[PATCH] arm64: traps: attempt to dump all instructions

[Ard Biesheuvel]

On Fri, 27 Jan 2023 at 13:13, Mark Rutland <mark.rutland at arm.com> wrote:
>
> Currently dump_kernel_instr() dumps a few instructions around the
> pt_regs::pc value, dumping 4 instructions before the PC before dumping
> the instruction at the PC. If an attempt to read an instruction fails,
> it gives up and does not attempt to dump any subsequent instructions.
>
> This is unfortunate when the pt_regs::pc value points to the start of a
> page with a leading guard page, where the instruction at the PC can be
> read, but prior instructions cannot.
>
> This patch makes dump_kernel_instr() attempt to dump each instruction
> regardless of whether reading a prior instruction could be read, which
> gives a more useful code dump in such cases. When an instruction cannot
> be read, it is reported as "????????", which cannot be confused with a
> hex value,
>
> For example, with a `UDF #0` (AKA 0x00000000) early in the kexec control
> page, we'll now get the following code dump:
>
> | Internal error: Oops - Undefined instruction: 0000000002000000 [#1] SMP
> | Modules linked in:
> | CPU: 0 PID: 261 Comm: kexec Not tainted 6.2.0-rc5+ #26
> | Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
> | pstate: 604003c5 (nZCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : 0x48c00000
> | lr : machine_kexec+0x190/0x200
> | sp : ffff80000d36ba80
> | x29: ffff80000d36ba80 x28: ffff000002dfc380 x27: 0000000000000000
> | x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
> | x23: ffff80000a9f7858 x22: 000000004c460000 x21: 0000000000000010
> | x20: 00000000ad821000 x19: ffff000000aa0000 x18: 0000000000000006
> | x17: ffff8000758a2000 x16: ffff800008000000 x15: ffff80000d36b568
> | x14: 0000000000000000 x13: ffff80000d36b707 x12: ffff80000a9bf6e0
> | x11: 00000000ffffdfff x10: ffff80000aaaf8e0 x9 : ffff80000815eff8
> | x8 : 000000000002ffe8 x7 : c0000000ffffdfff x6 : 00000000000affa8
> | x5 : 0000000000001fff x4 : 0000000000000001 x3 : ffff80000a263008
> | x2 : ffff80000a9e20f8 x1 : 0000000048c00000 x0 : ffff000000aa0000
> | Call trace:
> |  0x48c00000
> |  kernel_kexec+0x88/0x138
> |  __do_sys_reboot+0x108/0x288
> |  __arm64_sys_reboot+0x2c/0x40
> |  invoke_syscall+0x78/0x140
> |  el0_svc_common.constprop.0+0x4c/0x100
> |  do_el0_svc+0x34/0x80
> |  el0_svc+0x34/0x140
> |  el0t_64_sync_handler+0xf4/0x140
> |  el0t_64_sync+0x194/0x1c0
> | Code: ???????? ???????? ???????? ???????? (00000000)
> | ---[ end trace 0000000000000000 ]---
> | Kernel panic - not syncing: Oops - Undefined instruction: Fatal exception
> | Kernel Offset: disabled
> | CPU features: 0x002000,00050108,c8004203
> | Memory Limit: none
>
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Will Deacon <will at kernel.org>

Reviewed-by: Ard Biesheuvel <ardb at kernel.org>

> ---
>  arch/arm64/kernel/traps.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 4c0caa589e12d..0ccc063daccb8 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -162,10 +162,8 @@ static void dump_kernel_instr(const char *lvl, struct pt_regs *regs)
>
>                 if (!bad)
>                         p += sprintf(p, i == 0 ? "(%08x) " : "%08x ", val);
> -               else {
> -                       p += sprintf(p, "bad PC value");
> -                       break;
> -               }
> +               else
> +                       p += sprintf(p, i == 0 ? "(????????) " : "???????? ");
>         }
>
>         printk("%sCode: %s\n", lvl, str);
> --
> 2.30.2
>