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/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
index e92a10124935..01ffdd4964f0 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);