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

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Date:   Tue, 29 May 2018 11:37:11 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        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 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
>>> <christophe.leroy@....fr> wrote:
>>>>
>>>>     CC      arch/powerpc/kernel/nvram_64.o
>>>> arch/powerpc/kernel/nvram_64.c: In function 'nvram_create_partition':
>>>> arch/powerpc/kernel/nvram_64.c:1042:2: error: 'strncpy' specified bound
>>>> 12 equals destination size [-Werror=stringop-truncation]
>>>>     strncpy(new_part->header.name, name, 12);
>>>>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>
>>>>     CC      arch/powerpc/kernel/trace/ftrace.o
>>>> In function 'make_field',
>>>>       inlined from 'ps3_repository_read_boot_dat_address' at
>>>> arch/powerpc/platforms/ps3/repository.c:900:9:
>>>> arch/powerpc/platforms/ps3/repository.c:106:2: error: 'strncpy' output
>>>> truncated before terminating nul copying 8 bytes from a string of the same
>>>> length [-Werror=stringop-truncation]
>>>>     strncpy((char *)&n, text, 8);
>>>>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>>>
>>>
>>> Thanks for your patch!
>>>
>>>> --- 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().

Right but then, what is the best solution to elimate that warning ?

Would it be better to enclose those two lines in:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstringop-truncation"
...
#pragma GCC diagnostic pop


Christophe

> 
>>>>           new_part->header.checksum = nvram_checksum(&new_part->header);
>>>>
>>>>           rc = nvram_write_header(new_part);
>>>> diff --git a/arch/powerpc/platforms/ps3/repository.c
>>>> b/arch/powerpc/platforms/ps3/repository.c
>>>> index 50dbaf24b1ee..e49c887787c4 100644
>>>> --- a/arch/powerpc/platforms/ps3/repository.c
>>>> +++ b/arch/powerpc/platforms/ps3/repository.c
>>>> @@ -101,9 +101,9 @@ static u64 make_first_field(const char *text, u64
>>>> index)
>>>>
>>>>    static u64 make_field(const char *text, u64 index)
>>>>    {
>>>> -       u64 n;
>>>> +       u64 n = 0;
>>>>
>>>> -       strncpy((char *)&n, text, 8);
>>>> +       memcpy((char *)&n, text, strnlen(text, sizeof(n)));
>>>
>>>
>>> This changes behavior: strncpy() fills the remainder of the buffer with
>>> zeroes.  I don't remember the details of the PS3 repository structure,
>>> but given this writes to a fixed size u64 buffer, I'd expect the PS3
>>> hypervisor code to (1) rely on the zero padding, and (2) not need a zero
>>> terminator if there are 8 characters in the buffer, so probably the
>>> original code is correct, and the "fixed" code isn't.
>>
>> Here I have set n to 0 prior to the copy, so the buffer IS zero padded.
> 
> Sorry, I missed that part.
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>