Re: [PATCH] xfs: avoid null *src in memcpy call in xlog_write

[Eric Sandeen <sandeen@redhat.com>]

On 10/7/15 4:24 PM, Dave Chinner wrote:
> On Wed, Oct 07, 2015 at 11:33:43AM -0500, Eric Sandeen wrote:
>> The gcc undefined behavior sanitizer caught this; surely
>> any sane memcpy implementation will no-op if size == 0,
>> but behavior with a *src of NULL is technically undefined
>> (declared nonnull), so avoid it here.
>>
>> We are actually in this situation frequently via
>> xlog_commit_record(), because:
>>
>>         struct xfs_log_iovec reg = {
>>                 .i_addr = NULL,
>>                 .i_len = 0,
>>                 .i_type = XLOG_REG_TYPE_COMMIT,
>>         };
>>
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>> ---
>>
>> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
>> index 4012523..8897fd1 100644
>> --- a/fs/xfs/xfs_log.c
>> +++ b/fs/xfs/xfs_log.c
>> @@ -2424,7 +2424,10 @@ xlog_write(
>>  
>>  			/* copy region */
>>  			ASSERT(copy_len >= 0);
>> -			memcpy(ptr, reg->i_addr + copy_off, copy_len);
>> +			ASSERT(reg->i_addr + copy_off > 0 || copy_len == 0);
>> +			/* size == 0 is ok, but *src == NULL is undefined */
>> +			if (reg->i_addr + copy_off)
>> +				memcpy(ptr, reg->i_addr + copy_off, copy_len);
>>  			xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);
> 
> The comment doesn't explain anything about why copylen might be zero
> or reg->i_addr might be null.  If copylen is zero, we should really
> skip the copy, not rely on some magic pointer arithmetic to tell us
> it's ok to copy...
> 
> 			/*
> 			 * Copy region.
> 			 *
> 			 * Unmount records just log an opheader, so can have
> 			 * empty payloads with no data region to copy. Hence we only
> 			 * copy the payload if the vector says it has data to copy.
> 			 */
> 			ASSERT(copy_len >= 0);
> 			if (copy_len > 0) {
> 				memcpy(ptr, reg->i_addr + copy_off, copy_len);
> 				xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);
> 			}

Yeah, I thought about that, why didn't I do it that way?  Maybe I was thinking
about *src still being NULL, but I guess in that case we'd find out that there's
a problem very quickly.

You may as well just commit that version w/ your S-O-B and my Reported-by.

-Eric

> Cheers,
> 
> Dave.
> 

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