RE: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

Viacheslav Dubeyko
2025-11-10T19:57:24Z

On Mon, 2025-11-10 at 11:48 -0800, Gregory Farnum wrote:
> On Mon, Nov 10, 2025 at 11:42 AM Viacheslav Dubeyko
> <Slava.Dubeyko@ibm.com> wrote:
> >
> > On Mon, 2025-11-10 at 15:44 +0100, Andy Shevchenko wrote:
> > > In a few cases the code compares 32-bit value to a SIZE_MAX derived
> > > constant which is much higher than that value on 64-bit platforms,
> > > Clang, in particular, is not happy about this
> > >
> > > fs/ceph/snap.c:377:10: error: result of comparison of constant 2305843009213693948 with expression of type 'u32' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> > > 377 | if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> > > | ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > > Fix this by casting to size_t. Note, that possible replacement of SIZE_MAX
> > > by U32_MAX may lead to the behaviour changes on the corner cases.
> > >
> > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > > ---
> > > fs/ceph/snap.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> > > index c65f2b202b2b..521507ea8260 100644
> > > --- a/fs/ceph/snap.c
> > > +++ b/fs/ceph/snap.c
> > > @@ -374,7 +374,7 @@ static int build_snap_context(struct ceph_mds_client *mdsc,
> > >
> > > /* alloc new snap context */
> > > err = -ENOMEM;
> > > - if (num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> > > + if ((size_t)num > (SIZE_MAX - sizeof(*snapc)) / sizeof(u64))
> >
> > The same question is here. Does it makes sense to declare num as size_t? Could
> > it be more clean solution? Or could it introduce another warnings/errors?
>
> Given that the number of snaps is constrained over the wire as a
> 32-bit integer, you probably want to keep that mapping...(Though I
> guess it's the sum of two 32-bit integers which technically could
> overflow, and I'm not sure what happens if you actually hit those
> boundaries on the server — but nobody generates snapshots on the same
> file in that quantity).
>
> All that said, it'd be kind of nice if we could just annotate for
> clang that we are perfectly happy for the evaluation to always be true
> on a 64-bit architecture (as snapids are 64 bits, we will always be
> able to count them).

So, are you suggesting to declare num as u64 here? Am I correct?

Thanks,
Slava.

> >
> > > goto fail;
> > > snapc = ceph_create_snap_context(num, GFP_NOFS);
> > > if (!snapc)