Re: [PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped resolution

On 2019-11-13, Al Viro [off-list ref] wrote:

On Tue, Nov 05, 2019 at 08:05:49PM +1100, Aleksa Sarai wrote:

quoted

@@ -2277,12 +2277,20 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 
 	nd->m_seq = read_seqbegin(&mount_lock);
 
-	/* Figure out the starting path and root (if needed). */
-	if (*s == '/') {
+	/* Absolute pathname -- fetch the root. */
+	if (flags & LOOKUP_IN_ROOT) {
+		/* With LOOKUP_IN_ROOT, act as a relative path. */
+		while (*s == '/')
+			s++;

Er...  Why bother skipping slashes?  I mean, not only link_path_walk()
will skip them just fine, you are actually risking breakage in this:
                if (*s && unlikely(!d_can_lookup(dentry))) {
                        fdput(f);
                        return ERR_PTR(-ENOTDIR);
                }
which is downstream from there with you patch, AFAICS.

I switched to stripping the slashes at your suggestion a few revisions
ago[1], and had (wrongly) assumed we needed to handle "/" somehow in
path_init(). But you're quite right about link_path_walk() -- and I'd be
more than happy to drop it.

[1]: https://lore.kernel.org/lkml/20190712125552.GL17978@ZenIV.linux.org.uk/ (local)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>