Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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