Re: [RFC PATCH 10/10] powerpc/fsl_booke/kaslr: dump out kernel offset information on panic

[Jason Yan <yanaijie@huawei.com>]

On 2019/7/29 19:43, Christophe Leroy wrote:
> 
> 
> Le 17/07/2019 à 10:06, Jason Yan a écrit :
>> When kaslr is enabled, the kernel offset is different for every boot.
>> This brings some difficult to debug the kernel. Dump out the kernel
>> offset when panic so that we can easily debug the kernel.
>>
>> Signed-off-by: Jason Yan <yanaijie@huawei.com>
>> Cc: Diana Craciun <diana.craciun@nxp.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> ---
>>   arch/powerpc/include/asm/page.h     |  5 +++++
>>   arch/powerpc/kernel/machine_kexec.c |  1 +
>>   arch/powerpc/kernel/setup-common.c  | 23 +++++++++++++++++++++++
>>   3 files changed, 29 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/page.h 
>> b/arch/powerpc/include/asm/page.h
>> index 60a68d3a54b1..cd3ac530e58d 100644
>> --- a/arch/powerpc/include/asm/page.h
>> +++ b/arch/powerpc/include/asm/page.h
>> @@ -317,6 +317,11 @@ struct vm_area_struct;
>>   extern unsigned long kimage_vaddr;
>> +static inline unsigned long kaslr_offset(void)
>> +{
>> +    return kimage_vaddr - KERNELBASE;
>> +}
>> +
>>   #include <asm-generic/memory_model.h>
>>   #endif /* __ASSEMBLY__ */
>>   #include <asm/slice.h>
>> diff --git a/arch/powerpc/kernel/machine_kexec.c 
>> b/arch/powerpc/kernel/machine_kexec.c
>> index c4ed328a7b96..078fe3d76feb 100644
>> --- a/arch/powerpc/kernel/machine_kexec.c
>> +++ b/arch/powerpc/kernel/machine_kexec.c
>> @@ -86,6 +86,7 @@ void arch_crash_save_vmcoreinfo(void)
>>       VMCOREINFO_STRUCT_SIZE(mmu_psize_def);
>>       VMCOREINFO_OFFSET(mmu_psize_def, shift);
>>   #endif
>> +    vmcoreinfo_append_str("KERNELOFFSET=%lx\n", kaslr_offset());
>>   }
>>   /*
>> diff --git a/arch/powerpc/kernel/setup-common.c 
>> b/arch/powerpc/kernel/setup-common.c
>> index 1f8db666468d..49e540c0adeb 100644
>> --- a/arch/powerpc/kernel/setup-common.c
>> +++ b/arch/powerpc/kernel/setup-common.c
>> @@ -715,12 +715,35 @@ static struct notifier_block ppc_panic_block = {
>>       .priority = INT_MIN /* may not return; must be done last */
>>   };
>> +/*
>> + * Dump out kernel offset information on panic.
>> + */
>> +static int dump_kernel_offset(struct notifier_block *self, unsigned 
>> long v,
>> +                  void *p)
>> +{
>> +    const unsigned long offset = kaslr_offset();
>> +
>> +    if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && offset > 0)
>> +        pr_emerg("Kernel Offset: 0x%lx from 0x%lx\n",
>> +             offset, KERNELBASE);
>> +    else
>> +        pr_emerg("Kernel Offset: disabled\n");
> 
> Do we really need that else branch ?
> 
> Why not just make the below atomic_notifier_chain_register() 
> conditionnal to IS_ENABLED(CONFIG_RANDOMIZE_BASE) && offset > 0
> and not print anything otherwise ?
> 

I'm trying to keep the same fashion as x86/arm64 do. But I agree
with you that it's simpler to not print anything else if not randomized.

> Christophe
> 
>> +
>> +    return 0;
>> +}
>> +
>> +static struct notifier_block kernel_offset_notifier = {
>> +    .notifier_call = dump_kernel_offset
>> +};
>> +
>>   void __init setup_panic(void)
>>   {
>>       /* PPC64 always does a hard irq disable in its panic handler */
>>       if (!IS_ENABLED(CONFIG_PPC64) && !ppc_md.panic)
>>           return;
>>       atomic_notifier_chain_register(&panic_notifier_list, 
>> &ppc_panic_block);
>> +    atomic_notifier_chain_register(&panic_notifier_list,
>> +                       &kernel_offset_notifier);
>>   }
>>   #ifdef CONFIG_CHECK_CACHE_COHERENCY
>>
> 
> .
>