Re: [RFC PATCH v1 0/2] Add capabilities file to sysfs

[Casey Schaufler]

On 12/28/2021 5:34 AM, Francis Laniel wrote:
> Hi.
>
> Le lundi 27 décembre 2021, 23:22:41 CET Casey Schaufler a écrit :
> > On 12/27/2021 12:54 PM, Francis Laniel wrote:
> > > Hi.
> > >
> > >
> > > First, I hope you are fine and the same for your relatives.
> > >
> > > Capabilities are used to check if a thread has the right to perform a
> > > given
> > > action [1].
> > > For example, a thread with CAP_BPF set can use the bpf() syscall.
> > >
> > > Capabilities are used in the container world.
> > > In terms of code, several projects related to container maintain code
> > > where the capabilities are written alike include/uapi/linux/capability.h
> > > [2][3][4][5]. For these projects, their codebase should be updated when a
> > > new capability is added to the kernel.
> > > Some other projects rely on <sys/capability.h> [6].
> > > In this case, this header file should reflect the capabilities offered by
> > > the kernel.
> > >
> > > So, in this series, I added a new file to sysfs: /sys/kernel/capabilities.
> > This should be /sys/kernel/security/capabilities.
> I began to write code to move this under /sys/kernel/security/capabilities but
> I realized this directory is linked to CONFIG_SECURITYFS.
> This option is not required to be able to run container [1].

You're going to need to handle the case where the file is missing
regardless. It is hard to design a kernel feature based on what a
container expects when there are so many definitions of a container.

> Also, kernel/capability.c is always compiled, so I think it is better if this
> file (i.e. the one which prints capabilities to user) does not depend on any
> CONFIG_.

CONFIG_MULTUSER is going to be an issue if you really care.

> What do you think of it? Does this sound acceptable for you?

Meh. I'm not going to get worked up over it, but your rationale
is a little weak.

> > > The goal of this file is to be used by "container world" software to know
> > > kernel capabilities at run time instead of compile time.
> > >
> > > The underlying kernel attribute is read-only and its content is the
> > > capability number associated with the capability name:
> > > root@vm-amd64:~# cat /sys/kernel/capabilities
> > > 0       CAP_CHOWN
> > > 1       CAP_DAC_OVERRIDE
> > > ...
> > > 39      CAP_BPF
> > >
> > > The kernel already exposes the last capability number under:
> > > /proc/sys/kernel/cap_last_cap
> > > So, I think there should not be any issue exposing all the capabilities it
> > > offers.
> > > If there is any, please share it as I do not want to introduce issue with
> > > this series.
> > >
> > > Also, if you see any way to improve this series please share it as it
> > > would
> > > increase this contribution quality.
> > >
> > > Francis Laniel (2):
> > >     capability: Add cap_strings.
> > >     kernel/ksysfs.c: Add capabilities attribute.
> > >    
> > >    include/uapi/linux/capability.h |  1 +
> > >    kernel/capability.c             | 45 +++++++++++++++++++++++++++++++++
> > >    kernel/ksysfs.c                 | 18 +++++++++++++
> > >    3 files changed, 64 insertions(+)
> > >
> > > Best regards and thank you in advance for your reviews.
> > > ---
> > > [1] man capabilities
> > > [2]
> > > https://github.com/containerd/containerd/blob/1a078e6893d07fec10a4940a566
> > > 4fab21d6f7d1e/pkg/cap/cap_linux.go#L135 [3]
> > > https://github.com/moby/moby/commit/485cf38d48e7111b3d1f584d5e9eab46a902a
> > > abc#diff-2e04625b209932e74c617de96682ed72fbd1bb0d0cb9fb7c709cf47a86b6f9c1
> > > moby relies on containerd code.
> > > [4]
> > > https://github.com/syndtr/gocapability/blob/42c35b4376354fd554efc7ad35e0b
> > > 7f94e3a0ffb/capability/enum.go#L47 [5]
> > > https://github.com/opencontainers/runc/blob/00f56786bb220b55b41748231880b
> > > a0e6380519a/libcontainer/capabilities/capabilities.go#L12 runc relies on
> > > syndtr package.
> > > [6]
> > > https://github.com/containers/crun/blob/fafb556f09e6ffd4690c452ff51856b88
> > > 0c089f1/src/libcrun/linux.c#L35
>
> Best regards.
> ---
> [1] https://github.com/moby/moby/blob/
> 10aecb0e652d346130a37e5b4383eca28f594c21/contrib/check-config.sh