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Date: Tue, 10 Mar 2026 18:18:28 -0700
From: Eric Biggers <ebiggers@kernel.org>
To: Petr Pavlu <petr.pavlu@suse.com>
Cc: Thomas =?iso-8859-1?Q?Wei=DFschuh?= <linux@weissschuh.net>,
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Subject: Re: [PATCH v4 15/17] module: Introduce hash-based integrity checking
Message-ID: <20260311011828.GB212983@quark>
References: <20260113-module-hashes-v4-0-0b932db9b56b@weissschuh.net>
 <20260113-module-hashes-v4-15-0b932db9b56b@weissschuh.net>
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In-Reply-To: <fab2af64-e396-45f9-8876-feff4002e04b@suse.com>

On Tue, Feb 03, 2026 at 01:19:20PM +0100, Petr Pavlu wrote:
> > +static unsigned int get_pow2(unsigned int val)
> > +{
> > +	return 31 - __builtin_clz(val);
> > +}
> > +
> > +static unsigned int roundup_pow2(unsigned int val)
> > +{
> > +	return 1 << (get_pow2(val - 1) + 1);
> > +}
> > +
> > +static unsigned int log2_roundup(unsigned int val)
> > +{
> > +	return get_pow2(roundup_pow2(val));
> > +}
> 
> In the edge case when the kernel is built with only one module, the code
> calls log2_roundup(1) -> roundup_pow2(1) -> get_pow2(0) ->
> __builtin_clz(0). The return value of __builtin_clz() is undefined if
> the input is zero.

A suggestion:

        static unsigned int log2_roundup(unsigned int val)
        {
                if (val <= 1)
                        return 0;
                return 32 - __builtin_clz(val - 1);
        }

- Eric