linux-kernel - Re: [PATCH] nios2: __kuser

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Message-ID: <54C0B19D.4020804@codesourcery.com>
Date:	Thu, 22 Jan 2015 16:15:25 +0800
From:	Chung-Lin Tang <cltang@...esourcery.com>
To:	Ley Foon Tan <lftan@...era.com>,
	Chung-Lin Tang <chunglin.tang@...il.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tobias Klauser <tklauser@...tanz.ch>
Subject: Re: [PATCH] nios2: __kuser_sigtramp placement fix

On 2015/1/22 03:54 PM, Ley Foon Tan wrote:
> On Thu, Jan 22, 2015 at 3:14 PM, Chung-Lin Tang <chunglin.tang@...il.com> wrote:
>> The address of __kuser_sigtramp is wrong by one word, due to padding
>> __kuser_cmpxchg to fully 64 bytes. The version word at the start of the
>> page displaces __kuser_sigtramp to 0x1044, instead of the intended 0x1040.
>>
>> Fixed by counting the 64-byte frame from the start of the page instead
>> of __kuser_cmpxchg. This shortens the __kuser_cmpxchg frame to 60-bytes,
>> but that's likely enough.
>>
>> The zero-byte padding parts of the kuser_pad macro has been removed, due
>> to the .if command refusing to behave properly when there are other
>> .word directives in between. Probably a binutils gas bug.
>>
>> Thanks,
>> Chung-Lin
>>
>> Cc: Ley Foon Tan <lftan@...era.com>
>> Cc: Tobias Klauser <tklauser@...tanz.ch>
>> Signed-off-by: Chung-Lin Tang <cltang@...esourcery.com>
>>
>> diff --git a/arch/nios2/kernel/entry.S b/arch/nios2/kernel/entry.S
>> index 0bdfd13..3224839 100644
>> --- a/arch/nios2/kernel/entry.S
>> +++ b/arch/nios2/kernel/entry.S
>> @@ -492,14 +492,7 @@ ENTRY(ret_from_kernel_thread)
>>
>>   /* Filling pads with undefined instructions. */
>>  .macro kuser_pad sym size
>> -       .if     ((. - \sym) & 3)
>> -       .rept   (4 - (. - \sym) & 3)
>> -       .byte   0
>> -       .endr
>> -       .endif
>> -       .rept   ((\size - (. - \sym)) / 4)
>> -       .word   0xdeadbeef
>> -       .endr
>> +       .fill   ((\size - (. - \sym)) / 4), 4, 0xdeadbeef
>>  .endm
>>
>>         .align  6
>> @@ -526,7 +519,10 @@ cmpxchg_stw:
>>  cmpxchg_ret:
>>         ret
>>
>> -       kuser_pad __kuser_cmpxchg, 64
>> +       /* The first 64-byte frame contains the version word, so note
>> +          that the first padding entry is based from the start of the kuser
>> +          page, instead of __kuser_cmpxchg.  */
>> +       kuser_pad __kuser_helper_start, 64
>>
>>         .globl  __kuser_sigtramp
>>  __kuser_sigtramp:
>> --
> Hi Chung-Lin
> 
> I thought this is our original intention for it.
> 
> First 4 bytes is the __kuser_version and each __kuser function will
> have 64 bytes size.
> 
> 0x1000 __kuser_helper_version
> 0x1004 __kuser_cmpxchg
> 0x1044 __kuser_sigtramp
> 
> Any reason you can't use __kuser_sigtramp @ 0x1044?

Well, that's okay as well, as the main place which sets this is the
signal frame construction in arch/nios2/kernel/signal.c. The user space
is mostly oblivious to the exact location.  You'll need to update it
that way if you prefer 0x1044, as it's currently set at 0x1040.

I suggested 0x1040 because 64b frames aligned at 64b seemed more
intuitive; __kuser_cmpxchg would be a special case.

Chung-Lin

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