linux-kernel - Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA

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Date:   Wed, 17 Mar 2021 16:18:39 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Mark Brown <broonie@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Scull <ascull@...gle.com>,
        David Brazdil <dbrazdil@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

On Wed, Mar 17, 2021 at 02:37:57PM +0000, Catalin Marinas wrote:
> On Thu, Feb 25, 2021 at 12:20:56PM +0100, Arnd Bergmann wrote:
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> > index bad2b9eaab22..926cdb597a45 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -217,7 +217,7 @@ SECTIONS
> >  		INIT_CALLS
> >  		CON_INITCALL
> >  		INIT_RAM_FS
> > -		*(.init.altinstructions .init.bss .init.bss.*)	/* from the EFI stub */
> > +		*(.init.altinstructions .init.data.* .init.bss .init.bss.*)	/* from the EFI stub */
> 
> INIT_DATA already covers .init.data and .init.data.*, so I don't think
> we need this change.

Ah, INIT_DATA only covers init.data.* (so no dot in front). The above
is needed for the EFI stub.

However, I gave this a quick try and under Qemu with -cpu max and -smp 2
(or more) it fails as below. I haven't debugged but the lr points to
just after the switch_to() call. Maybe some section got discarded and we
patched in the wrong instructions. It is fine with -cpu host or -smp 1.

-------------------8<------------------------
smp: Bringing up secondary CPUs ...
Detected PIPT I-cache on CPU1
CPU1: Booted secondary processor 0x0000000001 [0x000f0510]
Unable to handle kernel paging request at virtual address eb91d81ad2971160
Mem abort info:
  ESR = 0x86000004
  EC = 0x21: IABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
[eb91d81ad2971160] address between user and kernel address ranges
Internal error: Oops: 86000004 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 16 Comm: migration/1 Not tainted 5.12.0-rc3-00002-g128e977c1322 #1
Stopper: 0x0 <- 0x0
pstate: 60000085 (nZCv daIf -PAN -UAO -TCO BTYPE=--)
pc : 0xeb91d81ad2971160
lr : __schedule+0x230/0x6b8
sp : ffff80001009bd60
x29: ffff80001009bd60 x28: 0000000000000000 
x27: ffff0000000a6760 x26: ffff0000000b7540 
x25: 0080000000000000 x24: ffffd81ad3969000 
x23: ffff0000000a6200 x22: 6ee0d81ad2971658 
x21: ffff0000000a6200 x20: ffff000000080000 
x19: ffff00007fbc6bc0 x18: 0000000000000030 
x17: 0000000000000000 x16: 0000000000000000 
x15: 00008952b30a9a9e x14: 0000000000000366 
x13: 0000000000000192 x12: 0000000000000000 
x11: 0000000000000003 x10: 00000000000009b0 
x9 : ffff80001009bd30 x8 : ffff0000000a6c10 
x7 : ffff00007fbc6cc0 x6 : 00000000fffedb30 
x5 : 00000000ffffffff x4 : 0000000000000000 
x3 : 0000000000000008 x2 : 0000000000000000 
x1 : ffff0000000a6200 x0 : ffff0000000a3800 
Call trace:
 0xeb91d81ad2971160
 schedule+0x70/0x108
 schedule_preempt_disabled+0x24/0x40
 __kthread_parkme+0x68/0xd0
 kthread+0x138/0x170
 ret_from_fork+0x10/0x30
Code: bad PC value
---[ end trace af3481062ecef3e7 ]---

-- 
Catalin