linux-kernel - [PATCH v2] powerpc/mm: make NULL pointer deferences explicit on bad page faults.

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Message-Id: <46e2533084dc3255a1d67553b682bd4b9642b176.1544800765.git.christophe.leroy@c-s.fr>
Date:   Fri, 14 Dec 2018 15:23:33 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2] powerpc/mm: make NULL pointer deferences explicit on bad
 page faults.

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

In the mean time, this page makes all bad_page_fault() messages shorter
so that they remain on one single line. And it prefixes them by "BUG: "
so that they get easily greped.

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 v2: Taking into account comments from Michael

 arch/powerpc/mm/fault.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 01b9bcc7fa85..3398291f4785 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -636,21 +636,24 @@ 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);
+		if (regs->dar < PAGE_SIZE)
+			pr_alert("BUG: Kernel NULL pointer dereference");
+		else
+			pr_alert("BUG: Unable to handle kernel data access");
+		pr_cont(" at 0x%08lx\n", regs->dar);
 		break;
 	case 0x400:
 	case 0x480:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"instruction fetch\n");
+		pr_alert("BUG: Unable to handle kernel instruction fetch%s",
+			 regs->nip < PAGE_SIZE ? " (NULL pointer ?)\n" : "\n");
 		break;
 	case 0x600:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"unaligned access at address 0x%08lx\n", regs->dar);
+		pr_alert("BUG: Unable to handle kernel unaligned access at 0x%08lx\n",
+			 regs->dar);
 		break;
 	default:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"unknown fault\n");
+		pr_alert("BUG: Unable to handle unknown paging fault at 0x%08lx\n",
+			 regs->dar);
 		break;
 	}
 	printk(KERN_ALERT "Faulting instruction address: 0x%08lx\n",
-- 
2.13.3