Re: [PATCH] ARM: Remove the special printing format of pc and lr in __show_regs()

[Leizhen (ThunderTown)]

On 2022/8/9 10:09, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/8/8 17:41, Russell King (Oracle) wrote:
>> On Mon, Aug 01, 2022 at 11:20:16AM +0800, Zhen Lei wrote:
>>> Currently, instruction pointers are printed in [<%08lx>] format to make
>>> them more visible. However, it is not necessary in __show_regs() because
>>> they have the prefix 'pc :' or 'lr :', and it is also inconsistent with
>>> that of other registers, which causes misalignment.
>>
>> The formatting is not "to make them more visible" - it was to mark the
>> addresses that we wanted the ksymoops utility to translate to kernel
>> symbols before we had kallsyms in the kernel. If one disables kallsyms,
>> then we still need a way to translate kernel addresses to symbols.
> 
> I searched the git log and found that the ksymoops utility is discarded.
> 
> See:
> 073a9ecb3a73401662430bb955aedeac1de643d1
> However, a commit in the pre-git era [1] had added the statement,
> "ksymoops is useless on 2.6.  Please use the Oops in its original format".
> 
> That statement existed until commit 4eb9241127a0 ("Documentation:
> admin-guide: update bug-hunting.rst") finally removed the stale
> ksymoops information.
> 
> 4eb9241127a0b5ac3aaaf1b246728009527ebc86
> - delete all references to ksymoops since it is no longer applicable;
> 
>>
>> I notice there is a script which helps with this that is part of the
>> kernel source - scripts/decode_stacktrace.sh. I haven't tried this on
>> arm32 since I always use kallsyms - and I suspect that is rather
>> universally true as it avoids needing System.map files etc to decode
>> the oops. That said, if you're building a kernel for small systems,
>> you probably don't want the overhead of kallsyms.

Hi, Russell:

I re-examined the code and found that 'pc' and 'lr' had extra printing
in __show_regs(). Therefore, maybe v2 should be changed as follows, as
is done in dump_backtrace_entry():

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 96f3fbd51764292..2b0b49821cbf2f5 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -134,9 +134,15 @@ void __show_regs(struct pt_regs *regs)

        show_regs_print_info(KERN_DEFAULT);

+#ifndef CONFIG_KALLSYMS
+       printk("PC is at [<%08lx>]\n", instruction_pointer(regs));
+       printk("LR is at [<%08lx>]\n", regs->ARM_lr);
+#else
        printk("PC is at %pS\n", (void *)instruction_pointer(regs));
        printk("LR is at %pS\n", (void *)regs->ARM_lr);
-       printk("pc : [<%08lx>]    lr : [<%08lx>]    psr: %08lx\n",
+#endif
+
+       printk("pc : %08lx  lr : %08lx  psr: %08lx\n",
               regs->ARM_pc, regs->ARM_lr, regs->ARM_cpsr);
        printk("sp : %08lx  ip : %08lx  fp : %08lx\n",
               regs->ARM_sp, regs->ARM_ip, regs->ARM_fp);


So that there are no concerns that you mentioned.

CONFIG_KALLSYMS=n:
PC is at [<801993d4>]
LR is at [<801993d4>]
pc : 801993d4  lr : 801993d4  psr: 60000013
sp : c49f9f28  ip : 00000001  fp : 00000001

CONFIG_KALLSYMS=y:
PC is at ktime_get+0x4c/0xe8
LR is at ktime_get+0x4c/0xe8
pc : 8019a4ac  lr : 8019a4ac  psr: 60000013
sp : c49f9f28  ip : 00000001  fp : 00000001


> 
> Yes, I read scripts/decode_stacktrace.sh, it requires the format "[<...>]".
> But if that's the only concern, maybe we can do the conversion from
> "pc: addr" and "lr: addr" to "[<addr>]" first in scripts/decode_stacktrace.sh
> 
> I'm usually "objdump -d vmlinux > asm_file", then search "addr:" in asm_file.
> 
> Honestly, I think format "[<...>]" is dump_backtrace()'s requirement, not __show_regs()'s.
> 
> 
>>
>> So, there's an argument for keeping it - it's an API in that it
>> provides hints to scripting to identify which values need to be
>> converted to symbols. There's also the argument for getting rid of it,
>> which is that hardly anyone does that anymore.
>>
>> The question is, which is the more important argument, and I don't
>> think there's a definite answer. So I'm inclined to leave this
>> as-is.
> 
> OK
> 
>>
> 

-- 
Regards,
  Zhen Lei