Re: [PATCH RFC 33/43] objtool: Add validation for x86 PIE support

[Hou Wenlong]

On Fri, Apr 28, 2023 at 07:43:38PM +0800, Peter Zijlstra wrote:
> On Fri, Apr 28, 2023 at 10:28:19AM +0000, Christophe Leroy wrote:
> 
> 
> > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > > index 5b600bbf2389..d67b80251eec 100644
> > > --- a/tools/objtool/check.c
> > > +++ b/tools/objtool/check.c
> > > @@ -131,6 +131,27 @@ static struct instruction *prev_insn_same_sym(struct objtool_file *file,
> > >          for (insn = next_insn_same_sec(file, insn); insn;               \
> > >               insn = next_insn_same_sec(file, insn))
> > > 
> > > +static struct instruction *find_insn_containing(struct objtool_file *file,
> > > +                                               struct section *sec,
> > > +                                               unsigned long offset)
> > > +{
> > > +       struct instruction *insn;
> > > +
> > > +       insn = find_insn(file, sec, 0);
> > > +       if (!insn)
> > > +               return NULL;
> > > +
> > > +       sec_for_each_insn_from(file, insn) {
> > > +               if (insn->offset > offset)
> > > +                       return NULL;
> > > +               if (insn->offset <= offset && (insn->offset + insn->len) > offset)
> > > +                       return insn;
> > > +       }
> > > +
> > > +       return NULL;
> > > +}
> 
> Urgh, this is horrendous crap. Yes you're only using it in case of a
> warning, but adding a function like this makes it appear like it's
> actually sane to use.
> 
> A far better implementation -- but still not stellar -- would be
> something like:
> 
> 	sym = find_symbol_containing(sec, offset);
> 	if (!sym)
> 		// fail
> 	sym_for_each_insn(file, sym, insn) {
> 		...
> 	}
> 
> But given insn_hash uses sec_offset_hash() you can do something similar
> to find_reloc_by_dest_range()
> 
> 	start = offset - (INSN_MAX_SIZE - 1);
> 	for_offset_range(o, start, start + INSN_MAX_SIZE) {
> 		hash_for_each_possible(file->insn_hash, insn, hash, sec_offset_hash(sec, o)) {
> 			if (insn->sec != sec)
> 				continue;
> 
> 			if (insn->offset <= offset &&
> 			    insn->offset + inns->len > offset)
> 				return insn;
> 		}
> 	}
> 	return NULL;
>
Thanks for your suggestion, I'll pick it in the next version.
 
> > > +
> > > +
> > >   static inline struct symbol *insn_call_dest(struct instruction *insn)
> > >   {
> > >          if (insn->type == INSN_JUMP_DYNAMIC ||
> > > @@ -4529,6 +4550,61 @@ static int validate_reachable_instructions(struct objtool_file *file)
> > >          return 0;
> > >   }
> > > 
> > > +static int is_in_pvh_code(struct instruction *insn)
> > > +{
> > > +       struct symbol *sym = insn->sym;
> > > +
> > > +       return sym && !strcmp(sym->name, "pvh_start_xen");
> > > +}
> > > +
> > > +static int validate_pie(struct objtool_file *file)
> > > +{
> > > +       struct section *sec;
> > > +       struct reloc *reloc;
> > > +       struct instruction *insn;
> > > +       int warnings = 0;
> > > +
> > > +       for_each_sec(file, sec) {
> > > +               if (!sec->reloc)
> > > +                       continue;
> > > +               if (!(sec->sh.sh_flags & SHF_ALLOC))
> > > +                       continue;
> > > +
> > > +               list_for_each_entry(reloc, &sec->reloc->reloc_list, list) {
> > > +                       switch (reloc->type) {
> > > +                       case R_X86_64_NONE:
> > > +                       case R_X86_64_PC32:
> > > +                       case R_X86_64_PLT32:
> > > +                       case R_X86_64_64:
> > > +                       case R_X86_64_PC64:
> > > +                       case R_X86_64_GOTPCREL:
> > > +                               break;
> > > +                       case R_X86_64_32:
> > > +                       case R_X86_64_32S:
> > 
> > That looks very specific to X86, should it go at another place ?
> > 
> > If it can work for any architecture, can you add generic macros, just 
> > like commit c1449735211d ("objtool: Use macros to define arch specific 
> > reloc types") then commit c984aef8c832 ("objtool/powerpc: Add --mcount 
> > specific implementation") ?
> 
> Yes, this should be something like arch_PIE_reloc() or so. Similar to
> arch_pc_relative_reloc().