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

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Message-ID: <20230428114338.GB1449475@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Apr 2023 13:43:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Hou Wenlong <houwenlong.hwl@...group.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Garnier <thgarnie@...omium.org>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH RFC 33/43] objtool: Add validation for x86 PIE support

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;

> > +
> > +
> >   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().