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Message-ID: <YsL2/uwU6j8jbzpP@worktop.programming.kicks-ass.net>
Date: Mon, 4 Jul 2022 16:19:42 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Sathvika Vasireddy <sv@...ux.ibm.com>,
"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
"jpoimboe@...hat.com" <jpoimboe@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"aik@...abs.ru" <aik@...abs.ru>,
"mpe@...erman.id.au" <mpe@...erman.id.au>,
"mingo@...hat.com" <mingo@...hat.com>,
"rostedt@...dmis.org" <rostedt@...dmis.org>,
"naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
"mbenes@...e.cz" <mbenes@...e.cz>,
"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
"paulus@...ba.org" <paulus@...ba.org>
Subject: Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
On Mon, Jul 04, 2022 at 12:44:30PM +0000, Christophe Leroy wrote:
>
>
> 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.
Right; I'm not against making that depend on yet-another OBJTOOL_$config
thing.
> 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 ?
For all the things where it needs to follow control flow, so stack
validation, ORC generation, unreachable instruction validation etc..
I'd need to double check code-gen on x86, but the way
__builtin_unreachable() makes code-gen stop dead, I'm not entirely sure
it's right for us either.
> > 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 ?
That should work I suppose.
> By the way, for __WARN_FLAGS, we use the __extable for the continuation.
> Is objtools able to follow __extable ?
Yes.
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