Re: [PATCH 2/2] xfs: make sure link path does not go away at access

From: Miklos Szeredi
Date: Tue Nov 16 2021 - 05:12:44 EST

On Tue, 16 Nov 2021 at 04:01, Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> I *think* that just zeroing the buffer means the race condition
> means the link resolves as either wholly intact, partially zeroed
> with trailing zeros in the length, wholly zeroed or zero length.
> Nothing will crash, the link string is always null terminated even
> if the length is wrong, and so nothing bad should happen as a result
> of zeroing the symlink buffer when it gets evicted from the VFS
> inode cache after unlink.

That's my thinking. However, modifying the buffer while it is being
processed does seem pretty ugly, and I have to admit that I don't
understand why this needs to be done in either XFS or EXT4.

> The root cause is "allowing an inode to be reused without waiting
> for an RCU grace period to expire". This might seem pedantic, but
> "without waiting for an rcu grace period to expire" is the important
> part of the problem (i.e. the bug), not the "allowing an inode to be
> reused" bit.