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, 3 insertions, 45 deletions

diff --git a/arch/x86_64/mm/fault.c b/arch/x86_64/mm/fault.c
index 2074bddd4f04..5e9ac70c135e 100644
--- a/arch/x86_64/mm/fault.c
+++ b/arch/x86_64/mm/fault.c
@@ -221,16 +221,6 @@ static int is_errata93(struct pt_regs *regs, unsigned long address)
         return 0;
 } 
 
-int unhandled_signal(struct task_struct *tsk, int sig)
-{
-        if (is_init(tsk))
-                return 1;
-        if (tsk->ptrace & PT_PTRACED)
-                return 0;
-        return (tsk->sighand->action[sig-1].sa.sa_handler == SIG_IGN) ||
-                (tsk->sighand->action[sig-1].sa.sa_handler == SIG_DFL);
-}
-
 static noinline void pgtable_bad(unsigned long address, struct pt_regs *regs,
                                  unsigned long error_code)
 {
@@ -302,7 +292,7 @@ static int vmalloc_fault(unsigned long address)
 }
 
 static int page_fault_trace;
-int exception_trace = 1;
+int show_unhandled_signals = 1;
 
 /*
  * This routine handles page faults.  It determines the address,
@@ -494,7 +484,8 @@ bad_area_nosemaphore:
                     (address >> 32))
                         return;
 
-                if (exception_trace && unhandled_signal(tsk, SIGSEGV)) {
+                if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+                    printk_ratelimit()) {
                         printk(
                        "%s%s[%d]: segfault at %016lx rip %016lx rsp %016lx error %lx\n",
                                         tsk->pid > 1 ? KERN_INFO : KERN_EMERG,
diff --git a/arch/x86_64/mm/init.c b/arch/x86_64/mm/init.c
index 381c2ecd407e..88678e82e23d 100644
--- a/arch/x86_64/mm/init.c
+++ b/arch/x86_64/mm/init.c
@@ -697,39 +697,6 @@ int kern_addr_valid(unsigned long addr)
         return pfn_valid(pte_pfn(*pte));
 }
 
-#ifdef CONFIG_SYSCTL
-#include <linux/sysctl.h>
-
-static ctl_table debug_table2[] = {
-        {
-                .ctl_name       = 99,
-                .procname       = "exception-trace",
-                .data           = &exception_trace,
-                .maxlen         = sizeof(int),
-                .mode           = 0644,
-                .proc_handler   = proc_dointvec
-        },
-        {}
-}; 
-
-static ctl_table debug_root_table2[] = { 
-        {
-                .ctl_name = CTL_DEBUG,
-                .procname = "debug",
-                .mode = 0555,
-                .child = debug_table2
-        },
-        {}
-}; 
-
-static __init int x8664_sysctl_init(void)
-{ 
-        register_sysctl_table(debug_root_table2);
-        return 0;
-}
-__initcall(x8664_sysctl_init);
-#endif
-
 /* A pseudo VMA to allow ptrace access for the vsyscall page.  This only
    covers the 64bit vsyscall page now. 32bit has a real VMA now and does
    not need special handling anymore. */