readlinkat(), fchownat() and fstatat() with empty relative pathnames

For readlinkat() we simply allow empty pathname; it will fail unless
we have dfd equal to O_PATH-opened symlink, so we are outside of
POSIX scope here.  For fchownat() and fstatat() we allow AT_EMPTY_PATH;
let the caller explicitly ask for such behaviour.

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

2 files changed, 11 insertions, 6 deletions

diff --git a/fs/open.c b/fs/open.c
index 14a51de01f5..3cac0bda46d 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -573,13 +573,15 @@ SYSCALL_DEFINE5(fchownat, int, dfd, const char __user *, filename, uid_t, user,
 {
         struct path path;
         int error = -EINVAL;
-        int follow;
+        int lookup_flags;
 
-        if ((flag & ~AT_SYMLINK_NOFOLLOW) != 0)
+        if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
                 goto out;
 
-        follow = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
-        error = user_path_at(dfd, filename, follow, &path);
+        lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
+        if (flag & AT_EMPTY_PATH)
+                lookup_flags |= LOOKUP_EMPTY;
+        error = user_path_at(dfd, filename, lookup_flags, &path);
         if (error)
                 goto out;
         error = mnt_want_write(path.mnt);
diff --git a/fs/stat.c b/fs/stat.c
index d5c61cf2b70..961039121cb 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -75,13 +75,16 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
         int error = -EINVAL;
         int lookup_flags = 0;
 
-        if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT)) != 0)
+        if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
+                      AT_EMPTY_PATH)) != 0)
                 goto out;
 
         if (!(flag & AT_SYMLINK_NOFOLLOW))
                 lookup_flags |= LOOKUP_FOLLOW;
         if (flag & AT_NO_AUTOMOUNT)
                 lookup_flags |= LOOKUP_NO_AUTOMOUNT;
+        if (flag & AT_EMPTY_PATH)
+                lookup_flags |= LOOKUP_EMPTY;
 
         error = user_path_at(dfd, filename, lookup_flags, &path);
         if (error)
@@ -297,7 +300,7 @@ SYSCALL_DEFINE4(readlinkat, int, dfd, const char __user *, pathname,
         if (bufsiz <= 0)
                 return -EINVAL;
 
-        error = user_path_at(dfd, pathname, 0, &path);
+        error = user_path_at(dfd, pathname, LOOKUP_EMPTY, &path);
         if (!error) {
                 struct inode *inode = path.dentry->d_inode;