taskstats: return -EBADF when cgroupstats receives an invalid fd

Patch series "taskstats: fix cgroupstats invalid fd handling and add
selftests".

This series fixes an issue where cgroupstats mishandles invalid file
descriptors, and introduces a functional kselftest to prevent regressions.

When an invalid file descriptor is passed via CGROUPSTATS_CMD_ATTR_FD,
cgroupstats_user_cmd() returns 0 instead of an error code. This leads to
two broken behaviors depending on netlink flags:
- Callers without NLM_F_ACK block indefinitely on recv().
- Callers with NLM_F_ACK receive a misleading success ACK (errno == 0)
  but no actual statistics payload.

The first patch addresses this by returning -EBADF when the fd cannot be
resolved.  The second patch adds a comprehensive kselftest covering both
the valid cgroup v1 query and the invalid fd paths (with and without
NLM_F_ACK) to ensure the fixes work as intended.


This patch (of 2):

cgroupstats_user_cmd() returns 0 without sending a reply or a netlink
error when the fd passed via CGROUPSTATS_CMD_ATTR_FD does not resolve to
an open file in the caller's table.  As a result:

- clients that did not set NLM_F_ACK block on recv() indefinitely
  waiting for a CGROUPSTATS_CMD_NEW message that is never emitted;

- clients that set NLM_F_ACK receive a misleading "success" ACK (errno
  == 0) with no statistics payload.

Return -EBADF instead so the netlink layer propagates the error to
userspace as expected.

Link: https://lore.kernel.org/cover.1783713230.git.cyyzero16@gmail.com
Link: https://lore.kernel.org/b4fd9e288e4a48efebaf41b4ffcdb204b06675c4.1783713230.git.cyyzero16@gmail.com
Signed-off-by: Yiyang Chen <cyyzero16@gmail.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Cc: Dr. Thomas Orgis <thomas.orgis@uni-hamburg.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 2cd0172d0516..8d115670a3ca 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -423,7 +423,7 @@ static int cgroupstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
 	fd = nla_get_u32(info->attrs[CGROUPSTATS_CMD_ATTR_FD]);
 	CLASS(fd, f)(fd);
 	if (fd_empty(f))
-		return 0;
+		return -EBADF;
 
 	size = nla_total_size(sizeof(struct cgroupstats));