Re: [Qemu-devel] [PATCH v3 for-2.2 3/3] raw-posix: The SEEK_HOLE code is flawed, rewrite it

[Max Reitz <mreitz@redhat.com>]

On 2014-11-17 at 11:18, Markus Armbruster wrote:
> On systems where SEEK_HOLE in a trailing hole seeks to EOF (Solaris,
> but not Linux), try_seek_hole() reports trailing data instead.
>
> Additionally, unlikely lseek() failures are treated badly:
>
> * When SEEK_HOLE fails, try_seek_hole() reports trailing data.  For
>    -ENXIO, there's in fact a trailing hole.  Can happen only when
>    something truncated the file since we opened it.
>
> * When SEEK_HOLE succeeds, SEEK_DATA fails, and SEEK_END succeeds,
>    then try_seek_hole() reports a trailing hole.  This is okay only
>    when SEEK_DATA failed with -ENXIO (which means the non-trailing hole
>    found by SEEK_HOLE has since become trailing somehow).  For other
>    failures (unlikely), it's wrong.
>
> * When SEEK_HOLE succeeds, SEEK_DATA fails, SEEK_END fails (unlikely),
>    then try_seek_hole() reports bogus data [-1,start), which its caller
>    raw_co_get_block_status() turns into zero sectors of data.  Could
>    theoretically lead to infinite loops in code that attempts to scan
>    data vs. hole forward.
>
> Rewrite from scratch, with very careful comments.
>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   block/raw-posix.c | 111 +++++++++++++++++++++++++++++++++++++++++-------------
>   1 file changed, 85 insertions(+), 26 deletions(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>