On Fri, Feb 10, 2017 at 03:03:35PM -0800, Omar Sandoval wrote:
> From: Omar Sandoval <osan...@fb.com>
>
> If btrfs_decompress_buf2page() is handed a bio with its page in the
> middle of the working buffer, then we adjust the offset into the working
> buffer. After we copy into the bio, we advance the iterator by the
> number of bytes we copied. Then, we have some logic to handle the case
> of discontiguous pages and adjust the offset into the working buffer
> again. However, if we didn't advance the bio to a new page, we may enter
> this case in error, essentially repeating the adjustment that we already
> made when we entered the function. The end result is bogus data in the
> bio.
>
> Previously, we only checked for this case when we advanced to a new
> page, but the conversion to bio iterators changed that. This restores
> the old, correct behavior.
>
> A case I saw when testing with zlib was:
>
> buf_start = 42769
> total_out = 46865
> working_bytes = total_out - buf_start = 4096
> start_byte = 45056
>
> The condition (total_out > start_byte && buf_start < start_byte) is
> true, so we adjust the offset:
>
> buf_offset = start_byte - buf_start = 2287
> working_bytes -= buf_offset = 1809
> current_buf_start = buf_start = 42769
>
> Then, we copy
>
> bytes = min(bvec.bv_len, PAGE_SIZE - buf_offset, working_bytes) = 1809
> buf_offset += bytes = 4096
> working_bytes -= bytes = 0
> current_buf_start += bytes = 44578
>
> After bio_advance(), we are still in the same page, so start_byte is the
> same. Then, we check (total_out > start_byte && current_buf_start <
> start_byte),
> which is true! So, we adjust the values again:
>
> buf_offset = start_byte - buf_start = 2287
> working_bytes = total_out - start_byte = 1809
> current_buf_start = buf_start + buf_offset = 45056
>
> But note that working_bytes was already zero before this, so we should
> have stopped copying.
>
> Fixes: 974b1adc3b10 ("btrfs: use bio iterators for the decompression
> handlers")
> Reported-by: Pat Erley <pat-l...@erley.org>
> Reviewed-by: Chris Mason <c...@fb.com>
> Signed-off-by: Omar Sandoval <osan...@fb.com>
Whoops, I sent this out before I got your other email, Bo, but I assume
it's still okay for Chris to add your reviewed-by and tested-by?
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
