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

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Message-ID: <CAJ4mKGbrXjC=kX5YDsX=RZUw6mK0PVPiinVsujhp+XfPVsrVVg@mail.gmail.com>
Date: Mon, 10 Nov 2025 12:00:06 -0800
From: Gregory Farnum <gfarnum@...hat.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>
Cc: Xiubo Li <xiubli@...hat.com>, "justinstitt@...gle.com" <justinstitt@...gle.com>, 
	"andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>, 
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "nathan@...nel.org" <nathan@...nel.org>, 
	"morbo@...gle.com" <morbo@...gle.com>, "idryomov@...il.com" <idryomov@...il.com>, 
	"nick.desaulniers+lkml@...il.com" <nick.desaulniers+lkml@...il.com>, 
	"ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>
Subject: Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build breakage

On Mon, Nov 10, 2025 at 11:57 AM Viacheslav Dubeyko
<Slava.Dubeyko@....com> wrote:
>
> 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@....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).
>
> So, are you suggesting to declare num as u64 here? Am I correct?

What? No, the whole point of this block is checking that it can track
all the snapshots and allocate them (on a <32-bit architecture, which
are the only ones at risk), isn't it?