[RFC][PATCH] getname_maybe_null() - the third variant of pathname copy-in

[Al Viro <viro@zeniv.linux.org.uk>]

[
in #work.getname; if nobody objects, I'm going to make #work.xattr pull that.
IMO it's saner than hacks around vfs_empty_path() and it does very similar
logics - with simpler handling on the caller side.
]

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_FLAGS 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>
--- 
diff --git a/fs/namei.c b/fs/namei.c
index 4a4a22a08ac2..27eb0a81d9b8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -211,22 +211,38 @@ getname_flags(const char __user *filename, int flags)
 	return result;
 }
 
-struct filename *
-getname_uflags(const char __user *filename, int uflags)
+struct filename *getname_uflags(const char __user *filename, int uflags)
 {
 	int flags = (uflags & AT_EMPTY_PATH) ? LOOKUP_EMPTY : 0;
 
 	return getname_flags(filename, flags);
 }
 
-struct filename *
-getname(const char __user * filename)
+struct filename *getname(const char __user * filename)
 {
 	return getname_flags(filename, 0);
 }
 
-struct filename *
-getname_kernel(const char * filename)
+struct filename *__getname_maybe_null(const char __user *pathname)
+{
+	struct filename *name;
+	char c;
+
+	/* try to save on allocations; loss on um, though */
+	if (get_user(c, pathname))
+		return ERR_PTR(-EFAULT);
+	if (!c)
+		return NULL;
+
+	name = getname_flags(pathname, LOOKUP_EMPTY);
+	if (!IS_ERR(name) && !(name->name[0])) {
+		putname(name);
+		name = NULL;
+	}
+	return name;
+}
+
+struct filename *getname_kernel(const char * filename)
 {
 	struct filename *result;
 	int len = strlen(filename) + 1;
diff --git a/fs/stat.c b/fs/stat.c
index 41e598376d7e..aa5bfc41a669 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)
 		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);
 
@@ -775,7 +768,7 @@ SYSCALL_DEFINE5(statx,
 {
 	int ret;
 	unsigned lflags;
-	struct filename *name;
+	struct filename *name = getname_maybe_null(filename, flags);
 
 	/*
 	 * Short-circuit handling of NULL and "" paths.
@@ -788,10 +781,9 @@ SYSCALL_DEFINE5(statx,
 	 * 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)
 		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);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e3c603d01337..403258ac2ea2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2766,6 +2766,16 @@ extern struct filename *getname_flags(const char __user *, int);
 extern struct filename *getname_uflags(const char __user *, int);
 extern struct filename *getname(const char __user *);
 extern struct filename *getname_kernel(const char *);
+extern struct filename *__getname_maybe_null(const char __user *);
+static inline struct filename *getname_maybe_null(const char __user *name, int flags)
+{
+	if (!(flags & AT_EMPTY_PATH))
+		return getname(name);
+
+	if (!name)
+		return NULL;
+	return __getname_maybe_null(name);
+}
 extern void putname(struct filename *name);
 
 extern int finish_open(struct file *file, struct dentry *dentry,