Re: [PATCH] arm64/traps: show fault address in exception trace

[Baruch Siach <baruch@tkos.co.il>]

Hi Will,

On Tue, Apr 29 2025, Will Deacon wrote:
> On Tue, Apr 29, 2025 at 10:15:24AM +0300, Baruch Siach wrote:
>> The FAR (fault address) register provides useful information when
>> debugging an unhandled exception.
>> 
>> Signed-off-by: Baruch Siach <baruch@tkos.co.il>
>> ---
>>  arch/arm64/kernel/traps.c | 11 ++++++-----
>>  1 file changed, 6 insertions(+), 5 deletions(-)
>> 
>> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
>> index 4e26bd356a48..14eb598e33a7 100644
>> --- a/arch/arm64/kernel/traps.c
>> +++ b/arch/arm64/kernel/traps.c
>> @@ -239,7 +239,7 @@ void die(const char *str, struct pt_regs *regs, long err)
>>  		make_task_dead(SIGSEGV);
>>  }
>>  
>> -static void arm64_show_signal(int signo, const char *str)
>> +static void arm64_show_signal(int signo, const char *str, unsigned long far)
>>  {
>>  	static DEFINE_RATELIMIT_STATE(rs, DEFAULT_RATELIMIT_INTERVAL,
>>  				      DEFAULT_RATELIMIT_BURST);
>> @@ -256,6 +256,7 @@ static void arm64_show_signal(int signo, const char *str)
>>  	pr_info("%s[%d]: unhandled exception: ", tsk->comm, task_pid_nr(tsk));
>>  	if (esr)
>>  		pr_cont("%s, ESR 0x%016lx, ", esr_get_class_string(esr), esr);
>> +	pr_cont("FAR 0x%016lx, ", far);
>
> Are you sure the FAR is always valid here? For example, if we're delivering
> a SIGILL due to an undefined instruction.

A user interested in hardware exception information should be in a
position to know whether this information is valid. But I guess that FAR
can't be valid when ESR isn't valid either.

> Also, once you have the PC and the registers (both of which we print),
> it's not too hard to dig the instruction out of the binary and figure out
> the faulting address that way.

I find FAR helpful to confirm the analysis of the instruction being
executed and its operands. The cost for adding this bit of information
isn't huge, I think.

Thanks for your review,
baruch

-- 
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