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SubjectRe: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
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Le 04/07/2022 à 14:05, Peter Zijlstra a écrit :
> On Sat, Jun 25, 2022 at 06:46:54AM +0000, Christophe Leroy wrote:
>>
>>
>> Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :
>>> objtool is throwing *unannotated intra-function call*
>>> warnings with a few instructions that are marked
>>> unreachable. Remove unreachable() from WARN_ON()
>>> to fix these warnings, as the codegen remains same
>>> with and without unreachable() in WARN_ON().
>>
>> Did you try the two exemples described in commit 1e688dd2a3d6
>> ("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with
>> asm goto") ?
>>
>> Without your patch:
>>
>> 00000640 <test>:
>> 640: 81 23 00 84 lwz r9,132(r3)
>> 644: 71 29 40 00 andi. r9,r9,16384
>> 648: 40 82 00 0c bne 654 <test+0x14>
>> 64c: 80 63 00 0c lwz r3,12(r3)
>> 650: 4e 80 00 20 blr
>> 654: 0f e0 00 00 twui r0,0
>>
>> 00000658 <test9w>:
>> 658: 2c 04 00 00 cmpwi r4,0
>> 65c: 41 82 00 0c beq 668 <test9w+0x10>
>> 660: 7c 63 23 96 divwu r3,r3,r4
>> 664: 4e 80 00 20 blr
>> 668: 0f e0 00 00 twui r0,0
>> 66c: 38 60 00 00 li r3,0
>> 670: 4e 80 00 20 blr
>
> Per this construct you should do as x86 does and assume twui terminates
> control flow and explicitly annotate the WARN case. That is, given the
> fact that BUG as no instructions following it, you can't very well
> annotate that.

That exactly the problem I guess. I'm fine with replacing the
unreachable() by __builtin_unreachable() with our __WARN_FLAGS() and
BUG() but we will still have a problem with some of the unrachable()
that are in core parts of the kernel.

Even the ones in arch/powerpc/, they are valid and should remain. The
point seems that the generic annotate_unreachable() is wrong for powerpc
as is, and activating CONFIG_OBJTOOL lead to bad code generation.

By the way, for which functionnalities of objtool is that analysis
necessary ? I understand it is not necessary to mcount accounting, so
maybe the not empty annotate_unreachable() should be limited to those
those functionnalities ?

>
> Alternatively, you can teach objtool to look at __bug_table to
> distinguish these cases.

Isn't it enough to tell objtool that execution never go past twui, using
INSN_BUG ?
By the way, for __WARN_FLAGS, we use the __extable for the continuation.
Is objtools able to follow __extable ?

Christophe
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