x86: i386-show-unhandled-signals-v3

This patch makes the i386 behave the same way that x86_64 does when a
segfault happens.  A line gets printed to the kernel log so that tools
that need to check for failures can behave more uniformly between
debug.show_unhandled_signals sysctl variable to 0 (or by doing echo 0 >
/proc/sys/debug/exception-trace)

Also, all of the lines being printed are now using printk_ratelimit() to
deny the ability of DoS from a local user with a program like the
following:

main()
{
       while (1)
               if (!fork()) *(int *)0 = 0;
}

This new revision also includes the fix that Andrew did which got rid of
new sysctl that was added to the system in earlier versions of this.
Also, 'show-unhandled-signals' sysctl has been renamed back to the old
'exception-trace' to avoid breakage of people's scripts.

AK: Enabling by default for i386 will be likely controversal, but let's see what happens
AK: Really folks, before complaining just fix your segfaults
AK: I bet this will find a lot of silent issues

Signed-off-by: Masoud Sharbiani <masouds@google.com>
Signed-off-by: Andi Kleen <ak@suse.de>
[ Personally, I've found the complaints useful on x86-64, so I'm all for
  this. That said, I wonder if we could do it more prettily..   -Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 222299844ad1..ddebf3f2affe 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1203,6 +1203,16 @@ static ctl_table fs_table[] = {
 };
 
 static ctl_table debug_table[] = {
+#ifdef CONFIG_X86
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "exception-trace",
+                .data           = &show_unhandled_signals,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec
+        },
+#endif
         { .ctl_name = 0 }
 };