linux-kernel - Re: [PATCH] powerpc/mm: tell if a bad page fault on data is read or write.

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Date:   Thu, 29 Aug 2019 14:59:00 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/mm: tell if a bad page fault on data is read or
 write.



Le 29/08/2019 à 14:14, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
>> DSISR has a bit to tell if the fault is due to a read or a write.
> 
> Except some CPUs don't have a DSISR?
> 
> Which is why we have page_fault_is_write() that's used in
> __do_page_fault().
> 
> Or is that old cruft?
> 
> I see eg. in head_40x.S we pass r5=0 for error code, and we don't set
> regs->dsisr anywhere AFAICS. So it might just contain some junk.

But then we have a problem with show_regs() as well, havent't we ?

	if (trap == 0x200 || trap == 0x300 || trap == 0x600)
#if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)
		pr_cont("DEAR: "REG" ESR: "REG" ", regs->dar, regs->dsisr);
#else
		pr_cont("DAR: "REG" DSISR: %08lx ", regs->dar, regs->dsisr);
#endif

I need to look closer.

Christophe


> 
> cheers
> 
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 8432c281de92..b5047f9b5dec 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -645,6 +645,7 @@ NOKPROBE_SYMBOL(do_page_fault);
>>   void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
>>   {
>>   	const struct exception_table_entry *entry;
>> +	int is_write = page_fault_is_write(regs->dsisr);
>>   
>>   	/* Are we prepared to handle this fault?  */
>>   	if ((entry = search_exception_tables(regs->nip)) != NULL) {
>> @@ -658,9 +659,10 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
>>   	case 0x300:
>>   	case 0x380:
>>   	case 0xe00:
>> -		pr_alert("BUG: %s at 0x%08lx\n",
>> +		pr_alert("BUG: %s on %s at 0x%08lx\n",
>>   			 regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
>> -			 "Unable to handle kernel data access", regs->dar);
>> +			 "Unable to handle kernel data access",
>> +			 is_write ? "write" : "read", regs->dar);
> 
>>   		break;
>>   	case 0x400:
>>   	case 0x480:
>> -- 
>> 2.13.3