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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/mm: tell if a bad page fault on data is read or write.
In-Reply-To: <4f88d7e6fda53b5f80a71040ab400242f6c8cb93.1566400889.git.christophe.leroy@c-s.fr>
References: <4f88d7e6fda53b5f80a71040ab400242f6c8cb93.1566400889.git.christophe.leroy@c-s.fr>
Date:   Thu, 29 Aug 2019 22:14:38 +1000
Message-ID: <87o908tbgx.fsf@mpe.ellerman.id.au>
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Sender: linux-kernel-owner@vger.kernel.org
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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().

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.

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