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 <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.le...@csgroup.eu>
Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> 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
>
