Re: [PATCH] fuse: don't truncate cached, mutated symlink

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Feb 23, 2025 at 08:12:21PM +0100, Miklos Szeredi wrote:
> On Sun, 23 Feb 2025 at 01:28, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Thu, Feb 20, 2025 at 11:02:58AM +0100, Miklos Szeredi wrote:
> >
> > > The solution is to just remove this truncation.  This can cause a
> > > regression in a filesystem that relies on supplying a symlink larger than
> > > the file size, but this is unlikely.  If that happens we'd need to make
> > > this behavior conditional.
> >
> > Note, BTW, that page *contents* must not change at all, so truncation is
> > only safe if we have ->i_size guaranteed to be stable.
> >
> > Core pathwalk really counts upon the string remaining immutable, and that
> > does include the string returned by ->get_link().
> 
> Page contents will not change after initial readlink, but page could
> get invalidated and a fresh page filled with new value for the same
> object.

*nod*

My point is that truncation of something that might be traversed by another
pathwalk is a hard bug - we only get away with doing that in page_get_link()
because it's idempotent.  If ->i_size can change, we are not allowed to
do that.