https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97928
Bug ID: 97928
Summary: -fstack-clash-protection probe miss
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: sguelton at redhat dot com
Target Milestone: ---
The compilation of the following code
```c
#include <alloca.h>
int square(int num) {
char foo[6000];
char* bar = alloca(num * num);
return foo[num] + bar[num];
}
```
with gcc -fstack-clash-protection trunk yields the following assembly (full
assembly here https://godbolt.org/z/95636K)
```asm
square:
push rbp
mov rbp, rsp
sub rsp, 4096
or QWORD PTR [rsp], 0
sub rsp, 1936
#...
.L2:
cmp rsp, rdx
je .L3
sub rsp, 4096
or QWORD PTR [rsp+4088], 0
jmp .L2
#...
```
there's a potential sequence here that jumps over a ``PAGE_SIZE`` guard:
```asm
sub rsp, 1936
...
sub rsp, 4096
<< signal here >>
or QWORD PTR [rsp+4088], 0
```
If a signal is received at << signal here >>, then the stack may points behind
the page guard.
It seems to me the following achieve the same protection level.
```asm
.L2:
cmp rsp, rdx
je .L3
or QWORD PTR [rsp], 0
sub rsp, 4096
jmp .L2
```
