[PATCH] x86_64: Remove CONFIG_CHECKING and add command line option for pagefault tracing

CONFIG_CHECKING covered some debugging code used in the early times
of the port. But it wasn't even SMP safe for quite some time
and the bugs it checked for seem to be gone.

This patch removes all the code to verify GS at kernel entry. There
haven't been any new bugs in this area for a long time.

Previously it also covered the sysctl for the page fault tracing.
That didn't make much sense because that code was unconditionally
compiled in. I made that a boot option now because it is typically
only useful at boot.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/x86_64/boot-options.txt

@@ -193,6 +193,9 @@ Debugging
 
   kstack=N   Print that many words from the kernel stack in oops dumps.
 
+  pagefaulttrace Dump all page faults. Only useful for extreme debugging
+		and will create a lot of output.
+
 Misc
 
   noreplacement  Don't replace instructions with more appropiate ones

arch/x86_64/Kconfig.debug

@@ -2,15 +2,6 @@ menu "Kernel hacking"
 
 source "lib/Kconfig.debug"
 
-# !SMP for now because the context switch early causes GPF in segment reloading
-# and the GS base checking does the wrong thing then, causing a hang.
-config CHECKING
-	bool "Additional run-time checks"
-	depends on DEBUG_KERNEL && !SMP
-	help
-	  Enables some internal consistency checks for kernel debugging.
-	  You should normally say N.
-
 config INIT_DEBUG
 	bool "Debug __init statements"
 	depends on DEBUG_KERNEL

arch/x86_64/kernel/traps.c

@@ -428,19 +428,6 @@ static void __kprobes do_trap(int trapnr, int signr, char *str,
 {
 	conditional_sti(regs);
 
-#ifdef CONFIG_CHECKING
-       { 
-               unsigned long gs; 
-               struct x8664_pda *pda = cpu_pda + safe_smp_processor_id(); 
-               rdmsrl(MSR_GS_BASE, gs); 
-               if (gs != (unsigned long)pda) { 
-                       wrmsrl(MSR_GS_BASE, pda); 
-                       printk("%s: wrong gs %lx expected %p rip %lx\n", str, gs, pda,
-			      regs->rip);
-               }
-       }
-#endif
-
 	if (user_mode(regs)) {
 		struct task_struct *tsk = current;
 
@@ -513,20 +500,6 @@ asmlinkage void __kprobes do_general_protection(struct pt_regs * regs,
 {
 	conditional_sti(regs);
 
-#ifdef CONFIG_CHECKING
-       { 
-               unsigned long gs; 
-               struct x8664_pda *pda = cpu_pda + safe_smp_processor_id(); 
-               rdmsrl(MSR_GS_BASE, gs); 
-               if (gs != (unsigned long)pda) { 
-                       wrmsrl(MSR_GS_BASE, pda); 
-		       oops_in_progress++;
-                       printk("general protection handler: wrong gs %lx expected %p\n", gs, pda);
-		       oops_in_progress--;
-               }
-       }
-#endif
-
 	if (user_mode(regs)) {
 		struct task_struct *tsk = current;
 
@@ -665,19 +638,6 @@ asmlinkage void __kprobes do_debug(struct pt_regs * regs,
 	struct task_struct *tsk = current;
 	siginfo_t info;
 
-#ifdef CONFIG_CHECKING
-       { 
-	       /* RED-PEN interaction with debugger - could destroy gs */
-               unsigned long gs; 
-               struct x8664_pda *pda = cpu_pda + safe_smp_processor_id(); 
-               rdmsrl(MSR_GS_BASE, gs); 
-               if (gs != (unsigned long)pda) { 
-                       wrmsrl(MSR_GS_BASE, pda); 
-                       printk("debug handler: wrong gs %lx expected %p\n", gs, pda);
-               }
-       }
-#endif
-
 	get_debugreg(condition, 6);
 
 	if (notify_die(DIE_DEBUG, "debug", regs, condition, error_code,

arch/x86_64/mm/fault.c

@@ -308,18 +308,6 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 	unsigned long flags;
 	siginfo_t info;
 
-#ifdef CONFIG_CHECKING
-	{ 
-		unsigned long gs; 
-		struct x8664_pda *pda = cpu_pda + stack_smp_processor_id(); 
-		rdmsrl(MSR_GS_BASE, gs); 
-		if (gs != (unsigned long)pda) { 
-			wrmsrl(MSR_GS_BASE, pda); 
-			printk("page_fault: wrong gs %lx expected %p\n", gs, pda);
-		}
-	}
-#endif
-
 	/* get the address */
 	__asm__("movq %%cr2,%0":"=r" (address));
 	if (notify_die(DIE_PAGE_FAULT, "page fault", regs, error_code, 14,
@@ -571,3 +559,10 @@ do_sigbus:
 	force_sig_info(SIGBUS, &info, tsk);
 	return;
 }
+
+static int __init enable_pagefaulttrace(char *str)
+{
+	page_fault_trace = 1;
+	return 0;
+}
+__setup("pagefaulttrace", enable_pagefaulttrace);

arch/x86_64/mm/init.c

@@ -565,10 +565,6 @@ extern int exception_trace, page_fault_trace;
 static ctl_table debug_table2[] = {
 	{ 99, "exception-trace", &exception_trace, sizeof(int), 0644, NULL,
 	  proc_dointvec },
-#ifdef CONFIG_CHECKING
-	{ 100, "page-fault-trace", &page_fault_trace, sizeof(int), 0644, NULL,
-	  proc_dointvec },
-#endif
 	{ 0, }
 };