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cgroupfs: Support user xattrs

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This patch turns on xattr support for cgroupfs. This is useful for
letting non-root owners of delegated subtrees attach metadata to
cgroups.

One use case is for subtree owners to tell a userspace out of memory
killer to bias away from killing specific subtrees.

Tests:

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice -n user.name$i -v wow; done
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice --remove user.name$i; done
    setfattr: workload.slice: No such attribute
    setfattr: workload.slice: No such attribute
    setfattr: workload.slice: No such attribute

    [/sys/fs/cgroup]# for i in $(seq 0 130); \
        do setfattr workload.slice -n user.name$i -v wow; done
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device
    setfattr: workload.slice: No space left on device

`seq 0 130` is inclusive, and 131 - 128 = 3, which is the number of
errors we expect to see.

    [/data]# cat testxattr.c
    #include <sys/types.h>
    #include <sys/xattr.h>
    #include <stdio.h>
    #include <stdlib.h>

    int main() {
      char name[256];
      char *buf = malloc(64 << 10);
      if (!buf) {
        perror("malloc");
        return 1;
      }

      for (int i = 0; i < 4; ++i) {
        snprintf(name, 256, "user.bigone%d", i);
        if (setxattr("/sys/fs/cgroup/system.slice", name, buf,
                     64 << 10, 0)) {
          printf("setxattr failed on iteration=%d\n", i);
          return 1;
        }
      }

      return 0;
    }

    [/data]# ./a.out
    setxattr failed on iteration=2

    [/data]# ./a.out
    setxattr failed on iteration=0

    [/sys/fs/cgroup]# setfattr -x user.bigone0 system.slice/
    [/sys/fs/cgroup]# setfattr -x user.bigone1 system.slice/

    [/data]# ./a.out
    setxattr failed on iteration=2

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Daniel Xu 2020-03-12 13:03:17 -07:00 committed by Tejun Heo
parent 0c47383ba3
commit 38aca3071c
1 changed files with 2 additions and 1 deletions
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3
kernel/cgroup/cgroup.c
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@ -1954,7 +1954,8 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
root->kf_root = kernfs_create_root(kf_sops,
KERNFS_ROOT_CREATE_DEACTIVATED |
KERNFS_ROOT_SUPPORT_EXPORTOP,
KERNFS_ROOT_SUPPORT_EXPORTOP |
KERNFS_ROOT_SUPPORT_USER_XATTR,
root_cgrp);
if (IS_ERR(root->kf_root)) {
ret = PTR_ERR(root->kf_root);