Re: [PATCH] recordmcount: Fix the wrong use of w* in arm64_is_fake_mcount()

From: Li Huafei <lihuafei1@huawei.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: <will@kernel.org>, <gregory.herrero@oracle.com>,
	<catalin.marinas@arm.com>, <christophe.leroy@csgroup.eu>,
	<linux-kernel@vger.kernel.org>,
	Zhangjinhao <zhangjinhao2@huawei.com>, <yangjihong1@huawei.com>,
	<linux-arm-kernel@lists.infradead.org>, <chenjun102@huawei.com>
Subject: Re: [PATCH] recordmcount: Fix the wrong use of w* in arm64_is_fake_mcount()
Date: Wed, 3 Mar 2021 09:25:05 +0800	[thread overview]
Message-ID: <54eb164c-695d-57c5-6bcd-51483b7998d3@huawei.com> (raw)
In-Reply-To: <20210302173058.28fd3d36@gandalf.local.home>

On 2021/3/3 6:30, Steven Rostedt wrote:
> On Thu, 25 Feb 2021 16:01:17 +0000
> Will Deacon <will@kernel.org> wrote:
> 
>> On Thu, Feb 25, 2021 at 09:44:26AM -0500, Steven Rostedt wrote:
>>> This requires an acked-by from one of the ARM64 maintainers.
>>>
>>> -- Steve
>>>
>>>
>>> On Thu, 25 Feb 2021 22:07:47 +0800
>>> Li Huafei <lihuafei1@huawei.com> wrote:
>>>    
>>>> When cross-compiling the kernel, the endian of the target machine and
>>>> the local machine may not match, at this time the recordmcount tool
>>>> needs byte reversal when processing elf's variables to get the correct
>>>> value. w* callback function is used to solve this problem, w is used for
>>>> 4-byte variable processing, while w8 is used for 8-byte.
>>>>
>>>> arm64_is_fake_mcount() is used to filter '_mcount' relocations that are
>>>> not used by ftrace. In arm64_is_fake_mcount(), rp->info is 8 bytes in
>>>> size, but w is used. This causes arm64_is_fake_mcount() to get the wrong
>>>> type of relocation when we cross-compile the arm64_be kernel image on an
>>>> x86_le machine, and all valid '_mcount' is filtered out. The
>>>> recordmcount tool does not collect any mcount function call locations.
>>>> At kernel startup, the following ftrace log is seen:
>>>>
>>>> 	ftrace: No functions to be traced?
>>>>
>>>> and thus ftrace cannot be used.
>>>>
>>>> Using w8 to get the value of rp->r_info will fix the problem.
>>>>
>>>> Fixes: ea0eada45632 ("recordmcount: only record relocation of type
>>>> R_AARCH64_CALL26 on arm64")
>>>> Signed-off-by: Li Huafei <lihuafei1@huawei.com>
>>>> ---
>>>>   scripts/recordmcount.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
>>>> index b9c2ee7ab43f..cce12e1971d8 100644
>>>> --- a/scripts/recordmcount.c
>>>> +++ b/scripts/recordmcount.c
>>>> @@ -438,7 +438,7 @@ static int arm_is_fake_mcount(Elf32_Rel const *rp)
>>>>   
>>>>   static int arm64_is_fake_mcount(Elf64_Rel const *rp)
>>>>   {
>>>> -	return ELF64_R_TYPE(w(rp->r_info)) != R_AARCH64_CALL26;
>>>> +	return ELF64_R_TYPE(w8(rp->r_info)) != R_AARCH64_CALL26;
>>
>> Acked-by: Will Deacon <will@kernel.org>
>>
>> But you know you could avoid these sorts of problems by moving to little
>> endian along with everybody else? ;)
>>
> 
> I just realized that I received this patch twice, and thought it was the
> same patch! Chen was three days ahead of you, so he get's the credit ;-)
> 
>   https://lore.kernel.org/r/20210222135840.56250-1-chenjun102@huawei.com
> 
> -- Steve
> .
> 

That's fine, thanks Steve and Will!

Huafei

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