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 64k+ limit check is kind of arbitrary. So the check is now removed to just let expand_stack() decide if a segmentation fault should happen. Signed-off-by: Waiman Long <long...@redhat.com> --- arch/x86/mm/fault.c | 12 ------------ 1 file changed, 12 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 71d4b9d..29525cf 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -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 - * 32 pointers and then decrements %sp by 65535.) - */ - if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) { - bad_area(regs, sw_error_code, address); - return; - } - } if (unlikely(expand_stack(vma, address))) { bad_area(regs, sw_error_code, address); return; -- 1.8.3.1