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cgroup: clarify cgroup_css_set_fork() 

With recent fixes for the permission checking when moving a task into a cgroup
using a file descriptor to a cgroup's cgroup.procs file and calling write() it
seems a good idea to clarify CLONE_INTO_CGROUP permission checking with a
comment.

Cc: Tejun Heo <tj@kernel.org>
Cc: <cgroups@vger.kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Christian Brauner 2022-02-21 16:16:39 +01:00 committed by Tejun Heo
parent 05c7b7a92c
commit 6d3971dab2
1 changed files with 14 additions and 0 deletions
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14
kernel/cgroup/cgroup.c
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@ -6161,6 +6161,20 @@ static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
if (ret)
goto err;
/*
* Spawning a task directly into a cgroup works by passing a file
* descriptor to the target cgroup directory. This can even be an O_PATH
* file descriptor. But it can never be a cgroup.procs file descriptor.
* This was done on purpose so spawning into a cgroup could be
* conceptualized as an atomic
*
* fd = openat(dfd_cgroup, "cgroup.procs", ...);
* write(fd, <child-pid>, ...);
*
* sequence, i.e. it's a shorthand for the caller opening and writing
* cgroup.procs of the cgroup indicated by @dfd_cgroup. This allows us
* to always use the caller's credentials.
*/
ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
!(kargs->flags & CLONE_THREAD),
current->nsproxy->cgroup_ns);