On Sun, Oct 05, 2014 at 11:42:40PM -0400, Sasha Levin wrote:
> On 10/05/2014 11:13 PM, Al Viro wrote:
> > On Sun, Oct 05, 2014 at 08:27:47PM -0400, Sasha Levin wrote:
> >
> >> [ 434.580818] BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
> >> [ 434.582208] IP: do_raw_spin_trylock (./arch/x86/include/asm/spinlock.h:108 kernel/locking/spinlock_debug.c:143)
> > [snip]
> > spin_lock((void *)0x90)
> >> [ 434.590025] ? _raw_spin_trylock (include/linux/spinlock_api_smp.h:89 kernel/locking/spinlock.c:135)
> >> [ 434.590025] ? lockref_put_or_lock (lib/lockref.c:131)
> >> [ 434.590025] dput (fs/dcache.c:513 fs/dcache.c:616)
> >
> > ummm... lockref_put_or_lock(&dentry->d_lockref) ending up with 0x90 passed
> > to lockref_put_or_lock()... What offset does d_lockref have on your build?
>
> 0x90
Huh??? It means that we got to that lockref_put_or_lock with dentry == NULL.
But that makes no sense at all - we have
void dput(struct dentry *dentry)
{
if (unlikely(!dentry))
return;
repeat:
if (lockref_put_or_lock(&dentry->d_lockref))
return;
...
and the only branch to repeat: is
if (dentry)
goto repeat;
If we get to that lockref_put_or_lock() with dentry == NULL, something's
very wrong with compiler. And the only other lockref_put_or_lock() in
there is
while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) {
which would also make NULL dentry a miscompile.
Could you put fs/dcache.s for your build on some anonftp? That really
smells like compiler breakage; had the address it tried to access been
close but not equal to that offsetof(), we would be dealing with bogus
->f_path.dentry (close to, but not quite NULL). As it is, it looks like
dput() somehow getting to that line with NULL dentry, which should've
been prevented by the checks there...
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