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

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Message-ID: <aSWCJhA3cNSEIUir@smile.fi.intel.com>
Date: Tue, 25 Nov 2025 12:17:10 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: david laight <david.laight@...box.com>
Cc: ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, Xiubo Li <xiubli@...hat.com>,
	Ilya Dryomov <idryomov@...il.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v1 1/1] ceph: Amend checking to fix `make W=1` build
 breakage

On Tue, Nov 25, 2025 at 09:55:16AM +0000, david laight wrote:
> On Mon, 10 Nov 2025 15:44:04 +0100
> Andy Shevchenko <andriy.shevchenko@...ux.intel.com> 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.
> 
> Did you really read the code?

I read the piece that prevents builds. The exercise on how to fix this properly
is delegated to the authors and maintainers.

> The test itself needs moving into ceph_create_snap_context().
> Possibly by using kmalloc_array() to do the multiply.
> 
> But in any case are large values sane at all?
> Allocating very large kernel memory blocks isn't a good idea at all.
> 
> In fact this does a kmalloc(... GFP_NOFS) which is pretty likely to
> fail for even moderate sized requests. I bet it fails 64k (order 4?)
> on a regular basis.
> 
> Perhaps all three value that get added to make 'num' need 'sanity limits'
> that mean a large allocation just can't happen.

Nice, can you send a followup to fix all that in a better way?
(I don't care about the fix as long as it doesn't break my builds)

-- 
With Best Regards,
Andy Shevchenko