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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 15 Mar 2022 09:48:19 -0700
Message-ID: <CAHk-=wiZnS6n1ROQg3FHd=bcVTHi-sKutKT+toiViQEH47ZACg@mail.gmail.com>
Subject: Re: [GIT PULL] SPI fixes for v5.17-rc7
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Mark Brown <broonie@kernel.org>,
        linux-spi <linux-spi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Mar 15, 2022 at 2:08 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> I had noticed while reviewing the patch, but changing to size_t wouldn't
> help much, as other related code paths treat the value as unsigned int
> anyway.

.. but it really would.

Note that the paths *after* this code don't matter. Because the result
is guaranteed to fit in 'unsigned int' anyway.

Put another way:

    min_t(unsigned int,x,y)

is buggy if one of x/y is 'size_t'. Why? Because if that one gets
truncated, you're doing 'min()' with a value that may be artificially
much too small (that was exactly the problem commit 1a4e53d2fc4f:
"spi: Fix invalid sgs value")fixed).

But the situation is _not_ true in the reverse. Look:

    min(size_t,x,y)

is guaranteed to fit in 'unsigned int' as long as _one_ of x,y fits in
'unsigned int' - even if the other doesn't. Because then 'min()' will
just pick the one that already had the right size.

To make it really concrete, compare

    min_t(unsigned int, 5, 0x100000001);
    min_t(size_t, 5, 0x100000001);

on a 64-bit machine (ie size_t is 64-bits, and unsigned int is 32-bit).

One returns 1. The other returns 5. Both fit the result in 'unsigned
int', but one of them is wrong.

                Linus