[PATCH 2/2] vfs: Make sure {statx,fstatat}(..., AT_EMPTY_PATH | ..., NULL, ...) behave as (..., AT_EMPTY_PATH | ..., "", ...)

[Xi Ruoyao]

We've supported {statx,fstatat}(real_fd, NULL, AT_EMPTY_PATH, ...) since
Linux 6.11 for better performance.  However there are other cases, for
example using AT_FDCWD as the fd or having AT_SYMLINK_NOFOLLOW in flags,
not covered by the fast path.  While it may be impossible, too
difficult, or not very beneficial to optimize these cases, we should
still turn NULL into "" for them in the slow path to make the API easier
to be documented and used.

Fixes: 0ef625bba6fb ("vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
---
 fs/stat.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/stat.c b/fs/stat.c
index ed9d4fd8ba2c..5d1b51c23c62 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -337,8 +337,11 @@ int vfs_fstatat(int dfd, const char __user *filename,
 	flags &= ~AT_NO_AUTOMOUNT;
 	if (flags == AT_EMPTY_PATH && vfs_empty_path(dfd, filename))
 		return vfs_fstat(dfd, stat);
+	else if ((flags & AT_EMPTY_PATH) && !filename)
+		name = getname_kernel("");
+	else
+		name = getname_flags(filename, getname_statx_lookup_flags(statx_flags));
 
-	name = getname_flags(filename, getname_statx_lookup_flags(statx_flags));
 	ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS);
 	putname(name);
 
@@ -791,8 +794,11 @@ SYSCALL_DEFINE5(statx,
 	lflags = flags & ~(AT_NO_AUTOMOUNT | AT_STATX_SYNC_TYPE);
 	if (lflags == AT_EMPTY_PATH && vfs_empty_path(dfd, filename))
 		return do_statx_fd(dfd, flags & ~AT_NO_AUTOMOUNT, mask, buffer);
+	else if ((lflags & AT_EMPTY_PATH) && !filename)
+		name = getname_kernel("");
+	else
+		name = getname_flags(filename, getname_statx_lookup_flags(flags));
 
-	name = getname_flags(filename, getname_statx_lookup_flags(flags));
 	ret = do_statx(dfd, name, flags, mask, buffer);
 	putname(name);
 
-- 
2.46.2