tmp/chofile.txt
(text summary). You can convert the former to a PDF call graph withgprof2dot -f pstats /tmp/chofile.p | dot -Tpdf -o /tmp/chofile.pdf
. This excludes time spend in subprocesses. Profile data should still be written on fatal errors, but not if the program crashes.--rebuild
Execute all instructions, even if they are build cache hits, except for
FROM
which is retrieved from cache on hit.--password-many
Re-prompt the user every time a registry password is needed.
-s
,--storage DIR
Set the storage directory (see below for important details).
--tls-no-verify
Don’t verify TLS certificates of the repository. (Do not use this option unless you understand the risks.)
-v
,--verbose
Print extra chatter; can be repeated.
6.3. Architecture¶
Charliecloud provides the option --arch ARCH
to specify the
architecture for architecture-aware registry operations. The argument
ARCH
can be: (1) yolo
, to bypass architecture-aware code and
use the registry’s default architecture; (2) host
, to use the host’s
architecture, obtained with the equivalent of uname -m
(default if
--arch
not specified); or (3) an architecture name. If the specified
architecture is not available, the error message will list which ones are.
Notes:
ch-image
is limited to one image per image reference in builder storage at a time, regardless of architecture. For example, if you saych-image pull --arch=foo baz
and thench-image pull --arch=bar baz
, builder storage will contain one image called “baz”, with architecture “bar”.Images’ default architecture is usually
amd64
, so this is usually what you get with--arch=yolo
. Similarly, if a registry image is architecture-unaware, it will still be pulled with--arch=amd64
and--arch=host
on x86-64 hosts (other host architectures must specify--arch=yolo
to pull architecture-unaware images).uname -m
and image registries often use different names for the same architecture. For example, whatuname -m
reports as “x86_64” is known to registries as “amd64”.--arch=host
should translate if needed, but it’s useful to know this is happening. Directly specified architecture names are passed to the registry without translation.Registries treat architecture as a pair of items, architecture and sometimes variant (e.g., “arm” and “v7”). Charliecloud treats architecture as a simple string and converts to/from the registry view transparently.
6.4. Authentication¶
Charliecloud does not have configuration files; thus, it has no separate
login
subcommand to store secrets. Instead, Charliecloud will prompt
for a username and password when authentication is needed. Note that some
repositories refer to the secret as something other than a “password”; e.g.,
GitLab calls it a “personal access token (PAT)”, Quay calls it an “application
token”, and nVidia NGC calls it an “API token”.
For non-interactive authentication, you can use environment variables
CH_IMAGE_USERNAME
and CH_IMAGE_PASSWORD
. Only do this if you
fully understand the implications for your specific use case, because it is
difficult to securely store secrets in environment variables.
By default for most subcommands, all registry access is anonymous. To instead
use authenticated access for everything, specify --auth
or set the
environment variable $CH_IMAGE_AUTH=yes
. The exception is
push
, which always runs in authenticated mode. Even for pulling public
images, it can be useful to authenticate for registries that have per-user
rate limits, such as Docker Hub. (Older versions
of Charliecloud started with anonymous access, then tried to upgrade to
authenticated if it seemed necessary. However, this turned out to be brittle;
see issue #1318.)
The username and password are remembered for the life of the process and
silently re-offered to the registry if needed. One case when this happens is
on push to a private registry: many registries will first offer a read-only
token when ch-image
checks if something exists, then re-authenticate
when upgrading the token to read-write for upload. If your site uses one-time
passwords such as provided by a security device, you can specify
--password-many
to provide a new secret each time.
These values are not saved persistently, e.g. in a file. Note that we do use normal Python variables for this information, without pinning them into physical RAM with mlock(2) or any other special treatment, so we cannot guarantee they will never reach non-volatile storage.
Technical details
Most registries use something called Bearer authentication, where the client (e.g., Charliecloud) includes a token in the headers of every HTTP request.
The authorization dance is different from the typical UNIX approach, where
there is a separate login sequence before any content requests are made.
The client starts by simply making the HTTP request it wants (e.g., to
GET
an image manifest), and if the registry doesn’t like the
client’s token (or if there is no token because the client doesn’t have one
yet), it replies with HTTP 401 Unauthorized, but crucially it also provides
instructions in the response header on how to get a token. The client then
follows those instructions, obtains a token, re-tries the request, and
(hopefully) all is well. This approach also allows a client to upgrade a
token if needed, e.g. when transitioning from asking if a layer exists to
uploading its content.
The distinction between Charliecloud’s anonymous mode and authenticated modes is that it will only ask for anonymous tokens in anonymous mode and authenticated tokens in authenticated mode. That is, anonymous mode does involve an authentication procedure to obtain a token, but this “authentication” is done anonymously. (Yes, it’s confusing.)
Registries also often reply HTTP 401 when an image does not exist, rather than the seemingly more correct HTTP 404 Not Found. This is to avoid information leakage about the existence of images the client is not allowed to pull, and it’s why Charliecloud never says an image simply does not exist.
6.5. Storage directory¶
ch-image
maintains state using normal files and directories located in
its storage directory; contents include various caches and temporary images
used for building.
In descending order of priority, this directory is located at:
-s
,--storage DIR
Command line option.
$CH_IMAGE_STORAGE
Environment variable. The path must be absolute, because the variable is likely set in a very different context than when it’s used, which seems error-prone on what a relative path is relative to.
/var/tmp/$USER.ch
Default. (Previously, the default was
/var/tmp/$USER/ch-image
. If a valid storage directory is found at the old default path,ch-image
tries to move it to the new default path.)
Unlike many container implementations, there is no notion of storage drivers, graph drivers, etc., to select and/or configure.
The storage directory can reside on any single filesystem (i.e., it cannot be
split across multiple filesystems). However, it contains lots of small files
and metadata traffic can be intense. For example, the Charliecloud test suite
uses approximately 400,000 files and directories in the storage directory as
of this writing. Place it on a filesystem appropriate for this; tmpfs’es such
as /var/tmp
are a good choice if you have enough RAM (/tmp
is
not recommended because ch-run
bind-mounts it into containers by
default).
While you can currently poke around in the storage directory and find unpacked
images runnable with ch-run
, this is not a supported use case. The
supported workflow uses ch-convert
to obtain a packed image; see the
tutorial for details.
The storage directory format changes on no particular schedule.
ch-image
is normally able to upgrade directories produced by a given
Charliecloud version up to one year after that version’s release. Upgrades
outside this window and downgrades are not supported. In these cases,
ch-image
will refuse to run until you delete and re-initialize the
storage directory with ch-image reset
.
Warning
Network filesystems, especially Lustre, are typically bad choices for the storage directory. This is a site-specific question and your local support will likely have strong opinions.
6.6. Build cache¶
6.6.1. Overview¶
Subcommands that create images, such as build
and pull
, can
use a build cache to speed repeated operations. That is, an image is created
by starting from the empty image and executing a sequence of instructions,
largely Dockerfile instructions but also some others like “pull” and “import”.
Some instructions are expensive to execute (e.g., RUN wget
http://slow.example.com/bigfile
or transferring data billed by the byte), so
it’s often cheaper to retrieve their results from cache instead.
The build cache uses a relatively new Git under the hood; see the installation
instructions for version requirements. Charliecloud implements workarounds for
Git’s various storage limitations, so things like file metadata and Git
repositories within the image should work. Important exception: No files
named .git*
or other Git metadata are permitted in the image’s root
directory.
The cache has three modes, enabled, disabled, and a hybrid mode called
rebuild where the cache is fully enabled for FROM
instructions, but
all other operations re-execute and re-cache their results. The purpose of
rebuild is to do a clean rebuild of a Dockerfile atop a known-good base
image.
Enabled mode is selected with --cache
or setting
$CH_IMAGE_CACHE
to enabled
, disabled mode with
--no-cache
or disabled
, and rebuild mode with
--rebuild
or rebuild
. The default mode is enabled if an
appropriate Git is installed, otherwise disabled.
6.6.2. Compared to other implementations¶
Other container implementations typically use build caches based on overlayfs, or fuse-overlayfs in unprivileged situations (configured via a “storage driver”). This works by creating a new tmpfs for each instruction, layered atop the previous instruction’s tmpfs using overlayfs. Each layer can then be tarred up separately to form a tar-based diff.
The Git-based cache has two advantages over the overlayfs approach. First, kernel-mode overlayfs is only available unprivileged in Linux 5.11 and higher, forcing the use of fuse-overlayfs and its accompanying FUSE overhead for unprivileged use cases. Second, Git de-duplicates and compresses files in a fairly sophisticated way across the entire build cache, not just between image states with an ancestry relationship (detailed in the next section).
A disadvantage is lowered performance in some cases. Preliminary experiments suggest this performance penalty is relatively modest, and sometimes Charliecloud is actually faster than alternatives. We have ongoing experiments to answer this performance question in more detail.
6.6.3. De-duplication and garbage collection¶
Charliecloud’s build cache takes advantage of Git’s file de-duplication features. This operates across the entire build cache, i.e., files are de-duplicated no matter where in the cache they are found or the relationship between their container images. Files are de-duplicated at different times depending on whether they are identical or merely similar.
Identical files are de-duplicated at git add
time; in
ch-image build
terms, that’s upon committing a successful instruction.
That is, it’s impossible to store two files with the same content in the build
cache. If you try — say with RUN yum install -y foo
in one Dockerfile
and RUN yum install -y foo bar
in another, which are different
instructions but both install RPM foo
’s files — the content is stored
once and each copy gets its own metadata and a pointer to the content, much
like filesystem hard links.
Similar files, however, are only de-duplicated during Git’s garbage
collection process. When files are initially added to a Git repository (with
git add
), they are stored inside the repository as (possibly
compressed) individual files, called objects in Git jargon. Upon garbage
collection, which happens both automatically when certain parameters are met
and explicitly with git gc
, these files are archived and
(re-)compressed together into a single file called a packfile. Also,
existing packfiles may be re-written into the new one.
During this process, similar files are identified, and each set of similar files is stored as one base file plus diffs to recover the others. (Similarity detection seems to be based primarily on file size.) This delta process is agnostic to alignment, which is an advantage over alignment-sensitive block-level de-duplicating filesystems. Exception: “Large” files are not compressed or de-duplicated. We use the Git default threshold of 512 MiB (as of this writing).
Charliecloud runs Git garbage collection at two different times. First, a lighter-weight garbage pass runs automatically when the number of loose files (objects) grows beyond a limit. This limit is in flux as we learn more about build cache performance, but it’s quite a bit higher than the Git default. This garbage runs in the background and can continue after the build completes; you may see Git processes using a lot of CPU.
An important limitation of the automatic garbage is that large packfiles
(again, this is in flux, but it’s several GiB) will not be re-packed, limiting
the scope of similar file detection. To address this, a heavier garbage
collection can be run manually with ch-image build-cache --gc
. This
will re-pack (and re-write) the entire build cache, de-duplicating all similar
files. In both cases, garbage uses all available cores.
git build-cache
prints the specific garbage collection parameters in
use, and -v
can be added for more detail.
6.6.4. Large file threshold¶
Because Git uses content-addressed storage, upon commit, it must read in full
all files modified by an instruction. This I/O cost can be a significant
fraction of build time for some large images. Regular files larger than the
experimental large file threshold are stored outside the Git repository,
somewhat like Git Large File Storage.
ch-image
uses hard links to bring large files in and out of images as
needed, which is a fast metadata operation that ignores file content.
Option --cache-large
sets the threshold in MiB; if not set,
environment variable CH_IMAGE_CACHE_LARGE
is used; if that is not set
either, the default value 0
indicates that no files are considered
large.
There are two trade-offs. First, large files in any image with the same path,
mode, size, and mtime (to nanosecond precision if possible) are considered
identical, even if their content is not actually identical; e.g.,
touch(1)
shenanigans can corrupt an image. Second, every version of a
large file is stored verbatim and uncompressed (e.g., a large file with a
one-byte change will be stored in full twice), and large files do not
participate in the build cache’s de-duplication, so more storage space will
likely be used. Unused versions are deleted by ch-image build-cache
--gc
.
(Note that Git has an unrelated setting called core.bigFileThreshold
.)
6.6.5. Example¶
Suppose we have this Dockerfile:
$ cat a.df
FROM alpine:3.17
RUN echo foo
RUN echo bar
On our first build, we get:
$ ch-image build -t foo -f a.df .
1. FROM alpine:3.17
[ ... pull chatter omitted ... ]
2. RUN echo foo
copying image ...
foo
3. RUN echo bar
bar
grown in 3 instructions: foo
Note the dot after each instruction’s line number. This means that the
instruction was executed. You can also see this by the output of the two
echo
commands.
But on our second build, we get:
$ ch-image build -t foo -f a.df .
1* FROM alpine:3.17
2* RUN echo foo
3* RUN echo bar
copying image ...
grown in 3 instructions: foo
Here, instead of being executed, each instruction’s results were retrieved
from cache. (Charliecloud uses lazy retrieval; nothing is actually retrieved
until the end, as seen by the “copying image” message.) Cache hit for each
instruction is indicated by an asterisk (*
) after the line number.
Even for such a small and short Dockerfile, this build is noticeably faster
than the first.
We can also try a second, slightly different Dockerfile. Note that the first three instructions are the same, but the third is different:
$ cat c.df
FROM alpine:3.17
RUN echo foo
RUN echo qux
$ ch-image build -t c -f c.df .
1* FROM alpine:3.17
2* RUN echo foo
3. RUN echo qux
copying image ...
qux
grown in 3 instructions: c
Here, the first two instructions are hits from the first Dockerfile, but the third is a miss, so Charliecloud retrieves that state and continues building.
We can also inspect the cache:
$ ch-image build-cache --tree
* (c) RUN echo qux
| * (a) RUN echo bar
|/
* RUN echo foo
* (alpine+3.9) PULL alpine:3.17
* (HEAD -> root) ROOT
named images: 4
state IDs: 5
commits: 5
files: 317
disk used: 3 MiB
Here there are four named images: a
and c
that we built, the
base image alpine:3.17
(written as alpine+3.9
because colon is
not allowed in Git branch names), and the empty base of everything
root
. Also note how a
and c
diverge after the last
common instruction RUN echo foo
.
6.7. build
¶
Build an image from a Dockerfile and put it in the storage directory.
6.7.1. Synopsis¶
$ ch-image [...] build [-t TAG] [-f DOCKERFILE] [...] CONTEXT
6.7.2. Description¶
Uses ch-run -w -u0 -g0 --no-passwd --unsafe
to execute RUN
instructions. Note that FROM
implicitly pulls the base image if
needed, so you may want to read about the pull
subcommand below as
well.
Required argument:
CONTEXT
Path to context directory. This is the root of
COPY
instructions in the Dockerfile. If a single hyphen (-
) is specified: (a) read the Dockerfile from standard input, (b) specifying--file
is an error, and (c) there is no context, soCOPY
will fail. (See--file
for how to provide the Dockerfile on standard input while also having a context.)
Options:
-b
,--bind SRC[:DST]
For
RUN
instructions only, bind-mountSRC
at guestDST
. The default destination if not specified is to use the same path as the host; i.e., the default is equivalent to--bind=SRC:SRC
. IfDST
does not exist, try to create it as an empty directory, though images do have ten directories/mnt/[0-9]
already available as mount points. Can be repeated.Note: See documentation for
ch-run --bind
for important caveats and gotchas.Note: Other instructions that modify the image filesystem, e.g.
COPY
, can only access host files from the context directory, regardless of this option.--build-arg KEY[=VALUE]
Set build-time variable
KEY
defined byARG
instruction toVALUE
. IfVALUE
not specified, use the value of environment variableKEY
.-f
,--file DOCKERFILE
Use
DOCKERFILE
instead ofCONTEXT/Dockerfile
. If a single hyphen (-
) is specified, read the Dockerfile from standard input; likedocker build
, the context directory is still available in this case.--force[=MODE]
Use unprivileged build workarounds of mode
MODE
, which can befakeroot
orseccomp
(the default). See section “Privilege model” below for details on what this does and when you might need it.--force-cmd=CMD,ARG1[,ARG2...]
If command
CMD
is found in aRUN
instruction, add the comma-separatedARGs
to it. For example,--force-cmd=foo,-a,--bar=baz
would transformRUN foo -c
intoRUN foo -a --bar=baz -c
. This is intended to suppress validation that defeats--force=seccomp
and implies that option. Can be repeated. If specified, replaces (does not extend) the default suppression options. Literal commas can be escaped with backslash; importantly however, backslash will need to be protected from the shell also. Section “Privilege model” below explains why you might need this.-n
,--dry-run
Don’t actually execute any Dockerfile instructions.
--parse-only
Stop after parsing the Dockerfile.
-t
,--tag TAG
Name of image to create. If not specified, infer the name:
If Dockerfile named
Dockerfile
with an extension: use the extension with invalid characters stripped, e.g.Dockerfile.@FOO.bar
→foo.bar
.If Dockerfile has extension
dockerfile
: use the basename with the same transformation, e.g.baz.@QUX.dockerfile
->baz.qux
.If context directory is not
/
: use its name, i.e. the last component of the absolute path to the context directory, with the same transformation,Otherwise (context directory is
/
): useroot
.If no colon present in the name, append
:latest
.
6.7.3. Privilege model¶
6.7.3.1. Overview¶
ch-image
is a fully unprivileged image builder. It does not use any
setuid or setcap helper programs, and it does not use configuration files
/etc/subuid
or /etc/subgid
. This contrasts with the “rootless”
or “fakeroot” modes
of some competing builders, which do require privileged supporting code or
utilities.
Without workarounds provided by --force
, this approach does confuse
programs that expect to have real root privileges, most notably distribution
package installers. This subsection describes why that happens and what you
can do about it.
ch-image
executes all instructions as the normal user who invokes it.
For RUN
, this is accomplished with ch-run
arguments including
-w --uid=0 --gid=0
. That is, your host EUID and EGID are both mapped
to zero inside the container, and only one UID (zero) and GID (zero) are
available inside the container. Under this arrangement, processes running in
the container for each RUN
appear to be running as root, but many
privileged system calls will fail without the workarounds described below.
This affects any fully unprivileged container build, not just
Charliecloud.
The most common time to see this is installing packages. For example, here is
RPM failing to chown(2)
a file, which makes the package update fail:
Updating : 1:dbus-1.10.24-13.el7_6.x86_64 2/4
Error unpacking rpm package 1:dbus-1.10.24-13.el7_6.x86_64
error: unpacking of archive failed on file /usr/libexec/dbus-1/dbus-daemon-launch-helper;5cffd726: cpio: chown
Cleanup : 1:dbus-libs-1.10.24-12.el7.x86_64 3/4
error: dbus-1:1.10.24-13.el7_6.x86_64: install failed
This one is (ironically) apt-get
failing to drop privileges:
E: setgroups 65534 failed - setgroups (1: Operation not permitted)
E: setegid 65534 failed - setegid (22: Invalid argument)
E: seteuid 100 failed - seteuid (22: Invalid argument)
E: setgroups 0 failed - setgroups (1: Operation not permitted)
Charliecloud provides two different mechanisms to avoid these problems. Both involve lying to the containerized process about privileged system calls, but at very different levels of complexity.
6.7.3.2. Workaround mode fakeroot
¶
This mode uses fakeroot(1)
to maintain an elaborate web of deceit that
is internally consistent. This program intercepts both privileged system calls
(e.g., setuid(2)
) as well as other system calls whose return values
depend on those calls (e.g., getuid(2)
), faking success for privileged
system calls (perhaps making no system call at all) and altering return values
to be consistent with earlier fake success. Charliecloud automatically
installs the fakeroot(1)
program inside the container and then wraps
RUN
instructions having known privilege needs with it. Thus, this mode
is only available for certain distributions.
The advantage of this mode is its consistency; e.g., careful programs that check the new UID after attempting to change it will not notice anything amiss. Its disadvantage is complexity: detailed knowledge and procedures for multiple Linux distributions.
This mode has three basic steps:
After
FROM
, analyze the image to see what distribution it contains, which determines the specific workarounds.Before the user command in the first
RUN
instruction where the injection seems needed, installfakeroot(1)
in the image, if one is not already installed, as well as any other necessary initialization commands. For example, we turn off theapt
sandbox (for Debian Buster) and configure EPEL but leave it disabled (for CentOS/RHEL).Prepend
fakeroot
toRUN
instructions that seem to need it, e.g. ones that containapt
,apt-get
,dpkg
for Debian derivatives anddnf
,rpm
, oryum
for RPM-based distributions.
RUN
instructions that do not seem to need modification are
unaffected by this mode.
The details are specific to each distribution. ch-image
analyzes image
content (e.g., grepping /etc/debian_version
) to select a
configuration; see lib/force.py
for details. ch-image
prints
exactly what it is doing.
Warning
Because of fakeroot
mode’s complexity, we plan to remove it if
seccomp
mode performs well enough. If you have a situation where
fakeroot
mode works and seccomp
does not, please let us
know.
6.7.3.3. Workaround mode seccomp
(default)¶
This mode uses the kernel’s seccomp(2)
system call filtering to
intercept certain privileged system calls, do absolutely nothing, and return
success to the program.
The quashed system calls are: capset(2)
; chown(2)
and friends;
mknod(2)
and mknodat(2)
; and setuid(2)
,
setgid(2)
, and setgroups(2)
along with the other system calls
that change user or group.
The advantages of this approach is that it’s much simpler, it’s faster, it’s
completely agnostic to libc, and it’s mostly agnostic to distribution. The
disadvantage is that it’s a very lazy liar; even the most cursory consistency
checks will fail, e.g., getuid(2)
after setuid(2)
.
While this mode does not provide consistency, it does offer a hook to help
prevent programs asking for consistency. For example, apt-get -o
APT::Sandbox::User=root
will prevent apt-get
from attempting to drop
privileges, which it verifies,
exiting with failure if the correct IDs are not found (which they won’t be
under this approach). This can be expressed with
--force-cmd=apt-get,-o,APT::Sandbox::User=root
, though this particular
case is built-in and does not need to be specified. The full default
configuration, which is applied regardless of the image distribution, can be
examined in the source file force.py
. If any --force-cmd
are
specified, this replaces (rather than extends) the default configuration.
Note that because the substitutions are a simple regex with no knowledge of
shell syntax, they can cause unwanted modifications. For example, RUN
apt-get install -y apt-get
will be run as /bin/sh -c "apt-get -o
APT::Sandbox::User=root install -y apt-get -o APT::Sandbox::User=root"
. One
workaround is to add escape syntax transparent to the shell; e.g., RUN
apt-get install -y a\pt-get
.
This mode executes all RUN
instructions with the seccomp(2)
filter and has no knowledge of which instructions actually used the
intercepted system calls. Therefore, the printed “instructions modified”
number is only a count of instructions with a hook applied as described above.
6.7.4. Compatibility with other Dockerfile interpreters¶
ch-image
is an independent implementation and shares no code with
other Dockerfile interpreters. It uses a formal Dockerfile parsing grammar
developed from the Dockerfile reference documentation and miscellaneous other
sources, which you can examine in the source code.
We believe this independence is valuable for several reasons. First, it helps the community examine Dockerfile syntax and semantics critically, think rigorously about what is really needed, and build a more robust standard. Second, it yields disjoint sets of bugs (note that Podman, Buildah, and Docker all share the same Dockerfile parser). Third, because it is a much smaller code base, it illustrates how Dockerfiles work more clearly. Finally, it allows straightforward extensions if needed to support scientific computing.
ch-image
tries hard to be compatible with Docker and other
interpreters, though as an independent implementation, it is not
bug-compatible.
The following subsections describe differences from the Dockerfile reference that we expect to be approximately permanent. For not-yet-implemented features and bugs in this area, see related issues on GitHub.
None of these are set in stone. We are very interested in feedback on our assessments and open questions. This helps us prioritize new features and revise our thinking about what is needed for HPC containers.
6.7.4.1. Context directory¶
The context directory is bind-mounted into the build, rather than copied like Docker. Thus, the size of the context is immaterial, and the build reads directly from storage like any other local process would. However, you still can’t access anything outside the context directory.
6.7.4.2. Variable substitution¶
Variable substitution happens for all instructions, not just the ones listed in the Dockerfile reference.
ARG
and ENV
cause cache misses upon definition, in contrast
with Docker where these variables miss upon use, except for certain
cache-excluded variables that never cause misses, listed below.
Note that ARG
and ENV
have different syntax despite very
similar semantics.
ch-image
passes the following proxy environment variables in to the
build. Changes to these variables do not cause a cache miss. They do not
require an ARG
instruction, as documented in the
Dockerfile reference. Unlike Docker, they are available if the same-named
environment variable is defined; --build-arg
is not required.
HTTP_PROXY
http_proxy
HTTPS_PROXY
https_proxy
FTP_PROXY
ftp_proxy
NO_PROXY
no_proxy
In addition to those listed in the Dockerfile reference, these environment variables are passed through in the same way:
SSH_AUTH_SOCK
USER
Finally, these variables are also pre-defined but are unrelated to the host environment:
PATH=/ch/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
TAR_OPTIONS=--no-same-owner
6.7.4.3. ARG
¶
Variables set with ARG
are available anywhere in the Dockerfile,
unlike Docker, where they only work in FROM
instructions, and possibly
in other ARG
before the first FROM
.
6.7.4.4. FROM
¶
The FROM
instruction accepts option --arg=NAME=VALUE
, which
serves the same purpose as the ARG
instruction. It can be repeated.
6.7.4.5. LABEL
¶
The LABEL
instruction accepts key=value
pairs to
add metadata for an image. Unlike Docker, multiline values are not supported;
see issue #1512.
Can be repeated.
6.7.4.6. COPY
¶
Especially for people used to UNIX cp(1)
, the semantics of the
Dockerfile COPY
instruction can be confusing.
Most notably, when a source of the copy is a directory, the contents of that
directory, not the directory itself, are copied. This is documented, but it’s
a real gotcha because that’s not what cp(1)
does, and it means that
many things you can do in one cp(1)
command require multiple
COPY
instructions.
Also, the reference documentation is incomplete. In our experience, Docker
also behaves as follows; ch-image
does the same in an attempt to be
bug-compatible.
You can use absolute paths in the source; the root is the context directory.
Destination directories are created if they don’t exist in the following situations:
If the destination path ends in slash. (Documented.)
If the number of sources is greater than 1, either by wildcard or explicitly, regardless of whether the destination ends in slash. (Not documented.)
If there is a single source and it is a directory. (Not documented.)
Symbolic links behave differently depending on how deep in the copied tree they are. (Not documented.)
Symlinks at the top level — i.e., named as the destination or the source, either explicitly or by wildcards — are dereferenced. They are followed, and whatever they point to is used as the destination or source, respectively.
Symlinks at deeper levels are not dereferenced, i.e., the symlink itself is copied.
If a directory appears at the same path in source and destination, and is at the 2nd level or deeper, the source directory’s metadata (e.g., permissions) are copied to the destination directory. (Not documented.)
If an object appears in both the source and destination, and is at the 2nd level or deeper, and is of different types in the source and destination, then the source object will overwrite the destination object. (Not documented.) For example, if
/tmp/foo/bar
is a regular file, and/tmp
is the context directory, then the following Dockerfile snippet will result in a file in the container at/foo/bar
(copied from/tmp/foo/bar
); the directory and all its contents will be lost.RUN mkdir -p /foo/bar && touch /foo/bar/baz COPY foo /foo
We expect the following differences to be permanent:
Wildcards use Python glob semantics, not the Go semantics.
COPY --chown
is ignored, because it doesn’t make sense in an unprivileged build.
6.7.4.7. Features we do not plan to support¶
Parser directives are not supported. We have not identified a need for any of them.
EXPOSE
: Charliecloud does not use the network namespace, so containerized processes can simply listen on a host port like other unprivileged processes.HEALTHCHECK
: This instruction’s main use case is monitoring server processes rather than applications. Also, implementing it requires a container supervisor daemon, which we have no plans to add.MAINTAINER
is deprecated.STOPSIGNAL
requires a container supervisor daemon process, which we have no plans to add.USER
does not make sense for unprivileged builds.VOLUME
: This instruction is not currently supported. Charliecloud has good support for bind mounts; we anticipate that it will continue to focus on that and will not introduce the volume management features that Docker has.
6.7.5. Examples¶
Build image bar
using ./foo/bar/Dockerfile
and context
directory ./foo/bar
:
$ ch-image build -t bar -f ./foo/bar/Dockerfile ./foo/bar
[...]
grown in 4 instructions: bar
Same, but infer the image name and Dockerfile from the context directory path:
$ ch-image build ./foo/bar
[...]
grown in 4 instructions: bar
Build using humongous vendor compilers you want to bind-mount instead of installing into the image:
$ ch-image build --bind /opt/bigvendor:/opt .
$ cat Dockerfile
FROM centos:7
RUN /opt/bin/cc hello.c
#COPY /opt/lib/*.so /usr/local/lib # fail: COPY doesn’t bind mount
RUN cp /opt/lib/*.so /usr/local/lib # possible workaround
RUN ldconfig
6.8. build-cache
¶
$ ch-image [...] build-cache [...]
Print basic information about the cache. If -v
is given, also print
some Git statistics and the Git repository configuration.
If any of the following options are given, do the corresponding operation before printing. Multiple options can be given, in which case they happen in this order.
--reset
Clear and re-initialize the build cache.
--gc
Run Git garbage collection on the cache, including full de-duplication of similar files. This will immediately remove all cache entries not currently reachable from a named branch (which is likely to cause corruption if the build cache is being accessed concurrently by another process). The operation can take a long time on large caches.
--text
Print a text tree of the cache using Git’s
git log --graph
feature. If-v
is also given, the tree has more detail.--dot
Create a DOT export of the tree named
./build-cache.dot
and a PDF rendering./build-cache.pdf
. Requiresgraphviz
andgit2dot
.
6.9. delete
¶
$ ch-image [...] delete IMAGE_GLOB
Delete the image(s) described by IMAGE_GLOB
from the storage directory
(including all build stages).
IMAGE_GLOB
can be either a plain image reference or an image reference
with glob characters to match multiple images. For example, ch-image
delete 'foo*'
will delete all images whose names start with foo
.
Importantly, this sub-command does not also remove the image from the build cache. Therefore, it can be used to reduce the size of the storage directory, trading off the time needed to retrieve an image from cache.
Warning
Glob characters must be quoted or otherwise protected from the shell, which also desires to interpret them and will do so incorrectly.
6.10. gestalt
¶
$ ch-image [...] gestalt [SELECTOR]
Provide information about the configuration and available features of ch-image
. End users
generally will not need this; it is intended for testing and debugging.
SELECTOR
is one of:
bucache
. Exit successfully if the build cache is available, unsuccessfully with an error message otherwise. With-v
, also print version information about dependencies.
bucache-dot
. Exit successfully if build cache DOT trees can be written, unsuccessfully with an error message otherwise. With-v
, also print version information about dependencies.
python-path
. Print the path to the Python interpreter in use and exit successfully.
storage-path
. Print the storage directory path and exit successfully.
6.11. list
¶
Print information about images. If no argument given, list the images in builder storage.
6.11.1. Synopsis¶
$ ch-image [...] list [-l] [IMAGE_REF]
6.11.2. Description¶
Optional argument:
-l
,--long
Use long format (name, last change timestamp) when listing images.
IMAGE_REF
Print details of what’s known about
IMAGE_REF
, both locally and in the remote registry, if any.
6.11.3. Examples¶
List images in builder storage:
$ ch-image list
alpine:3.17 (amd64)
alpine:latest (amd64)
debian:buster (amd64)
Print details about Debian Buster image:
$ ch-image list debian:buster
details of image: debian:buster
in local storage: no
full remote ref: registry-1.docker.io:443/library/debian:buster
available remotely: yes
remote arch-aware: yes
host architecture: amd64
archs available: 386 bae2738ed83
amd64 98285d32477
arm/v7 97247fd4822
arm64/v8 122a0342878
For remotely available images like Debian Buster, the associated digest is listed beside each available architecture. Importantly, this feature does not provide the hash of the local image, which is only calculated on push.
6.12. import
¶
$ ch-image [...] import PATH IMAGE_REF
Copy the image at PATH
into builder storage with name
IMAGE_REF
. PATH
can be:
an image directory
a tarball with no top-level directory (a.k.a. a “tarbomb”)
a standard tarball with one top-level directory
If the imported image contains Charliecloud metadata, that will be imported
unchanged, i.e., images exported from ch-image
builder storage will be
functionally identical when re-imported.
Note
Every import creates a new cache entry, even if the file or directory has already been imported.
6.13. pull
¶
Pull the image described by the image reference IMAGE_REF
from a
repository to the local filesystem.
6.13.1. Synopsis¶
$ ch-image [...] pull [...] IMAGE_REF [DEST_REF]
See the FAQ for the gory details on specifying image references.
6.13.2. Description¶
Destination:
DEST_REF
If specified, use this as the destination image reference, rather than
IMAGE_REF
. This lets you pull an image with a complicated reference while storing it locally with a simpler one.
Options:
--last-layer N
Unpack only
N
layers, leaving an incomplete image. This option is intended for debugging.--parse-only
Parse
IMAGE_REF
, print a parse report, and exit successfully without talking to the internet or touching the storage directory.
This script does a fair amount of validation and fixing of the layer tarballs
before flattening in order to support unprivileged use despite image problems
we frequently see in the wild. For example, device files are ignored, and file
and directory permissions are increased to a minimum of rwx------
and
rw-------
respectively. Note, however, that symlinks pointing outside
the image are permitted, because they are not resolved until runtime within a
container.
The following metadata in the pulled image is retained; all other metadata is currently ignored. (If you have a need for additional metadata, please let us know!)
Current working directory set with
WORKDIR
is effective in downstream Dockerfiles.Environment variables set with
ENV
are effective in downstream Dockerfiles and also written to/ch/environment
for use inch-run --set-env
.Mount point directories specified with
VOLUME
are created in the image if they don’t exist, but no other action is taken.
Note that some images (e.g., those with a “version 1 manifest”) do not contain metadata. A warning is printed in this case.
6.13.3. Examples¶
Download the Debian Buster image matching the host’s architecture and place it in the storage directory:
$ uname -m
aarch32
pulling image: debian:buster
requesting arch: arm64/v8
manifest list: downloading
manifest: downloading
config: downloading
layer 1/1: c54d940: downloading
flattening image
layer 1/1: c54d940: listing
validating tarball members
resolving whiteouts
layer 1/1: c54d940: extracting
image arch: arm64
done
Same, specifying the architecture explicitly:
$ ch-image --arch=arm/v7 pull debian:buster
pulling image: debian:buster
requesting arch: arm/v7
manifest list: downloading
manifest: downloading
config: downloading
layer 1/1: 8947560: downloading
flattening image
layer 1/1: 8947560: listing
validating tarball members
resolving whiteouts
layer 1/1: 8947560: extracting
image arch: arm (may not match host arm64/v8)
6.14. push
¶
Push the image described by the image reference IMAGE_REF
from the
local filesystem to a repository.
6.14.1. Synopsis¶
$ ch-image [...] push [--image DIR] IMAGE_REF [DEST_REF]
See the FAQ for the gory details on specifying image references.
6.14.2. Description¶
Destination:
DEST_REF
If specified, use this as the destination image reference, rather than
IMAGE_REF
. This lets you push to a repository without permanently adding a tag to the image.
Options:
--image DIR
Use the unpacked image located at
DIR
rather than an image in the storage directory namedIMAGE_REF
.
Because Charliecloud is fully unprivileged, the owner and group of files in
its images are not meaningful in the broader ecosystem. Thus, when pushed,
everything in the image is flattened to user:group root:root
. Also,
setuid/setgid bits are removed, to avoid surprises if the image is pulled by a
privileged container implementation.
6.14.3. Examples¶
Push a local image to the registry example.com:5000
at path
/foo/bar
with tag latest
. Note that in this form, the local
image must be named to match that remote reference.
$ ch-image push example.com:5000/foo/bar:latest
pushing image: example.com:5000/foo/bar:latest
layer 1/1: gathering
layer 1/1: preparing
preparing metadata
starting upload
layer 1/1: a1664c4: checking if already in repository
layer 1/1: a1664c4: not present, uploading
config: 89315a2: checking if already in repository
config: 89315a2: not present, uploading
manifest: uploading
cleaning up
done
Same, except use local image alpine:3.17
. In this form, the local image
name does not have to match the destination reference.
$ ch-image push alpine:3.17 example.com:5000/foo/bar:latest
pushing image: alpine:3.17
destination: example.com:5000/foo/bar:latest
layer 1/1: gathering
layer 1/1: preparing
preparing metadata
starting upload
layer 1/1: a1664c4: checking if already in repository
layer 1/1: a1664c4: not present, uploading
config: 89315a2: checking if already in repository
config: 89315a2: not present, uploading
manifest: uploading
cleaning up
done
Same, except use unpacked image located at /var/tmp/image
rather than
an image in ch-image
storage. (Also, the sole layer is already present
in the remote registry, so we don’t upload it again.)
$ ch-image push --image /var/tmp/image example.com:5000/foo/bar:latest
pushing image: example.com:5000/foo/bar:latest
image path: /var/tmp/image
layer 1/1: gathering
layer 1/1: preparing
preparing metadata
starting upload
layer 1/1: 892e38d: checking if already in repository
layer 1/1: 892e38d: already present
config: 546f447: checking if already in repository
config: 546f447: not present, uploading
manifest: uploading
cleaning up
done
6.15. reset
¶
$ ch-image [...] reset
Delete all images and cache from ch-image builder storage.
6.16. undelete
¶
$ ch-image [...] undelete IMAGE_REF
If IMAGE_REF
has been deleted but is in the build cache, recover it
from the cache. Only available when the cache is enabled, and will not
overwrite IMAGE_REF
if it exists.
6.17. Environment variables¶
CH_IMAGE_USERNAME
,CH_IMAGE_PASSWORD
Username and password for registry authentication. See important caveats in section “Authentication” above.
CH_LOG_FILE
If set, append log chatter to this file, rather than standard error. This is useful for debugging situations where standard error is consumed or lost.
Also sets verbose mode if not already set (equivalent to
--verbose
).CH_LOG_FESTOON
If set, prepend PID and timestamp to logged chatter.