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SubjectRe: [PATCH] powerpc/mm: tell if a bad page fault on data is read or write.
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Le 29/08/2019 à 14:14, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@c-s.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().


And that's why I'm also using page_fault_is_write() in my patch.

>
> 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.


We pass r5=0 in ISI but r5=SPRN_ESR in DSI.
And r5 is also saved into _ESR(r11)

And in asm-offset.c, we have:

STACK_PT_REGS_OFFSET(_ESR, dsisr);

So regs->dsisr has the expected content.

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

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