vdso: print fatal signals

Add the print-fatal-signals=1 boot option and the
/proc/sys/kernel/print-fatal-signals runtime switch.

This feature prints some minimal information about userspace segfaults to
the kernel console.  This is useful to find early bootup bugs where
userspace debugging is very hard.

Defaults to off.

[akpm@linux-foundation.org: Don't add new sysctl numbers]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 48 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 182c6a39d5a2..ab38322111c4 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1383,6 +1383,12 @@ and is between 256 and 4096 characters. It is defined in the file
                         autoconfiguration.
                         Ranges are in pairs (memory base and size).
 
+        print-fatal-signals=
+                        [KNL] debug: print fatal signals
+                        print-fatal-signals=1: print segfault info to
+                        the kernel console.
+                        default: off.
+
         profile=        [KNL] Enable kernel profiling via /proc/profile
                         Format: [schedule,]<number>
                         Param: "schedule" - profile schedule points.
diff --git a/kernel/signal.c b/kernel/signal.c
index f9405609774e..39d122753bac 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -718,6 +718,37 @@ out_set:
 #define LEGACY_QUEUE(sigptr, sig) \
         (((sig) < SIGRTMIN) && sigismember(&(sigptr)->signal, (sig)))
 
+int print_fatal_signals;
+
+static void print_fatal_signal(struct pt_regs *regs, int signr)
+{
+        printk("%s/%d: potentially unexpected fatal signal %d.\n",
+                current->comm, current->pid, signr);
+
+#ifdef __i386__
+        printk("code at %08lx: ", regs->eip);
+        {
+                int i;
+                for (i = 0; i < 16; i++) {
+                        unsigned char insn;
+
+                        __get_user(insn, (unsigned char *)(regs->eip + i));
+                        printk("%02x ", insn);
+                }
+        }
+#endif
+        printk("\n");
+        show_regs(regs);
+}
+
+static int __init setup_print_fatal_signals(char *str)
+{
+        get_option (&str, &print_fatal_signals);
+
+        return 1;
+}
+
+__setup("print-fatal-signals=", setup_print_fatal_signals);
 
 static int
 specific_send_sig_info(int sig, struct siginfo *info, struct task_struct *t)
@@ -1855,6 +1886,8 @@ relock:
                  * Anything else is fatal, maybe with a core dump.
                  */
                 current->flags |= PF_SIGNALED;
+                if ((signr != SIGKILL) && print_fatal_signals)
+                        print_fatal_signal(regs, signr);
                 if (sig_kernel_coredump(signr)) {
                         /*
                          * If it was able to dump core, this kills all
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index ccaebbbd75ae..2cce2286bdcd 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -61,6 +61,7 @@ extern int proc_nr_files(ctl_table *table, int write, struct file *filp,
 
 /* External variables not in a header file. */
 extern int C_A_D;
+extern int print_fatal_signals;
 extern int sysctl_overcommit_memory;
 extern int sysctl_overcommit_ratio;
 extern int sysctl_panic_on_oom;
@@ -340,6 +341,14 @@ static ctl_table kern_table[] = {
                 .proc_handler   = &proc_dointvec,
         },
 #endif
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "print-fatal-signals",
+                .data           = &print_fatal_signals,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
 #ifdef __sparc__
         {
                 .ctl_name       = KERN_SPARC_REBOOT,