On 10/06/2014 12:25 AM, Al Viro wrote:
> 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...
I think you've misread the stack trace. The lockref_put_or_lock isn't reliable
(probably just a derelict from the previous, successful call to it), and the
stack trace's reliable symbols takes us elsewhere.
Stack trace shows that:
[ 434.590025] dput (fs/dcache.c:513 fs/dcache.c:616)
[ 434.590025] __fput (fs/file_table.c:235)
[ 434.590025] ____fput (fs/file_table.c:253)
Looking at fs/dcache.c:616 we see:
kill_it:
dentry = dentry_kill(dentry); <=== This
if (dentry)
goto repeat;
And within dentry_kill() (fs/dcache.c:513):
if (!IS_ROOT(dentry)) {
parent = dentry->d_parent;
if (unlikely(!spin_trylock(&parent->d_lock))) { <=== here
if (inode)
spin_unlock(&inode->i_lock);
goto failed;
We're trying to deref a NULL 'parent'.
Looking at git history, this check was slightly changed to remove the
'parent != NULL' condition in e55fd01154 ("split dentry_kill()"):
- }
if (!IS_ROOT(dentry))
parent = dentry->d_parent;
- if (parent && !spin_trylock(&parent->d_lock)) {
- if (inode)
- spin_unlock(&inode->i_lock);
[...]
+ if (!IS_ROOT(dentry)) {
+ parent = dentry->d_parent;
+ if (unlikely(!spin_trylock(&parent->d_lock))) {
+ if (inode)
+ spin_unlock(&inode->i_lock);
+ goto failed;
+ }
+ }
So this is a case where dentry is not root, but has parent set to NULL.
Although I don't see how that can happen.
Thanks,
Sasha
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