Re: [PATCH 2/3] fs: namei: Allow follow_down() to uncover auto mounts

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On 18/11/22 05:01, Jeff Layton wrote:
On Thu, 2022-11-17 at 20:11 +0100, Richard Weinberger wrote:
This function is only used by NFSD to cross mount points.
If a mount point is of type auto mount, follow_down() will
not uncover it. Add LOOKUP_AUTOMOUNT to the lookup flags
to have ->d_automount() called when NFSD walks down the
mount tree.

Signed-off-by: Richard Weinberger <richard@xxxxxx>
---
  fs/namei.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 578c2110df02..000c4b84e6be 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1462,7 +1462,7 @@ int follow_down(struct path *path)
  {
  	struct vfsmount *mnt = path->mnt;
  	bool jumped;
-	int ret = traverse_mounts(path, &jumped, NULL, 0);
+	int ret = traverse_mounts(path, &jumped, NULL, LOOKUP_AUTOMOUNT);
if (path->mnt != mnt)
  		mntput(mnt);

What happens when CROSSMOUNT isn't enabled and someone tries to stroll
into an automount point? I'm guessing the automount happens but the
export is denied? It seems like LOOKUP_AUTOMOUNT ought to be conditional
on the parent export having CROSSMOUNT set.

There's also another caller of follow_down too, the UNIX98 pty code.
This may be harmless for it, but it'd be best not to perturb that if we
can help it.

Maybe follow_down can grow a lookupflags argument?es, I think that's needed too.


Changing the core VFS unconditionally ricks breaking things.


For example this:

        if (!(lookup_flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
                           LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_AUTOMOUNT)) &&
            dentry->d_inode)

will never be true now so that, at the least, the handling of this case

will change for automount(8). I don't remember now the reasons behind

doing this but I do remember there was a special case that needed to

be handled by it.


Ian




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