linux-kernel - Re: [PATCH] static_call: Don't make __static_call

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Message-ID: <eabd82b5-6617-1965-e65b-25abae19e1d5@csgroup.eu>
Date:   Wed, 16 Mar 2022 07:11:14 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] static_call: Don't make __static_call_return0 static



Le 15/03/2022 à 22:11, Josh Poimboeuf a écrit :
> On Mon, Mar 14, 2022 at 08:21:30AM +0100, Christophe Leroy wrote:
>> System.map shows that vmlinux contains several instances of
>> __static_call_return0():
>>
>> 	c0004fc0 t __static_call_return0
>> 	c0011518 t __static_call_return0
>> 	c00d8160 t __static_call_return0
>>
>> arch_static_call_transform() uses the middle one to check whether we are
>> setting a call to __static_call_return0 or not:
>>
>> 	c0011520 <arch_static_call_transform>:
>> 	c0011520:       3d 20 c0 01     lis     r9,-16383	<== r9 =  0xc001 << 16
>> 	c0011524:       39 29 15 18     addi    r9,r9,5400	<== r9 += 0x1518
>> 	c0011528:       7c 05 48 00     cmpw    r5,r9		<== r9 has value 0xc0011518 here
>>
>> So if static_call_update() is called with one of the other instances of
>> __static_call_return0(), arch_static_call_transform() won't recognise it.
>>
>> In order to work properly, global single instance of __static_call_return0() is required.
>>
>> Fixes: 3f2a8fc4b15d ("static_call/x86: Add __static_call_return0()")
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> 
> Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
> 

Thanks for this Ack.

By the way I sent version v2 before your Ack for a missing #include

Christophe