Re: [Qemu-block] [Qemu-devel] [PATCH v4 2/4] block: Guarantee that *file is set on bdrv_get_block_status()

[John Snow]

On 06/05/2017 04:38 PM, Eric Blake wrote:
> We document that *file is valid if the return is not an error and
> includes BDRV_BLOCK_OFFSET_VALID, but forgot to obey this contract
> when a driver (such as blkdebug) lacks a callback.  Messed up in
> commit 67a0fd2 (v2.6), when we added the file parameter.
> 
> Enhance qemu-iotest 177 to cover this, using a sequence that would
> print garbage or even SEGV, because it was dererefencing through
> uninitialized memory.  [The resulting test output shows that we
> have less-than-ideal block status from the blkdebug driver, but
> that's a separate fix coming up soon.]
> 
> Setting *file on all paths that return BDRV_BLOCK_OFFSET_VALID is
> enough to fix the crash, but we can go one step further: always
> setting *file, even on error, means that a broken caller that
> blindly dereferences file without checking for error is now more
> likely to get a reliable SEGV instead of randomly acting on garbage,
> making it easier to diagnose such buggy callers.  Adding an
> assertion that file is set where expected doesn't hurt either.
> 
> CC: address@hidden
> Signed-off-by: Eric Blake <address@hidden>
> Reviewed-by: Fam Zheng <address@hidden>
> Reviewed-by: Max Reitz <address@hidden>

Reviewed-by: John Snow <address@hidden>