linux-kernel - Re: [PATCH v06 14/36] arm uapi asm/signal.h: include <stddef.h> for size

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Date:   Wed, 9 Aug 2017 14:41:59 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Dmitry V. Levin" <ldv@...linux.org>
Cc:     Mikko Rapeli <mikko.rapeli@....fi>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v06 14/36] arm uapi asm/signal.h: include <stddef.h> for
 size_t in userspace

On Wed, Aug 9, 2017 at 12:57 AM, Dmitry V. Levin <ldv@...linux.org> wrote:
> On Sun, Aug 06, 2017 at 06:44:05PM +0200, Mikko Rapeli wrote:
>> Arnd Bergmann <arnd@...db.de> doubts that __kernel_size_t could be used here
>> so trying to fall back to gcc's <stddef.h>.
>
> The only architecture where you cannot do this safely is x86 family
> because of x32 exception.  If there is no chance that the change will
> affect x32, feel free to replace size_t with __kernel_size_t like I did
> some time ago, see
> http://lkml.kernel.org/r/20170302002022.GB27097@altlinux.org

There is another problem: on some 32-bit architectures, size_t is
defined as 'unsigned int', while '__kernel_size_t' is defined as 'unsigned
long'. These obviously have the same size, but the man page
explicitly defines it as 'size_t ss_size'.

If a user space program accesses the field in a way requires an
exact type match, it gets a warning or error, e.g.

1. printf("signal with %zd bytes\n", stack->ss_size);
2. size_t *pointer_to_size_t = &stack->ss_size;
3. assert(__builtin_types_compatible_p(size_t, typeof(stack->ss_size)))

Not sure how important those are, but I think there is at least a risk
of any of those showing up in user space.

         Arnd