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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix remote symlinks on V5/CRC filesystems
From: Eric Sandeen <sandeen@xxxxxxxxxx>
Date: Mon, 15 Jun 2015 17:35:27 -0500
Cc: xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150615222157.GD10224@dastard>
References: <557F4E1E.8000505@xxxxxxxxxx> <20150615222157.GD10224@dastard>
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@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>
>> 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@xxxxxxxxxx>
> 
> 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.

-Eric


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