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Commit 258c7605 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Thomas Gleixner
Browse files

objtool: Allow alternatives to be ignored 



Getting objtool to understand retpolines is going to be a bit of a
challenge.  For now, take advantage of the fact that retpolines are
patched in with alternatives.  Just read the original (sane)
non-alternative instruction, and ignore the patched-in retpoline.

This allows objtool to understand the control flow *around* the
retpoline, even if it can't yet follow what's inside.  This means the
ORC unwinder will fail to unwind from inside a retpoline, but will work
fine otherwise.

Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarDavid Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: thomas.lendacky@amd.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Kees Cook <keescook@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: https://lkml.kernel.org/r/1515707194-20531-3-git-send-email-dwmw@amazon.co.uk
parent 39b73533

tools/objtool/check.c

+56 −6
Original line number Diff line number Diff line
@@ -427,6 +427,40 @@ static void add_ignores(struct objtool_file *file) 
	}

}



/*

 * FIXME: For now, just ignore any alternatives which add retpolines.  This is

 * a temporary hack, as it doesn't allow ORC to unwind from inside a retpoline.

 * But it at least allows objtool to understand the control flow *around* the

 * retpoline.

 */

static int add_nospec_ignores(struct objtool_file *file)

{

	struct section *sec;

	struct rela *rela;

	struct instruction *insn;



	sec = find_section_by_name(file->elf, ".rela.discard.nospec");

	if (!sec)

		return 0;



	list_for_each_entry(rela, &sec->rela_list, list) {

		if (rela->sym->type != STT_SECTION) {

			WARN("unexpected relocation symbol type in %s", sec->name);

			return -1;

		}



		insn = find_insn(file, rela->sym->sec, rela->addend);

		if (!insn) {

			WARN("bad .discard.nospec entry");

			return -1;

		}



		insn->ignore_alts = true;

	}



	return 0;

}



/*

 * Find the destination instructions for all jumps.

 */
@@ -509,11 +543,18 @@ static int add_call_destinations(struct objtool_file *file) 
			dest_off = insn->offset + insn->len + insn->immediate;

			insn->call_dest = find_symbol_by_offset(insn->sec,

								dest_off);

			/*

			 * FIXME: Thanks to retpolines, it's now considered

			 * normal for a function to call within itself.  So

			 * disable this warning for now.

			 */

#if 0

			if (!insn->call_dest) {

				WARN_FUNC("can't find call dest symbol at offset 0x%lx",

					  insn->sec, insn->offset, dest_off);

				return -1;

			}

#endif

		} else if (rela->sym->type == STT_SECTION) {

			insn->call_dest = find_symbol_by_offset(rela->sym->sec,

								rela->addend+4);
@@ -678,12 +719,6 @@ static int add_special_section_alts(struct objtool_file *file) 
		return ret;



	list_for_each_entry_safe(special_alt, tmp, &special_alts, list) {

		alt = malloc(sizeof(*alt));

		if (!alt) {

			WARN("malloc failed");

			ret = -1;

			goto out;

		}



		orig_insn = find_insn(file, special_alt->orig_sec,

				      special_alt->orig_off);
@@ -694,6 +729,10 @@ static int add_special_section_alts(struct objtool_file *file) 
			goto out;

		}



		/* Ignore retpoline alternatives. */

		if (orig_insn->ignore_alts)

			continue;



		new_insn = NULL;

		if (!special_alt->group || special_alt->new_len) {

			new_insn = find_insn(file, special_alt->new_sec,
@@ -719,6 +758,13 @@ static int add_special_section_alts(struct objtool_file *file) 
				goto out;

		}



		alt = malloc(sizeof(*alt));

		if (!alt) {

			WARN("malloc failed");

			ret = -1;

			goto out;

		}



		alt->insn = new_insn;

		list_add_tail(&alt->list, &orig_insn->alts);
@@ -1035,6 +1081,10 @@ static int decode_sections(struct objtool_file *file) 


	add_ignores(file);



	ret = add_nospec_ignores(file);

	if (ret)

		return ret;



	ret = add_jump_destinations(file);

	if (ret)

		return ret;

tools/objtool/check.h

+1 −1
Original line number Diff line number Diff line
@@ -44,7 +44,7 @@ struct instruction { 
	unsigned int len;

	unsigned char type;

	unsigned long immediate;

	bool alt_group, visited, dead_end, ignore, hint, save, restore;

	bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;

	struct symbol *call_dest;

	struct instruction *jump_dest;

	struct list_head alts;