Re: [PATCH v3 7/7] block: detect DKIOCGETBLOCKCOUNT/SIZE before use

[Daniel P . Berrangé]

On Tue, Jun 15, 2021 at 06:50:57PM +0200, Max Reitz wrote:
> On 03.06.21 15:37, Paolo Bonzini wrote:
> > From: Joelle van Dyne <j@getutm.app>
> > 
> > iOS hosts do not have these defined so we fallback to the
> > default behaviour.
> > 
> > Co-authored-by: Warner Losh <imp@bsdimp.com>
> > Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
> > Signed-off-by: Joelle van Dyne <j@getutm.app>
> > Message-Id: <20210315180341.31638-4-j@getutm.app>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >   block/file-posix.c | 21 +++++++++------------
> >   1 file changed, 9 insertions(+), 12 deletions(-)
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 5821e1afed..4e2f7cf508 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -2322,8 +2322,11 @@ static int64_t raw_getlength(BlockDriverState *bs)
> >   again:
> >   #endif
> >       if (!fstat(fd, &sb) && (S_IFCHR & sb.st_mode)) {
> > +        size = 0;
> >   #ifdef DIOCGMEDIASIZE
> > -        if (ioctl(fd, DIOCGMEDIASIZE, (off_t *)&size))
> > +        if (ioctl(fd, DIOCGMEDIASIZE, (off_t *)&size)) {
> 
> Pre-existing, but I feel compelled to express my unease about this cast.

We set -D_FILE_OFFSET_BITS=64, so IIUC,  off_t should be 64-bits
on both 32 and 64 bit build hosts. IIUC, it is defined to be a
signed integer.  So while off_t may not have the same typedef
as int64_t, it should be the same size and signedness. I expect
we have other code with this same assumption about off-t/int64_t
interchangeability.

We could assert sizeof(int64_t) == sizeof(off_t) in a header
somewhere if we want to be super paranoid.

> 
> > +            size = 0;
> > +        }
> >   #elif defined(DIOCGPART)
> >           {
> >                   struct partinfo pi;
> > @@ -2332,9 +2335,7 @@ again:
> >                   else
> >                           size = 0;
> >           }
> > -        if (size == 0)
> > -#endif
> > -#if defined(__APPLE__) && defined(__MACH__)
> > +#elif defined(DKIOCGETBLOCKCOUNT) && defined(DKIOCGETBLOCKSIZE)
> 
> As far a I can tell, before this patch, if the DIOCGMEDIASIZE ioctl failed,
> we fell back to this DKIOCGETBLOCKCOUNT/SIZE block (if compiled in). Now
> this is an #elif and so will not be used if DIOCGMEDIASIZE was defined. Is
> that intentional?
> 
> This may be fine, and apart from that, this patch looks good to me, but this
> change in behavior wasn’t mentioned in the commit message, hence me asking.
> 
> >           {
> >               uint64_t sectors = 0;
> >               uint32_t sector_size = 0;
> > @@ -2342,19 +2343,15 @@ again:
> >               if (ioctl(fd, DKIOCGETBLOCKCOUNT, &sectors) == 0
> >                  && ioctl(fd, DKIOCGETBLOCKSIZE, &sector_size) == 0) {
> >                   size = sectors * sector_size;
> > -            } else {
> > -                size = lseek(fd, 0LL, SEEK_END);
> > -                if (size < 0) {
> > -                    return -errno;
> > -                }
> >               }
> >           }
> > -#else
> > -        size = lseek(fd, 0LL, SEEK_END);
> > +#endif
> > +        if (size == 0) {
> > +            size = lseek(fd, 0LL, SEEK_END);
> > +        }
> >           if (size < 0) {
> >               return -errno;
> >           }
> > -#endif
> >   #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
> >           switch(s->type) {
> >           case FTYPE_CD:
> 
> 

Regards,
Daniel
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