Commit bd841d61 authored Apr 01, 2020 by Josh Poimboeuf Committed by Borislav Petkov Apr 14, 2020

objtool: Fix CONFIG_UBSAN_TRAP unreachable warnings



CONFIG_UBSAN_TRAP causes GCC to emit a UD2 whenever it encounters an
unreachable code path.  This includes __builtin_unreachable().  Because
the BUG() macro uses __builtin_unreachable() after it emits its own UD2,
this results in a double UD2.  In this case objtool rightfully detects
that the second UD2 is unreachable:

  init/main.o: warning: objtool: repair_env_string()+0x1c8: unreachable instruction

We weren't able to figure out a way to get rid of the double UD2s, so
just silence the warning.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/6653ad73c6b59c049211bd7c11ed3809c20ee9f5.1585761021.git.jpoimboe@redhat.com

parent 8f3d9f35

tools/objtool/check.c

+15 −2

Original line number	Diff line number	Diff line
		@@ -2364,14 +2364,27 @@ static bool ignore_unreachable_insn(struct instruction *insn)
		!strcmp(insn->sec->name, ".altinstr_aux"))
		return true;

		if (!insn->func)
		return false;

		/*
		* CONFIG_UBSAN_TRAP inserts a UD2 when it sees
		* __builtin_unreachable(). The BUG() macro has an unreachable() after
		* the UD2, which causes GCC's undefined trap logic to emit another UD2
		* (or occasionally a JMP to UD2).
		*/
		if (list_prev_entry(insn, list)->dead_end &&
		(insn->type == INSN_BUG \|\|
		(insn->type == INSN_JUMP_UNCONDITIONAL &&
		insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
		return true;

		/*
		* Check if this (or a subsequent) instruction is related to
		* CONFIG_UBSAN or CONFIG_KASAN.
		*
		* End the search at 5 instructions to avoid going into the weeds.
		*/
		if (!insn->func)
		return false;
		for (i = 0; i < 5; i++) {

		if (is_kasan_insn(insn) \|\| is_ubsan_insn(insn))

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