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Subject[PATCH v3 4/4] cgroupfs: Support user xattrs
Date
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>
---
kernel/cgroup/cgroup.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 7a39dc882095..ae1d808c0b9b 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -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);
--
2.21.1
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