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

[Li Huafei]

On 2021/3/3 6:30, Steven Rostedt wrote:
> On Thu, 25 Feb 2021 16:01:17 +0000
> Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >   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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx
>
> -- Steve
> .

That's fine, thanks Steve and Will!

Huafei