x86/mm/fault: Allow stack access below %rsp

The current x86 page fault handler allows stack access below the stack pointer if it is no more than 64k+256 bytes. Any access beyond the 64k+ limit will cause a segmentation fault.

The gcc -fstack-check option generates code to probe the stack for large stack allocation to see if the stack is accessible. The newer gcc does that while updating the %rsp simultaneously. Older gcc's like gcc4 doesn't do that. As a result, an application compiled with an old gcc and the -fstack-check option may fail to start at all.

% cat test.c

int main() {
	char tmp[1024*128];
	printf("### ok\n");
	return 0;
}

% gcc -fstack-check -g -o test test.c
% ./test
Segmentation fault

The old binary was working in older kernels where expand_stack() was somehow called before the check. But it is not working in newer kernels. Besides, the 64k+ limit check is kind of crude and will not catch a lot of mistakes that userspace applications may be misbehaving anyway. I think the kernel isn't the right place for this kind of tests. We should leave it to userspace instrumentation tools to perform them.

The 64k+ limit check is now removed to just let expand_stack() decide if a segmentation fault should happen, when the RLIMIT_STACK limit is exceeded, for example.

Signed-off-by Waiman Long <[email protected]>

---
 arch/x86/mm/fault.c | 12 ------------
 1 file changed, 12 deletions(-)

arch/x86/mm/fault.c +0 -5

index 71d4b9d..29525cf 100644
@@ -1380,18 +1380,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 		bad_area(regs, sw_error_code, address);
 		return;
 	}
-	if (sw_error_code & X86_PF_USER) {
-		/*
-		 * Accessing the stack below %sp is always a bug.
-		 * The large cushion allows instructions like enter
-		 * and pusha to work. ("enter $65535, $31" pushes