getname_maybe_null() - the third variant of pathname copy-in

Semantics used by statx(2) (and later *xattrat(2)): without AT_EMPTY_PATH
it's standard getname() (i.e. ERR_PTR(-ENOENT) on empty string,
ERR_PTR(-EFAULT) on NULL), with AT_EMPTY_PATH both empty string and
NULL are accepted.

Calling conventions: getname_maybe_null(user_pointer, flags) returns
	* pointer to struct filename when non-empty string had been
successfully read
	* ERR_PTR(...) on error
	* NULL if an empty string or NULL pointer had been given
with AT_EMPTY_PATH in the flags argument.

It tries to avoid allocation in the last case; it's not always
able to do so, in which case the temporary struct filename instance
is freed and NULL returned anyway.

Fast path is inlined.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 4 insertions, 24 deletions

diff --git a/fs/stat.c b/fs/stat.c
index 41e598376d7e..b74831dc7ae6 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -326,18 +326,11 @@ int vfs_fstatat(int dfd, const char __user *filename,
 {
 	int ret;
 	int statx_flags = flags | AT_NO_AUTOMOUNT;
-	struct filename *name;
+	struct filename *name = getname_maybe_null(filename, flags);
 
-	/*
-	 * Work around glibc turning fstat() into fstatat(AT_EMPTY_PATH)
-	 *
-	 * If AT_EMPTY_PATH is set, we expect the common case to be that
-	 * empty path, and avoid doing all the extra pathname work.
-	 */
-	if (flags == AT_EMPTY_PATH && vfs_empty_path(dfd, filename))
+	if (!name && dfd >= 0)
 		return vfs_fstat(dfd, stat);
 
-	name = getname_flags(filename, getname_statx_lookup_flags(statx_flags));
 	ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS);
 	putname(name);
 
@@ -774,24 +767,11 @@ SYSCALL_DEFINE5(statx,
 		struct statx __user *, buffer)
 {
 	int ret;
-	unsigned lflags;
-	struct filename *name;
+	struct filename *name = getname_maybe_null(filename, flags);
 
-	/*
-	 * Short-circuit handling of NULL and "" paths.
-	 *
-	 * For a NULL path we require and accept only the AT_EMPTY_PATH flag
-	 * (possibly |'d with AT_STATX flags).
-	 *
-	 * However, glibc on 32-bit architectures implements fstatat as statx
-	 * with the "" pathname and AT_NO_AUTOMOUNT | AT_EMPTY_PATH flags.
-	 * Supporting this results in the uglification below.
-	 */
-	lflags = flags & ~(AT_NO_AUTOMOUNT | AT_STATX_SYNC_TYPE);
-	if (lflags == AT_EMPTY_PATH && vfs_empty_path(dfd, filename))
+	if (!name && dfd >= 0)
 		return do_statx_fd(dfd, flags & ~AT_NO_AUTOMOUNT, mask, buffer);
 
-	name = getname_flags(filename, getname_statx_lookup_flags(flags));
 	ret = do_statx(dfd, name, flags, mask, buffer);
 	putname(name);