linux-kernel - Re: [PATCH v2] powerpc/64: Fix build failure with GCC 8.1

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Date:   Thu, 31 May 2018 07:57:48 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Geoff Levand <geoff@...radead.org>
Subject: Re: [PATCH v2] powerpc/64: Fix build failure with GCC 8.1



Le 31/05/2018 à 07:54, Michael Ellerman a écrit :
> Christophe LEROY <christophe.leroy@....fr> writes:
>> Le 29/05/2018 à 11:05, Geert Uytterhoeven a écrit :
>>> Hi Christophe,
>>> On Tue, May 29, 2018 at 10:56 AM, Christophe LEROY
>>> <christophe.leroy@....fr> wrote:
>>>> Le 29/05/2018 à 09:47, Geert Uytterhoeven a écrit :
>>>>> On Tue, May 29, 2018 at 8:03 AM, Christophe Leroy
>>>>>> --- a/arch/powerpc/kernel/nvram_64.c
>>>>>> +++ b/arch/powerpc/kernel/nvram_64.c
>>>>>> @@ -1039,7 +1039,7 @@ loff_t __init nvram_create_partition(const char
>>>>>> *name, int sig,
>>>>>>            new_part->index = free_part->index;
>>>>>>            new_part->header.signature = sig;
>>>>>>            new_part->header.length = size;
>>>>>> -       strncpy(new_part->header.name, name, 12);
>>>>>> +       memcpy(new_part->header.name, name, strnlen(name,
>>>>>> sizeof(new_part->header.name)));
>>>>>
>>>>>
>>>>> The comment for nvram_header.lgnth says:
>>>>>
>>>>>            /* Terminating null required only for names < 12 chars. */
>>>>>
>>>>> This will not terminate the string with a zero (the struct is
>>>>> allocated with kmalloc).
>>>>> So the original code is correct, the new one isn't.
>>>>
>>>> Right, then I have to first zeroize the destination.
>>>
>>> Using kzalloc() instead of kmalloc() will do.
>>>
>>> Still, papering around these warnings seems to obscure things, IMHO.
>>> And it increases code size, as you had to add a call to strnlen().
> 
> 
> The right fix is to not try and mirror the on-device structure in the
> kernel struct. We should just use a proper NULL terminated string, which
> would avoid the need to explicitly do strncmp(.., .., 12) in the code
> and be less bug prone in general.
> 
> The only place where we should need to worry about the 12 byte buffer is
> in nvram_write_header().
> 
> Anyway that's a bigger change, so I'll take this for now with kzalloc().

Thanks. You take it as is and add the kzalloc() or you expect a v3 from 
me with the kzalloc()

Christophe