/* Background. */ Userspace cannot easily resolve a path without resolving symlinks, and would have to manually resolve each path component with O_PATH and O_NOFOLLOW. This is clearly inefficient, and can be fairly easy to screw up (resulting in possible security bugs). Linus has mentioned that Git has a particular need for this kind of flag[1]. It also resolves a fairly long-standing perceived deficiency in O_NOFOLLOw -- that it only blocks the opening of trailing symlinks.
This is part of a refresh of Al's AT_NO_JUMPS patchset[2] (which was a variation on David Drysdale's O_BENEATH patchset[3], which in turn was based on the Capsicum project[4]).
/* Userspace API. */ LOOKUP_NO_SYMLINKS will be exposed to userspace through openat2(2).
/* Semantics. */ Unlike most other LOOKUP flags (most notably LOOKUP_FOLLOW), LOOKUP_NO_SYMLINKS applies to all components of the path.
With LOOKUP_NO_SYMLINKS, any symlink path component encountered during path resolution will yield -ELOOP. If the trailing component is a symlink (and no other components were symlinks), then O_PATH|O_NOFOLLOW will not error out and will instead provide a handle to the trailing symlink -- without resolving it.
/* Testing. */ LOOKUP_NO_SYMLINKS is tested as part of the openat2(2) selftests.
[1]: https://lore.kernel.org/lkml/CA+55aFyOKM7DW7+0sdDFKdZFXgptb5r1id9=Wvhd8AgSP7... [2]: https://lore.kernel.org/lkml/20170429220414.GT29622@ZenIV.linux.org.uk/ [3]: https://lore.kernel.org/lkml/1415094884-18349-1-git-send-email-drysdale@goog... [4]: https://lore.kernel.org/lkml/1404124096-21445-1-git-send-email-drysdale@goog...
Cc: Christian Brauner christian.brauner@ubuntu.com Suggested-by: Al Viro viro@zeniv.linux.org.uk Suggested-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Aleksa Sarai cyphar@cyphar.com --- fs/namei.c | 3 +++ include/linux/namei.h | 3 +++ 2 files changed, 6 insertions(+)
diff --git a/fs/namei.c b/fs/namei.c index 259652667881..14d6d3afb9d3 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1052,6 +1052,9 @@ const char *get_link(struct nameidata *nd) int error; const char *res;
+ if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS)) + return ERR_PTR(-ELOOP); + if (!(nd->flags & LOOKUP_RCU)) { touch_atime(&last->link); cond_resched(); diff --git a/include/linux/namei.h b/include/linux/namei.h index 758e9b47db6f..0d86e75c04a7 100644 --- a/include/linux/namei.h +++ b/include/linux/namei.h @@ -39,6 +39,9 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND}; #define LOOKUP_ROOT 0x2000 #define LOOKUP_ROOT_GRABBED 0x0008
+/* Scoping flags for lookup. */ +#define LOOKUP_NO_SYMLINKS 0x010000 /* No symlink crossing. */ + extern int path_pts(struct path *path);
extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);