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>

2 files changed, 5 insertions, 3 deletions

diff --git a/arch/x86_64/kernel/signal.c b/arch/x86_64/kernel/signal.c
index 4886afcd6287..739175b01e06 100644
--- a/arch/x86_64/kernel/signal.c
+++ b/arch/x86_64/kernel/signal.c
@@ -487,7 +487,7 @@ do_notify_resume(struct pt_regs *regs, void *unused, __u32 thread_info_flags)
 void signal_fault(struct pt_regs *regs, void __user *frame, char *where)
 { 
         struct task_struct *me = current; 
-        if (exception_trace)
+        if (show_unhandled_signals && printk_ratelimit())
                 printk("%s[%d] bad frame in %s frame:%p rip:%lx rsp:%lx orax:%lx\n",
                me->comm,me->pid,where,frame,regs->rip,regs->rsp,regs->orig_rax); 
 
diff --git a/arch/x86_64/kernel/traps.c b/arch/x86_64/kernel/traps.c
index 8713ad4a4db1..03888420775d 100644
--- a/arch/x86_64/kernel/traps.c
+++ b/arch/x86_64/kernel/traps.c
@@ -584,7 +584,8 @@ static void __kprobes do_trap(int trapnr, int signr, char *str,
                 tsk->thread.error_code = error_code;
                 tsk->thread.trap_no = trapnr;
 
-                if (exception_trace && unhandled_signal(tsk, signr))
+                if (show_unhandled_signals && unhandled_signal(tsk, signr) &&
+                    printk_ratelimit())
                         printk(KERN_INFO
                                "%s[%d] trap %s rip:%lx rsp:%lx error:%lx\n",
                                tsk->comm, tsk->pid, str,
@@ -688,7 +689,8 @@ asmlinkage void __kprobes do_general_protection(struct pt_regs * regs,
                 tsk->thread.error_code = error_code;
                 tsk->thread.trap_no = 13;
 
-                if (exception_trace && unhandled_signal(tsk, SIGSEGV))
+                if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+                    printk_ratelimit())
                         printk(KERN_INFO
                        "%s[%d] general protection rip:%lx rsp:%lx error:%lx\n",
                                tsk->comm, tsk->pid,