linux-kernel - Re: [PATCH] recordmcount: Fix the wrong use of w* in arm64_is_fake

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Date:   Thu, 25 Feb 2021 09:44:26 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Li Huafei <lihuafei1@...wei.com>
Cc:     <gregory.herrero@...cle.com>, <catalin.marinas@....com>,
        <christophe.leroy@...roup.eu>, <linux-kernel@...r.kernel.org>,
        <zhangjinhao2@...wei.com>, <yangjihong1@...wei.com>,
        <xukuohai@...wei.com>, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] recordmcount: Fix the wrong use of w* in
 arm64_is_fake_mcount()

This requires an acked-by from one of the ARM64 maintainers.

-- Steve


On Thu, 25 Feb 2021 22:07:47 +0800
Li Huafei <lihuafei1@...wei.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@...wei.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;
>  }
>  
>  /* 64-bit EM_MIPS has weird ELF64_Rela.r_info.