linux-kernel - Re: [PATCH] uaccess: Add missing __must

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Date:   Mon, 30 Sep 2019 12:33:19 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] uaccess: Add missing __must_check attributes

On Wed, Aug 28, 2019 at 7:38 PM Kees Cook <keescook@...omium.org> wrote:
>
> The usercopy implementation comments describe that callers of the
> copy_*_user() family of functions must always have their return values
> checked. This can be enforced at compile time with __must_check, so add
> it where needed.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>

I can't find any other reports, so I'd point out here that this found what
looks like a bug in the x86 math-emu code:

arch/x86/math-emu/reg_ld_str.c:88:2: error: ignoring return value of
function declared with 'warn_unused_result' attribute
[-Werror,-Wunused-result]
        __copy_from_user(sti_ptr, s, 10);
        ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~
arch/x86/math-emu/reg_ld_str.c:1129:2: error: ignoring return value of
function declared with 'warn_unused_result' attribute
[-Werror,-Wunused-result]
        __copy_from_user(register_base + offset, s, other);
        ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/math-emu/reg_ld_str.c:1131:3: error: ignoring return value of
function declared with 'warn_unused_result' attribute
[-Werror,-Wunused-result]
                __copy_from_user(register_base, s + other, offset);
                ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Moreover, the same code also ignores the return value from most
get_user()/put_user()/FPU_get_user()/FPU_put_user() calls,
which have no warn_unused_result annotation (they are macros,
but I think something could be done if we want to have that
annotation to catch some of the other such users).

      Arnd