objtool: Skip unreachable warnings for 'alt' instructions

When a whitelisted function uses one of the ALTERNATIVE macros, it
produces false positive warnings like:

  arch/x86/kvm/vmx.o: warning: objtool: .altinstr_replacement+0x0: unreachable instruction
  arch/x86/kvm/svm.o: warning: objtool: .altinstr_replacement+0x6e: unreachable instruction

There's no easy way to whitelist alternative instructions, so instead
just skip any 'unreachable' warnings associated with them.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/a5d0a8c60155f03b36a31fac871e12cf75f35fd0.1501188854.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index d07bf4a62b4599094a7fe016e3817c4575cc1376..4f0c4aea8f6fd3bf665180c143aafd94f18dac97 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1746,8 +1746,13 @@ static bool ignore_unreachable_insn(struct instruction *insn)
        /*
         * Ignore any unused exceptions.  This can happen when a whitelisted
         * function has an exception table entry.
+        *
+        * Also ignore alternative replacement instructions.  This can happen
+        * when a whitelisted function uses one of the ALTERNATIVE macros.
         */
-       if (!strcmp(insn->sec->name, ".fixup"))
+       if (!strcmp(insn->sec->name, ".fixup") ||
+           !strcmp(insn->sec->name, ".altinstr_replacement") ||
+           !strcmp(insn->sec->name, ".altinstr_aux"))
                return true;
 
        /*