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Message-Id: <42e7e2e9b74af526523fb5a1451fdb29cfe2687a.1539060806.git.christophe.leroy@c-s.fr>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Subject: [PATCH] powerpc/mm: make NULL pointer deferences explicit on bad page
 faults.
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Date:   Tue,  9 Oct 2018 05:21:32 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

As several other arches including x86, this patch makes it explicit
that a bad page fault is a NULL pointer dereference when the fault
address is lower than PAGE_SIZE

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
 arch/powerpc/mm/fault.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index d51cf5f4e45e..501a1eadb3e9 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -631,13 +631,16 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
 	switch (TRAP(regs)) {
 	case 0x300:
 	case 0x380:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"data at address 0x%08lx\n", regs->dar);
+		pr_alert("Unable to handle kernel %s for data at address 0x%08lx\n",
+			 regs->dar < PAGE_SIZE ? "NULL pointer dereference" :
+						 "paging request",
+			 regs->dar);
 		break;
 	case 0x400:
 	case 0x480:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"instruction fetch\n");
+		pr_alert("Unable to handle kernel %s for instruction fetch\n",
+			 regs->nip < PAGE_SIZE ? "NULL pointer dereference" :
+						 "paging request");
 		break;
 	case 0x600:
 		printk(KERN_ALERT "Unable to handle kernel paging request for "
-- 
2.13.3