Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

[Naveen N. Rao]

Peter Zijlstra wrote:
> On Fri, Mar 18, 2022 at 04:21:40PM +0530, Sathvika Vasireddy wrote:
> > This patch adds powerpc specific functions required for
> > 'objtool mcount' to work, and enables mcount for ppc.
>
> I would love to see more objtool enablement for Power :-)
>
>
> > diff --git a/tools/objtool/arch/powerpc/include/arch/elf.h b/tools/objtool/arch/powerpc/include/arch/elf.h
> > new file mode 100644
> > index 000000000000..3c8ebb7d2a6b
> > --- /dev/null
> > +++ b/tools/objtool/arch/powerpc/include/arch/elf.h
> > @@ -0,0 +1,8 @@
> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
> > +
> > +#ifndef _OBJTOOL_ARCH_ELF
> > +#define _OBJTOOL_ARCH_ELF
> > +
> > +#define R_NONE R_PPC_NONE
> > +
> > +#endif /* _OBJTOOL_ARCH_ELF */
> > diff --git a/tools/objtool/utils.c b/tools/objtool/utils.c
> > index d1fc6a123a6e..c9c14fa0dfd7 100644
> > --- a/tools/objtool/utils.c
> > +++ b/tools/objtool/utils.c
> > @@ -179,11 +179,29 @@ int create_mcount_loc_sections(struct objtool_file *file)
> >  		loc = (unsigned long *)sec->data->d_buf + idx;
> >  		memset(loc, 0, sizeof(unsigned long));
> >  
> > -		if (elf_add_reloc_to_insn(file->elf, sec,
> > -					  idx * sizeof(unsigned long),
> > -					  R_X86_64_64,
> > -					  insn->sec, insn->offset))
> > -			return -1;
> > +		if (file->elf->ehdr.e_machine == EM_X86_64) {
> > +			if (elf_add_reloc_to_insn(file->elf, sec,
> > +						  idx * sizeof(unsigned long),
> > +						  R_X86_64_64,
> > +						  insn->sec, insn->offset))
> > +				return -1;
> > +		}
> > +
> > +		if (file->elf->ehdr.e_machine == EM_PPC64) {
> > +			if (elf_add_reloc_to_insn(file->elf, sec,
> > +						  idx * sizeof(unsigned long),
> > +						  R_PPC64_ADDR64,
> > +						  insn->sec, insn->offset))
> > +				return -1;
> > +		}
> > +
> > +		if (file->elf->ehdr.e_machine == EM_PPC) {
> > +			if (elf_add_reloc_to_insn(file->elf, sec,
> > +						  idx * sizeof(unsigned long),
> > +						  R_PPC_ADDR32,
> > +						  insn->sec, insn->offset))
> > +				return -1;
> > +		}
>
> It appears to me that repeating this code 3 times doesn't really scale
> well, how about we introduce a helper like:
>
>
> int elf_reloc_type_long(struct elf *elf)
> {
> 	switch (elf->ehdr.e_machine) {
> 	case EM_X86_64:
> 		return R_X86_64_64;
> 	case EM_PPC64:
> 		return R_PPC64_ADDR64;
> 	case EM_PPC:
> 		return R_PPC_ADDR32;
> 	default:
> 		WARN("unknown machine...")
> 		exit(-1);
> 	}
> }
>
> 		if (elf_add_reloc_to_insn(file->elf, sec,
> 					  idx * sizeof(unsigned long),
> 					  elf_reloc_type_long(file->elf),
> 					  insn->sec, insn->offset))
> 			return -1;

Good point. I suppose we'll need a similar helper, say, 
elf_reloc_type_none(), to replace R_NONE usage if we want to have 
objtool work for cross-arch builds.


- Naveen