Re: [PATCH] xfs: do not call xfs_buf_hash_destroy on a NULL pag

[Colin Ian King]

On 20/01/17 20:47, Darrick J. Wong wrote:
> On Fri, Jan 20, 2017 at 01:26:12PM -0600, Eric Sandeen wrote:
>> On 1/20/17 8:26 AM, Colin King wrote:
>>> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>>>
>>> If pag cannot be allocated, the current error exit path will trip
>>> a null pointer deference error when calling xfs_buf_hash_destroy
>>> with a null pag.  Fix this by adding a new error exit lable and
>>> jumping to this, avoiding the hash destroy and unnecessary kmem_free
>>> on pag.
>>>
>>> Fixes CoverityScan CID#1397628 ("Dereference after null check")
>>>
>>> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>>
>> Hm, I think this leaves the code with issues.
>>
>>> ---
>>>  fs/xfs/xfs_mount.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
>>> index 9b9540d..4e66cd19 100644
>>> --- a/fs/xfs/xfs_mount.c
>>> +++ b/fs/xfs/xfs_mount.c
>>> @@ -207,7 +207,7 @@ xfs_initialize_perag(
>>>  
>>>  		pag = kmem_zalloc(sizeof(*pag), KM_MAYFAIL);
>>>  		if (!pag)
>>> -			goto out_unwind;
>>> +			goto out_unwind_pags;
>>
>> So let's say we got to index == 3 at the top of the loop, and
>> this fails.
>>
>> We succeeded in initializing 0, 1, and 2, but 3 failed.
>>
>> So we go to out_unwind_pags with index == 3...
>>
>>>  		pag->pag_agno = index;
>>>  		pag->pag_mount = mp;
>>>  		spin_lock_init(&pag->pag_ici_lock);
>>> @@ -242,6 +242,7 @@ xfs_initialize_perag(
>>>  out_unwind:
>>>  	xfs_buf_hash_destroy(pag);
>>>  	kmem_free(pag);
>>> +out_unwind_pags:
>>
>> ... where index == 3, and:
>>
>>>  	for (; index > first_initialised; index--) {
>>>  		pag = radix_tree_delete(&mp->m_perag_tree, index);
>>
>> this should fail, because it never got inserted, and...
>>
>>>  		xfs_buf_hash_destroy(pag);
>>
>> this still tries to destroy a NULL pag, no?
>>
>> There also seems to be an existing issue w/the code where ag 0 is
>> never torn down in the error case, because first_initialized doesn't
>> stay set to 0:
>>
>>                 if (!first_initialised)
>>                         first_initialised = index;
>>
>> And we don't even tear down ag 1, because:
>>
>>>  	for (; index > first_initialised; index--) {
>>>  		pag = radix_tree_delete(&mp->m_perag_tree, index);
>>
>> when the loop reaches the first initialized AG, it stops.
>>
>> So we seem to always leak at least 2 if we managed to get far enough
>> to initialize them.
> 
> Ugh, yeah, the the whole error exit from that function is fubar...
> Anyone want to clean this up?

I may step back on this if somebody else wants to fix this up properly.

> 
> --D
> 
>>
>> -Eric
>>
>>>
>> --
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