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>

3 files changed, 24 insertions, 0 deletions

diff --git a/arch/i386/kernel/signal.c b/arch/i386/kernel/signal.c
index d574e38..f5dd856 100644
--- a/arch/i386/kernel/signal.c
+++ b/arch/i386/kernel/signal.c
@@ -199,6 +199,13 @@ asmlinkage int sys_sigreturn(unsigned long __unused)
 	return eax;
 
 badframe:
+	if (show_unhandled_signals && printk_ratelimit())
+		printk("%s%s[%d] bad frame in sigreturn frame:%p eip:%lx"
+		       " esp:%lx oeax:%lx\n",
+		    current->pid > 1 ? KERN_INFO : KERN_EMERG,
+		    current->comm, current->pid, frame, regs->eip,
+		    regs->esp, regs->orig_eax);
+
 	force_sig(SIGSEGV, current);
 	return 0;
 }	
diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c
index 57772a1..438949d 100644
--- a/arch/i386/kernel/traps.c
+++ b/arch/i386/kernel/traps.c
@@ -618,6 +618,13 @@ fastcall void __kprobes do_general_protection(struct pt_regs * regs,
 
 	current->thread.error_code = error_code;
 	current->thread.trap_no = 13;
+	if (show_unhandled_signals && unhandled_signal(current, SIGSEGV) &&
+	    printk_ratelimit())
+		printk(KERN_INFO
+		    "%s[%d] general protection eip:%lx esp:%lx error:%lx\n",
+		    current->comm, current->pid,
+		    regs->eip, regs->esp, error_code);
+
 	force_sig(SIGSEGV, current);
 	return;
 
diff --git a/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
index e92a101..01ffdd4 100644
--- a/arch/i386/mm/fault.c
+++ b/arch/i386/mm/fault.c
@@ -283,6 +283,8 @@ static inline int vmalloc_fault(unsigned long address)
 	return 0;
 }
 
+int show_unhandled_signals = 1;
+
 /*
  * This routine handles page faults.  It determines the address,
  * and the problem, and then passes it off to one of the appropriate
@@ -469,6 +471,14 @@ bad_area_nosemaphore:
 		if (is_prefetch(regs, address, error_code))
 			return;
 
+		if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+		    printk_ratelimit()) {
+			printk("%s%s[%d]: segfault at %08lx eip %08lx "
+			    "esp %08lx error %lx\n",
+			    tsk->pid > 1 ? KERN_INFO : KERN_EMERG,
+			    tsk->comm, tsk->pid, address, regs->eip,
+			    regs->esp, error_code);
+		}
 		tsk->thread.cr2 = address;
 		/* Kernel addresses are always protection faults */
 		tsk->thread.error_code = error_code | (address >= TASK_SIZE);