linux-kernel - Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

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Message-ID: <CAJ4mKGYPoxPS62yFigmqFPPTHOSwgtj+WKwEtdpNGsu3BJya3w@mail.gmail.com>
Date: Mon, 10 Nov 2025 11:48:57 -0800
From: Gregory Farnum <gfarnum@...hat.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>
Cc: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>, 
	"ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>, "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "nathan@...nel.org" <nathan@...nel.org>, 
	"justinstitt@...gle.com" <justinstitt@...gle.com>, Xiubo Li <xiubli@...hat.com>, 
	"idryomov@...il.com" <idryomov@...il.com>, 
	"nick.desaulniers+lkml@...il.com" <nick.desaulniers+lkml@...il.com>, "morbo@...gle.com" <morbo@...gle.com>
Subject: Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

On Mon, Nov 10, 2025 at 11:42 AM Viacheslav Dubeyko
<Slava.Dubeyko@....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@...ux.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).
-Greg

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