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Thu, 22 Jun 2023 13:45:04 +0200 (CEST) Date: Thu, 22 Jun 2023 13:45:04 +0200 From: Peter Zijlstra To: Christophe Leroy Cc: Michael Ellerman , Nicholas Piggin , Josh Poimboeuf , Sathvika Vasireddy , Naveen N Rao , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v2 09/14] objtool: Add INSN_RETURN_CONDITIONAL Message-ID: <20230622114504.GK4253@hirez.programming.kicks-ass.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 22, 2023 at 12:54:31PM +0200, Christophe Leroy wrote: > Most functions have an unconditional return at the end, like > this one: > > 00000000 : > 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 : > 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 > 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 > 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 > 98: 39 00 00 04 li r8,4 > 9c: 4b ff ff bc b 58 > > 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 Acked-by: Peter Zijlstra (Intel) > --- > 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 >