On Thu, Feb 14, 2008 at 09:25:35PM +0100, Ingo Molnar wrote:
> The per function call overhead from stackprotector is already pretty
> serious IMO, but at least that's something that GCC _could_ be doing
> (much) smarter (why doesnt it jne forward out to __check_stk_failure,
> instead of generating 4 instructions, one of them a default-mispredicted
> branch instruction??), so that overhead could in theory be something
> like 4 fall-through instructions per function, instead of the current 6.
Where do you see a mispredicted branch?
int foo (void)
{
char buf[64];
bar (buf);
return 6;
}
-O2 -fstack-protector -m64:
subq $88, %rsp
movq %fs:40, %rax
movq %rax, 72(%rsp)
xorl %eax, %eax
movq %rsp, %rdi
call bar
movq 72(%rsp), %rdx
xorq %fs:40, %rdx
movl $6, %eax
jne .L5
addq $88, %rsp
ret
.L5:
.p2align 4,,6
.p2align 3
call __stack_chk_fail
-O2 -fstack-protector -m32:
pushl %ebp
movl %esp, %ebp
subl $88, %esp
movl %gs:20, %eax
movl %eax, -4(%ebp)
xorl %eax, %eax
leal -68(%ebp), %eax
movl %eax, (%esp)
call bar
movl $6, %eax
movl -4(%ebp), %edx
xorl %gs:20, %edx
jne .L5
leave
ret
.L5:
.p2align 4,,7
.p2align 3
call __stack_chk_fail
-O2 -fstack-protector -m64 -mcmodel=kernel:
subq $88, %rsp
movq %gs:40, %rax
movq %rax, 72(%rsp)
xorl %eax, %eax
movq %rsp, %rdi
call bar
movq 72(%rsp), %rdx
xorq %gs:40, %rdx
movl $6, %eax
jne .L5
addq $88, %rsp
ret
.L5:
.p2align 4,,6
.p2align 3
call __stack_chk_fail
both with gcc 4.1.x and 4.3.0.
BTW, you can use -fstack-protector --param=ssp-buffer-size=4
etc. to tweak the size of buffers to trigger stack protection, the
default is 8, but e.g. whole Fedora is compiled with 4.
Jakub
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