Re: [PATCH v5] btrfs: Don't submit any btree write bio if the fs has error

[Qu Wenruo <wqu@suse.com>]

On 2020/2/21 下午9:35, David Sterba wrote:
> On Wed, Feb 12, 2020 at 02:12:44PM +0800, Qu Wenruo wrote:
>> @@ -4036,7 +4037,39 @@ int btree_write_cache_pages(struct address_space *mapping,
>>  		end_write_bio(&epd, ret);
>>  		return ret;
>>  	}
>> -	ret = flush_write_bio(&epd);
>> +	/*
>> +	 * If something went wrong, don't allow any metadata write bio to be
>> +	 * submitted.
>> +	 *
>> +	 * This would prevent use-after-free if we had dirty pages not
>> +	 * cleaned up, which can still happen by fuzzed images.
>> +	 *
>> +	 * - Bad extent tree
>> +	 *   Allowing existing tree block to be allocated for other trees.
>> +	 *
>> +	 * - Log tree operations
>> +	 *   Exiting tree blocks get allocated to log tree, bumps its
>> +	 *   generation, then get cleaned in tree re-balance.
>> +	 *   Such tree block will not be written back, since it's clean,
>> +	 *   thus no WRITTEN flag set.
>> +	 *   And after log writes back, this tree block is not traced by
>> +	 *   any dirty extent_io_tree.
>> +	 *
>> +	 * - Offending tree block gets re-dirtied from its original owner
>> +	 *   Since it has bumped generation, no WRITTEN flag, it can be
>> +	 *   reused without COWing. This tree block will not be traced
>> +	 *   by btrfs_transaction::dirty_pages.
>> +	 *
>> +	 *   Now such dirty tree block will not be cleaned by any dirty
>> +	 *   extent io tree. Thus we don't want to submit such wild eb
>> +	 *   if the fs already has error.
>> +	 */
>> +	if (!test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state)) {
>> +		ret = flush_write_bio(&epd);
>> +	} else {
>> +		ret = -EUCLEAN;
>> +		end_write_bio(&epd, ret);
>> +	}
> 
> This replaces one instance of flush_write_bio, would it make sense to
> wrap it to flush_write_bio or some other helper? There might be places
> where not handling the fs error state would be acceptable, so eg.
> 
> flush_write_bio = as it is now
> 
> flush_write_bio_or_end = does the above
> 

I don't believe there are other call sites needs such special handling,
thus a wrapper only used once doesn't make much sense.

Unless we're going to introduce more path for btree writeback, current
one would be good enough I guess.

Thanks,
Qu