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

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Date:   Tue, 2 Mar 2021 17:30:58 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Will Deacon <will@...nel.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,
        Chen Jun <chenjun102@...wei.com>
Subject: Re: [PATCH] recordmcount: Fix the wrong use of w* in
 arm64_is_fake_mcount()

On Thu, 25 Feb 2021 16:01:17 +0000
Will Deacon <will@...nel.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@...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? ;)
> 

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