linux-kernel - [PATCH v2 09/14] objtool: Add INSN_RETURN

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Message-Id: <c25524d42cb03b1acfb7ab325b9e8881befba53b.1687430631.git.christophe.leroy@csgroup.eu>
Date:   Thu, 22 Jun 2023 12:54:31 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Naveen N Rao <naveen@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 09/14] objtool: Add INSN_RETURN_CONDITIONAL

Most functions have an unconditional return at the end, like
this one:

	00000000 <is_exec_fault>:
	   0:	81 22 04 d0 	lwz     r9,1232(r2)
	   4:	38 60 00 00 	li      r3,0
	   8:	2c 09 00 00 	cmpwi   r9,0
	   c:	4d 82 00 20 	beqlr		<== Conditional return
	  10:	80 69 00 a0 	lwz     r3,160(r9)
	  14:	54 63 00 36 	clrrwi  r3,r3,4
	  18:	68 63 04 00 	xori    r3,r3,1024
	  1c:	7c 63 00 34 	cntlzw  r3,r3
	  20:	54 63 d9 7e 	srwi    r3,r3,5
	  24:	4e 80 00 20 	blr		<== Unconditional return

But other functions like this other one below only have
conditional returns:

	00000028 <pte_update.isra.0>:
	  28:	81 25 00 00 	lwz     r9,0(r5)
	  2c:	2c 08 00 00 	cmpwi   r8,0
	  30:	7d 29 30 78 	andc    r9,r9,r6
	  34:	7d 27 3b 78 	or      r7,r9,r7
	  38:	54 84 65 3a 	rlwinm  r4,r4,12,20,29
	  3c:	81 23 00 18 	lwz     r9,24(r3)
	  40:	41 82 00 58 	beq     98 <pte_update.isra.0+0x70>
	  44:	7d 29 20 2e 	lwzx    r9,r9,r4
	  48:	55 29 07 3a 	rlwinm  r9,r9,0,28,29
	  4c:	2c 09 00 0c 	cmpwi   r9,12
	  50:	41 82 00 08 	beq     58 <pte_update.isra.0+0x30>
	  54:	39 00 00 80 	li      r8,128
	  58:	2c 08 00 01 	cmpwi   r8,1
	  5c:	90 e5 00 00 	stw     r7,0(r5)
	  60:	4d a2 00 20 	beqlr+		<== Conditional return
	  64:	7c e9 3b 78 	mr      r9,r7
	  68:	39 40 00 00 	li      r10,0
	  6c:	39 4a 00 04 	addi    r10,r10,4
	  70:	7c 0a 40 00 	cmpw    r10,r8
	  74:	91 25 00 04 	stw     r9,4(r5)
	  78:	91 25 00 08 	stw     r9,8(r5)
	  7c:	38 a5 00 10 	addi    r5,r5,16
	  80:	91 25 ff fc 	stw     r9,-4(r5)
	  84:	4c 80 00 20 	bgelr		<== Conditional return
	  88:	55 49 60 26 	slwi    r9,r10,12
	  8c:	7d 29 3a 14 	add     r9,r9,r7
	  90:	91 25 00 00 	stw     r9,0(r5)
	  94:	4b ff ff d8 	b       6c <pte_update.isra.0+0x44>
	  98:	39 00 00 04 	li      r8,4
	  9c:	4b ff ff bc 	b       58 <pte_update.isra.0+0x30>

If conditional returns are decoded as INSN_OTHER, objtool considers
that the second function never returns.

If conditional returns are decoded as INSN_RETURN, objtool considers
that code after that conditional return is dead.

To overcome this situation, introduce INSN_RETURN_CONDITIONAL which
is taken as a confirmation that a function is not noreturn but still
sees following code as reachable.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 tools/objtool/check.c                | 2 +-
 tools/objtool/include/objtool/arch.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0fcf99c91400..8977cdf93f54 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -259,7 +259,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 	func_for_each_insn(file, func, insn) {
 		empty = false;
 
-		if (insn->type == INSN_RETURN)
+		if (insn->type == INSN_RETURN || insn->type == INSN_RETURN_CONDITIONAL)
 			return false;
 	}
 
diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
index 2b6d2ce4f9a5..84ba75112934 100644
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -19,6 +19,7 @@ enum insn_type {
 	INSN_CALL,
 	INSN_CALL_DYNAMIC,
 	INSN_RETURN,
+	INSN_RETURN_CONDITIONAL,
 	INSN_CONTEXT_SWITCH,
 	INSN_BUG,
 	INSN_NOP,
-- 
2.40.1