linux-kernel - Re: [PATCH v2] static_call: Don't make __static_call

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Date:   Fri, 18 Mar 2022 18:40:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v2] static_call: Don't make __static_call_return0 static

On Mon, Mar 14, 2022 at 12:49:36PM +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>

Thanks! I've transferred Josh' ack from v1 in the presumption he's fine
with this one too :-)