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From:   Andreas Schwab <schwab@suse.de>
To:     Palmer Dabbelt <palmer@sifive.com>
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] RISC-V: Handle R_RISCV_32 in modules
References: <mhng-3f91f2da-55ae-41b8-b5b2-956ea638bed0@palmer-si-x1c4>
Date:   Mon, 11 Jun 2018 09:54:00 +0200
In-Reply-To: <mhng-3f91f2da-55ae-41b8-b5b2-956ea638bed0@palmer-si-x1c4>
        (Palmer Dabbelt's message of "Fri, 08 Jun 2018 15:33:35 -0700 (PDT)")
Message-ID: <mvm8t7liwrr.fsf@suse.de>
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On Jun 08 2018, Palmer Dabbelt <palmer@sifive.com> wrote:

> On Thu, 07 Jun 2018 03:27:27 PDT (-0700), schwab@suse.de wrote:
>> With CONFIG_MODVERSIONS=y the R_RISCV_32 relocation is used by the
>> __kcrctab section.
>>
>> Signed-off-by: Andreas Schwab <schwab@suse.de>
>> ---
>>  arch/riscv/kernel/module.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
>> index 5dddba301d..1d5e9b934b 100644
>> --- a/arch/riscv/kernel/module.c
>> +++ b/arch/riscv/kernel/module.c
>> @@ -17,6 +17,17 @@
>>  #include <linux/errno.h>
>>  #include <linux/moduleloader.h>
>>
>> +static int apply_r_riscv_32_rela(struct module *me, u32 *location, Elf_Addr v)
>> +{
>> +	if (v != (u32)v) {
>
> My worry with this kind of check is that it relies on some sort of
> undefined behavior in C and that at some point in the future GCC will just
> go decide the check can never fail.  I checked and GCC doesn't elide these
> checks now, so I might be wrong.
>
> Is this defined to do what it looks like it's doing?

This is unsigned arithmetic, thus fully defined.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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"And now for something completely different."