linux-kernel - Re: [PATCH v2 1/1] ARM: kprobes: Explicitly reserve r7 for local variables

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Date:   Mon, 20 Nov 2023 10:00:40 +0800
From:   "Aiqun(Maria) Yu" <quic_aiquny@...cinc.com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>
CC:     <linux@...linux.org.uk>, <mhiramat@...nel.org>,
        <kernel@...cinc.com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_lijuang@...cinc.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] ARM: kprobes: Explicitly reserve r7 for local
 variables

On 11/17/2023 1:36 AM, Ard Biesheuvel wrote:
> On Fri, 17 Nov 2023 at 03:24, Nathan Chancellor <nathan@...nel.org> wrote:
>>
>> On Wed, Nov 15, 2023 at 05:58:30PM +0800, Maria Yu wrote:
>>> Registers r7 is removed in clobber list, so compiler may choose r7 for
>>> local variables usage, while r7 will be actually updated by the inline asm
>>> code. This caused the runtime behavior wrong.
>>> While those kind of reserved registers cannot be set to clobber list
>>> because of error like "inline asm clobber list contains reserved
>>> registers".
>>> Explicitly reserve r7 by adding attribute no-omit-frame-pointer for needed
>>> function, then in T32 asm code r7 is used as a frame pointer and is not
>>> available for use as a general-purpose register.
>>> Note that "no-omit-frame-pointer" will make the code size a little bigger
>>> to store the stack frame pointer. So limited to needed functions can have
>>> the less impact than the full source file.
>>>
>>> Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
>>> Signed-off-by: Maria Yu <quic_aiquny@...cinc.com>
>>> Cc: stable@...r.kernel.org
>>
>> This causes warnings with clang:
>>
>>    arch/arm/probes/kprobes/actions-thumb.c:441:47: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
>>      441 | static unsigned long __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>>          |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>    arch/arm/probes/kprobes/actions-thumb.c:524:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
>>      524 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>>          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>    arch/arm/probes/kprobes/actions-thumb.c:560:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
>>      560 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>>          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>    arch/arm/probes/kprobes/actions-thumb.c:579:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
>>      579 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>>          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>    4 warnings generated.
>>
>> Furthermore, as far as I am aware, the optimize attribute has other issues so
>> its use is discouraged, see commits 080b6f407635 ("bpf: Don't rely on GCC
>> __attribute__((optimize)) to disable GCSE") and a7223f5bfcae ("powerpc: Avoid
>> broken GCC __attribute__((optimize))").

Thx for trying this. I was tested with arm-linux-gnueabihf-gcc and not 
notice this.
>>
> 
> Ah yes, apalogies for missing that. I did a test build with Clang but
> I did not spot the errors. So
> 
> Unreviewed-by: ....
> 
> Maria, please use the Makefile based per-file CFLAGS override that I
> suggested before. There is really no reason to make this per-function.
Ok. I will upload a new patchset.

-- 
Thx and BRs,
Aiqun(Maria) Yu