linux-kernel - Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI

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Date:   Thu, 14 Oct 2021 12:20:43 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI_CLANG support

On Thu, Oct 14, 2021 at 3:23 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Oct 13, 2021 at 11:16:44AM -0700, Sami Tolvanen wrote:
> > diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> > index 1f2ae708b223..5fe31523e51f 100644
> > --- a/tools/objtool/arch/x86/decode.c
> > +++ b/tools/objtool/arch/x86/decode.c
> > @@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg)
> >       }
> >  }
> >
> > +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc)
> > +{
> > +     if (!reloc->addend)
> > +             return 0;
> > +
> > +     if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32)
> > +             return reloc->addend + 4;
> > +
> > +     return reloc->addend;
> > +}
> > +
> > +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset)
> > +{
> > +     /* offset to the relocation in a jmp instruction */
> > +     return offset + 1;
> > +}
> > +
> >  unsigned long arch_dest_reloc_offset(int addend)
> >  {
> >       return addend + 4;
> > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> > index b18f0055b50b..cd09c93c34fb 100644
> > --- a/tools/objtool/elf.c
> > +++ b/tools/objtool/elf.c
>
> > @@ -575,6 +580,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
> >       return 0;
> >  }
> >
> > +/*
> > + * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate
> > + * jump table. Undo the conversion so objtool can make sense of things.
> > + */
> > +static int fix_cfi_relocs(const struct elf *elf)
> > +{
> > +     struct section *sec;
> > +     struct reloc *reloc;
> > +
> > +     list_for_each_entry(sec, &elf->sections, list) {
> > +             list_for_each_entry(reloc, &sec->reloc_list, list) {
> > +                     struct reloc *cfi_reloc;
> > +                     unsigned long offset;
> > +
> > +                     if (!reloc->sym->sec->cfi_jt)
> > +                             continue;
> > +
> > +                     if (reloc->sym->type == STT_SECTION)
> > +                             offset = arch_cfi_section_reloc_offset(reloc);
> > +                     else
> > +                             offset = reloc->sym->offset;
> > +
> > +                     /*
> > +                      * The jump table immediately jumps to the actual function,
> > +                      * so look up the relocation there.
> > +                      */
> > +                     offset = arch_cfi_jump_reloc_offset(offset);
> > +                     cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset);
> > +
> > +                     if (!cfi_reloc || !cfi_reloc->sym) {
> > +                             WARN("can't find a CFI jump table relocation at %s+0x%lx",
> > +                                     reloc->sym->sec->name, offset);
> > +                             return -1;
> > +                     }
> > +
> > +                     reloc->sym = cfi_reloc->sym;
> > +                     reloc->addend = 0;
> > +             }
> > +     }
> > +
> > +     return 0;
> > +}
>
> If that section ever gets modified and we end up running
> elf_rebuild_reloc_section() on it, we're in trouble, right?
>
> Do we want a fatal error in elf_rebuild_reloc_section() when ->cfi_jt is
> set?

That's a valid point. Since ->cfi_jt is only set for jump table
sections, I think we'll need a different flag for this though.

Sami