Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Arnd Bergmann]

On Wed, Mar 17, 2021 at 5:18 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> 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.

I wonder if that is just a typo in INIT_DATA. Nico introduced it as part of
266ff2a8f51f ("kbuild: Fix asm-generic/vmlinux.lds.h for
LD_DEAD_CODE_DATA_ELIMINATION"), so perhaps that should have
been .init.data.* instead.

> 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.

Ah, interesting.

> -------------------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 ]---

This looks like it has just returned from __schedule() to schedule()
and is trying to return from that as well, through code like this:

.L562:
// /git/arm-soc/kernel/sched/core.c:5159: }
        ldp     x19, x20, [sp, 16]      //,,
        ldp     x29, x30, [sp], 32      //,,,
        hint    29 // autiasp
        ret

It looks like pointer authentication gone wrong, which ended up
with dereferencing the broken pointer in x22, and it explains why
it only happens with -cpu max. Presumably this also only happens
on secondary CPUs, so maybe the bit that initializes PAC on
secondary CPUs got discarded?

        Arnd