Re: [PATCH] xfs: fix remote symlinks on V5/CRC filesystems

[Eric Sandeen <sandeen@sandeen.net>]

On 6/15/15 6:16 PM, Dave Chinner wrote:
> On Mon, Jun 15, 2015 at 05:49:55PM -0500, Eric Sandeen wrote:
>> On 6/15/15 5:47 PM, Dave Chinner wrote:
>>> On Mon, Jun 15, 2015 at 05:35:27PM -0500, Eric Sandeen wrote:
>>>> On 6/15/15 5:21 PM, Dave Chinner wrote:
>>>>> On Mon, Jun 15, 2015 at 05:13:50PM -0500, Eric Sandeen wrote:
>>>>>> If we create a CRC filesystem, mount it, and create a symlink with
>>>>>> a path long enough that it can't live in the inode, we get a very
>>>>>> strange result upon remount:
>>>>>>
>>>>>> # ls -l mnt
>>>>>> total 4
>>>>>> lrwxrwxrwx. 1 root root 929 Jun 15 16:58 link -> XSLM
>>>>>>
>>>>>> XSLM is the V5 symlink block header magic (which happens to be
>>>>>> followed by a NUL, so the string looks terminated).
>>>>>>
>>>>>> xfs_readlink_bmap() advanced cur_chunk by the size of the header
>>>>>> for CRC filesystems, but never actually used that pointer; it
>>>>>> kept reading from bp->b_addr, which is the start of the block,
>>>>>> rather than the start of the symlink data after the header.
>>>>>>
>>>>>> Looks like this problem goes back to v3.10.
>>>>>>
>>>>>> Fixing this gets us reading the proper link target, again.
>>>>>>
>>>>>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>>>>>> Cc: stable@vger.kernel.org
>>>>>> ---
>>>>>>
>>>>>> diff --git a/fs/xfs/xfs_symlink.c b/fs/xfs/xfs_symlink.c
>>>>>> index 3df411e..40c0765 100644
>>>>>> --- a/fs/xfs/xfs_symlink.c
>>>>>> +++ b/fs/xfs/xfs_symlink.c
>>>>>> @@ -104,7 +104,7 @@ xfs_readlink_bmap(
>>>>>>  			cur_chunk += sizeof(struct xfs_dsymlink_hdr);
>>>>>>  		}
>>>>>>  
>>>>>> -		memcpy(link + offset, bp->b_addr, byte_cnt);
>>>>>> +		memcpy(link + offset, cur_chunk, byte_cnt);
>>>>>>  
>>>>>>  		pathlen -= byte_cnt;
>>>>>>  		offset += byte_cnt;
>>>>>
>>>>> Looks like the correct fix, so:
>>>>>
>>>>> Reviewed-by: Dave Chinner <dchinner@redhat.com>
>>>>>
>>>>> However, it raises a more disturbing question: how did we not trip
>>>>> over this until now? I though we had long symlink test coverage in
>>>>> xfstests but clearly we haven't - do you have a test that closes
>>>>> this verification hole?
>>>>
>>>> It was a smaller part of a larger test harness I was using with xfs_metadump,
>>>> which was trying to create every type of on-disk metadata.  However, even with
>>>> that I only stumbled on it, because I was only verifying that the results were
>>>> uncorrupted and consistent with the original, not actually verifying that
>>>> what I created was still there (on the original!)
>>>>
>>>> So, I don't have a test specific to this, no, but could certainly write one;
>>>> I suppose a quick targeted fstest for just this bug would be ok, although
>>>> a test w/ broader scope might make sense too.
>>>
>>> Sure, the metadump test is a good idea, but my question is more
>>> asking why our broader tests haven't already covered verifying
>>> MAXPATHLEN symlinks work correctly or not. Surely symlink
>>> correctness is verified *somewhere* (even outside xfstests,
>>> e.g. LTP?), and if so why haven't we seen this before now? If not,
>>> then I'd suggest we've just uncovered a potential Nest O' Bugs...
>>
>> A) CRCs aren't default
> 
> Yet many people have been testing them and putting them in
> production (e.g. SLES 12), so they *should* have been tested.
> 
>> B) I bet LTP doesn't do a remount to verify on-disk persistence
> 
> Just reading back the symlink should expose the bug, right?
> Or is it being hidden by the dentry cache or something else?

it does seem to be cached, yes.

-Eric

> Cheers,
> 
> Dave.
> 

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