[SRU][EOAN][PATCH 2/2] UBUNTU: SAUCE: shiftfs: prevent ESTALE for LOOKUP_JUMP lookups

[Christian Brauner]

BugLink: https://bugs.launchpad.net/bugs/1872757

Users reported that creating temporary files shiftfs reports ESTALE.
This can be reproduced via:

import tempfile
import os

def test():
    with tempfile.TemporaryFile() as fd:
        fd.write("data".encode('utf-8'))
        # re-open the file to get a read-only file descriptor
        return open(f"/proc/self/fd/{fd.fileno()}", "r")

def main():
   fd = test()
   fd.close()

if __name__ == "__main__":
    main()

a similar issue was reported here:
https://github.com/systemd/systemd/issues/14861

Our revalidate methods were very opinionated about whether or not a
lower dentry was valid especially when it became unlinked we simply
invalidated the lower dentry which caused above bug to surface. This has
led to bugs where a ESTALE was returned for e.g.  temporary files that
were created and directly re-opened afterwards through
/proc/<pid>/fd/<nr-of-deleted-file>. When a file is re-opened through
/proc/<pid>/fd/<nr> LOOKUP_JUMP is set and the vfs will revalidate via
d_weak_revalidate(). Since the file has been unhashed or even already
gone negative we'd fail the open when we should've succeeded.

Reported-by: Christian Kellner <ckellner at redhat.com>
Reported-by: Evgeny Vereshchagin <evvers at ya.ru>
Cc: Seth Forshee <seth.forshee at canonical.com>
Link: https://github.com/systemd/systemd/issues/14861
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
 fs/shiftfs.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index 5d88193b41db..6f40cb9272be 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -251,8 +251,6 @@ static int shiftfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
 		struct inode *loweri = d_inode(lowerd);
 
 		shiftfs_copyattr(loweri, inode);
-		if (!inode->i_nlink)
-			err = 0;
 	}
 
 	return err;
@@ -278,8 +276,6 @@ static int shiftfs_d_revalidate(struct dentry *dentry, unsigned int flags)
 		struct inode *loweri = d_inode(lowerd);
 
 		shiftfs_copyattr(loweri, inode);
-		if (!inode->i_nlink)
-			err = 0;
 	}
 
 	return err;
-- 
2.27.0