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Hengyu Liang 7b15d6cf25 kernfs: preserve security xattrs without allocating iattrs

Commit d5e81a5650 ("kernfs: avoid iattr allocation in listxattr")
made kernfs_iop_listxattr() return an empty list when the kernfs node
has no allocated kernfs_iattrs.

However, this also skips security xattr names provided by
simple_xattr_list(). As of now, applications can retrieve the SELinux
label of a sysfs file with getxattr(), but cannot do it through
listxattr().

A similar issue happened before in commit b09e0fa4b4 ("tmpfs:
implement generic xattr support"). It was fixed by commit 8b0ba61df5
("fs/xattr.c: fix simple_xattr_list to always include security.* xattrs").
Perhaps this recent commit needs a fix as well.

The issue can be reproduced with a simple python program:

    python3 - <<'PY'
    import os

    path = "/sys/kernel/warn_count"

    print("getxattr:", os.getxattr(path, "security.selinux"))
    print("listxattr:", os.listxattr(path))
    PY

Before commit d5e81a5650 ("kernfs: avoid iattr allocation in listxattr"),
the result is:

    getxattr: b'system_u:object_r:sysfs_t:s0\x00'
    listxattr: ['security.selinux']

After that commit, the result is:

    getxattr: b'system_u:object_r:sysfs_t:s0\x00'
    listxattr: []

This patch will keep listxattr() consistent with getxattr() when security
xattrs are available.

Fixes: d5e81a5650 ("kernfs: avoid iattr allocation in listxattr")
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20260822051705.1761850-1-hengyul@cs.unc.edu
Signed-off-by: Danilo Krummrich <dakr@kernel.org>