Re: [RFC] [PATCH] Fix warning at fs/dcache.c:430 dentry_free

[Helge Deller]

Hi Al,

On 4/6/26 22:07, Al Viro wrote:
> On Mon, Apr 06, 2026 at 09:52:16PM +0200, Helge Deller wrote:
> > The debian buildd servers for the parisc architecture crash reproduceably when
> > building the webkit2gtk debian package, shortly after having shown the warning
> > below.
> >
> > This patch keeps the lock of the dentry up until when the dentry is given back
> > to the cache and after having freed the "external dentry name".
> >
> > I'm not sure if this patch is really correct, but it seems to have fixed the
> > problem, although more testing is needed.
>
> Hard NAK.  You are turning every place that grabs ->d_lock on a dentry scheduled
> for freeing (like, say it, any RCU pathwalk trying to check if the end result can
> be grabbed) into a UAF.

Thanks for looking into the patch!
I assume UAF means User-after-free?
As I'm not an expert here, could you please point me to where
this use-after-free happens?
The kfree() is used on the external dentry name, and the lock is
unlocked before calling kmem_cache_free(), so I'd not expect that I
introduced an UAF here. But of course I could be wrong....
 
> Do you have a better localized reproducer?

Sadly not yet. I will try, but since the package is huge and the machines are
relatively slow it's not easy to track down.

Thanks!
Helge