x86: make traps on entry code be debuggable in user space, 64-bit

Unify the x86-64 behavior for 32-bit processes that set
bogus %cs/%ss values (the only ones that can fault in iret)
match what the native i386 behavior is. (do not kill the task
via do_exit but generate a SIGSEGV signal)

[ tglx@linutronix.de: build fix ]
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -593,13 +593,22 @@ ENTRY(native_iret)
 	.quad native_iret, bad_iret
 	.previous
 	.section .fixup,"ax"
-	/* force a signal here? this matches i386 behaviour */
-	/* running with kernel gs */
 bad_iret:
-	movq $11,%rdi	/* SIGSEGV */
-	TRACE_IRQS_ON
-	ENABLE_INTERRUPTS(CLBR_ANY | ~(CLBR_RDI))
-	jmp do_exit
+	/*
+	 * The iret traps when the %cs or %ss being restored is bogus.
+	 * We've lost the original trap vector and error code.
+	 * #GPF is the most likely one to get for an invalid selector.
+	 * So pretend we completed the iret and took the #GPF in user mode.
+	 *
+	 * We are now running with the kernel GS after exception recovery.
+	 * But error_entry expects us to have user GS to match the user %cs,
+	 * so swap back.
+	 */
+	pushq $0
+
+	SWAPGS
+	jmp general_protection
+
 	.previous
 
 	/* edi: workmask, edx: work */