From a4cb4b1277a499ebced7f7534d5cfb11a30eb224 Mon Sep 17 00:00:00 2001
From: Grazvydas Ignotas <notasas@gmail.com>
Date: Tue, 30 Sep 2014 02:33:59 +0300
Subject: [PATCH] ARM: show short message on segfault

kernels on some x86 distros do this, it's pretty useful.
---
 arch/arm/include/asm/system.h |  1 +
 arch/arm/kernel/traps.c       |  2 +-
 arch/arm/mm/fault.c           | 41 +++++++++++++++++++++++++++++++++--
 3 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index 53785828744c..1344f5f7bc79 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -117,6 +117,7 @@ extern void (*arm_pm_restart)(char str, const char *cmd);
 #define UDBG_BADABORT	(1 << 2)
 #define UDBG_SEGV	(1 << 3)
 #define UDBG_BUS	(1 << 4)
+#define UDBG_SEGV_SHORT	(1 << 8)
 
 extern unsigned int user_debug;
 
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 7d12e732c2be..071772c42e9c 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -48,7 +48,7 @@ static const char *handler[]= {
 void *vectors_page;
 
 #ifdef CONFIG_DEBUG_USER
-unsigned int user_debug;
+unsigned int user_debug = UDBG_SEGV_SHORT;
 
 static int __init user_debug_setup(char *str)
 {
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 4c9c3b063194..ea34eb9d694b 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -152,6 +152,39 @@ __do_kernel_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
 	do_exit(SIGKILL);
 }
 
+#ifdef CONFIG_DEBUG_USER
+static void
+print_user_faulter_location(const char *name, struct pt_regs *regs)
+{
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma;
+	char *p, *t, buf[128];
+
+	printk(KERN_DEBUG "%s: pc=%08lx",
+	       name, regs->ARM_pc);
+
+	do {
+		if (!mm)
+			break;
+		vma = find_vma(mm, regs->ARM_pc);
+		if (!vma || !vma->vm_file)
+			break;
+
+		p = d_path(&vma->vm_file->f_path, buf, sizeof(buf));
+		if (IS_ERR(p))
+			break;
+
+		t = strrchr(p, '/');
+		if (t)
+			p = t + 1;
+
+		printk(KERN_CONT " (%s+%lx)", p, regs->ARM_pc - vma->vm_start);
+	} while (0);
+
+	printk(KERN_CONT ", lr=%08lx\n", regs->ARM_lr);
+}
+#endif
+
 /*
  * Something tried to access memory that isn't in our memory map..
  * User mode accesses just cause a SIGSEGV
@@ -164,9 +197,13 @@ __do_user_fault(struct task_struct *tsk, unsigned long addr,
 	struct siginfo si;
 
 #ifdef CONFIG_DEBUG_USER
+	if (user_debug & (UDBG_SEGV | UDBG_SEGV_SHORT)) {
+		printk(KERN_DEBUG "%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x (%s)\n",
+		       tsk->comm, sig, addr, fsr,
+		       (fsr & FSR_WRITE) ? "write" : "read");
+		print_user_faulter_location(tsk->comm, regs);
+	}
 	if (user_debug & UDBG_SEGV) {
-		printk(KERN_DEBUG "%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x\n",
-		       tsk->comm, sig, addr, fsr);
 		show_pte(tsk->mm, addr);
 		show_regs(regs);
 	}
-- 
2.39.2