[Qemu-devel] Re: [PATCH 08/15] blkdebug: fix enum comparison'

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Sep 05, 2010 at 07:37:54PM +0000, Blue Swirl wrote:
> On Sun, Sep 5, 2010 at 5:57 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Sun, Sep 05, 2010 at 03:06:32PM +0000, Blue Swirl wrote:
> >> The signedness of enum types depend on the compiler implementation.
> >> Therefore the check for negative values may or may not be meaningful.
> >>
> >> Fix by explicitly casting to a signed integer.
> >>
> >> Since the values are also checked earlier against event_names
> >> table, this is an internal error. Change the 'if' to 'assert'.
> >>
> >> This also fixes a warning with GCC flag -Wtype-limits.
> >>
> >> Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
> >> ---
> >>  block/blkdebug.c |    4 +---
> >>  1 files changed, 1 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/block/blkdebug.c b/block/blkdebug.c
> >> index 2a63df9..4d6ff0a 100644
> >> --- a/block/blkdebug.c
> >> +++ b/block/blkdebug.c
> >> @@ -439,9 +439,7 @@ static void blkdebug_debug_event(BlockDriverState
> >> *bs, BlkDebugEvent event)
> >>      struct BlkdebugRule *rule;
> >>      BlkdebugVars old_vars = s->vars;
> >>
> >> -    if (event < 0 || event >= BLKDBG_EVENT_MAX) {
> >> -        return;
> >> -    }
> >> +    assert((int)event >= 0 && event < BLKDBG_EVENT_MAX);
> >
> > I am not sure all compilers must generate a negative value from
> > a very large unsigned integer cast to int.
> 
> The enum rules seem to be vague. The type of enums may also be signed
> (on GCC when the enum set includes negative values, on other compilers
> in other cases). Do any machines or compilers exist (on which QEMU
> runs) where this could happen?

I remember reading that GCC sometimes assumes signed integers don't overflow,
and generates code behaves incorrectly if they do.
No idea whether this is ever the case for casts.

-- 
MST