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

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Date:   Thu, 25 Feb 2021 16:01:17 +0000
From:   Will Deacon <will@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Li Huafei <lihuafei1@...wei.com>, 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,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] recordmcount: Fix the wrong use of w* in
 arm64_is_fake_mcount()

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@...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;

Acked-by: Will Deacon <will@...nel.org>

But you know you could avoid these sorts of problems by moving to little
endian along with everybody else? ;)

Will