objtool: Fix IBT tail-call detection

Objtool reports:

  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_blocks_avx() falls through to next function poly1305_blocks_x86_64()
  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_emit_avx() falls through to next function poly1305_emit_x86_64()
  arch/x86/crypto/poly1305-x86_64.o: warning: objtool: poly1305_blocks_avx2() falls through to next function poly1305_blocks_x86_64()

Which reads like:

0000000000000040 <poly1305_blocks_x86_64>:
	 40:       f3 0f 1e fa             endbr64
	...

0000000000000400 <poly1305_blocks_avx>:
	400:       f3 0f 1e fa             endbr64
	404:       44 8b 47 14             mov    0x14(%rdi),%r8d
	408:       48 81 fa 80 00 00 00    cmp    $0x80,%rdx
	40f:       73 09                   jae    41a <poly1305_blocks_avx+0x1a>
	411:       45 85 c0                test   %r8d,%r8d
	414:       0f 84 2a fc ff ff       je     44 <poly1305_blocks_x86_64+0x4>
	...

These are simple conditional tail-calls and *should* be recognised as
such by objtool, however due to a mistake in commit 08f87a93
("objtool: Validate IBT assumptions") this is failing.

Specifically, the jump_dest is +4, this means the instruction pointed
at will not be ENDBR and as such it will fail the second clause of
is_first_func_insn() that was supposed to capture this exact case.

Instead, have is_first_func_insn() look at the previous instruction.

Fixes: 08f87a93 ("objtool: Validate IBT assumptions")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220322115125.811582125@infradead.org

tools/objtool/check.c

@@ -1239,11 +1239,20 @@ static bool same_function(struct instruction *insn1, struct instruction *insn2)
 	return insn1->func->pfunc == insn2->func->pfunc;
 }
 
-static bool is_first_func_insn(struct instruction *insn)
+static bool is_first_func_insn(struct objtool_file *file, struct instruction *insn)
 {
-	return insn->offset == insn->func->offset ||
-	       (insn->type == INSN_ENDBR &&
-		insn->offset == insn->func->offset + insn->len);
+	if (insn->offset == insn->func->offset)
+		return true;
+
+	if (ibt) {
+		struct instruction *prev = prev_insn_same_sym(file, insn);
+
+		if (prev && prev->type == INSN_ENDBR &&
+		    insn->offset == insn->func->offset + prev->len)
+			return true;
+	}
+
+	return false;
 }
 
 /*
@@ -1327,7 +1336,7 @@ static int add_jump_destinations(struct objtool_file *file)
 				insn->jump_dest->func->pfunc = insn->func;
 
 			} else if (!same_function(insn, insn->jump_dest) &&
-				   is_first_func_insn(insn->jump_dest)) {
+				   is_first_func_insn(file, insn->jump_dest)) {
 				/* internal sibling call (without reloc) */
 				add_call_dest(file, insn, insn->jump_dest->func, true);
 			}