https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85927
Bug ID: 85927
Summary: ud2 instruction generated starting with gcc 8
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ndesaulniers at google dot com
Target Milestone: ---
From: https://lkml.org/lkml/2018/5/25/636
It seems that:
__attribute__((naked))
unsigned long save_flags(void) {
unsigned long flags;
asm volatile("pushf; pop %0"
: "=rm" (flags)
:
: "memory");
return flags;
}
compiled with:
$ gcc-8 temp.c -c -fstack-protector-strong -O3
generates:
$ objdump -d temp.o
temp.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <save_flags>:
0: 9c pushfq
1: 58 pop %rax
2: 0f 0b ud2
Note that `ud2` is an explicit invalid opcode:
https://x86.puri.sm/html/file_module_x86_id_318.html
>From playing around with this in godbolt, this seems to have regressed from
gcc-7, which generates:
0000000000000000 <save_flags>:
0: 9c pushfq
1: 58 pop %rax
2: c3 retq
(but also gcc-7 warns:
temp.c:10:1: warning: ‘naked’ attribute directive ignored [-Wattributes]
).
