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

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Date:   Tue, 29 May 2018 11:05:47 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Christophe LEROY <christophe.leroy@....fr>
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

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().

>>>          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

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds