Re: [RFC PATCH v1 1/2] selftests/x86: sysret_rip: Handle syscall in a FRED system

[Ammar Faizi]

On Mon, Jan 23, 2023 at 05:40:23PM -0800, H. Peter Anvin wrote:
> So as per Andrew's comment, add:
> 
> register void * rsp asm("%rsp");
> 
> ...
> 
> "+r" (rsp)	/* clobber the redzone */
> 
> ... as the right way to avoid redzone problems.

I played with this more. I found something wrong with this. This doesn't
work for me. The compiler still uses red zone despite I use "+r" (rsp).

What did I do wrong?

-----

ammarfaizi2@integral2:/tmp$ gcc -fno-stack-protector -O2 -Wall -Wextra test.c -o test
ammarfaizi2@integral2:/tmp$ objdump --no-show-raw-insn -d test | grep "a_leaf_func_with_red_zone>:" -A8
0000000000001180 <a_leaf_func_with_red_zone>:
    1180:  endbr64 
    1184:  mov    $0x1,%eax
    1189:  mov    %rax,-0x8(%rsp)   ## BUG!!!
    118e:  pushf  
    118f:  pop    %rax
    1190:  mov    -0x8(%rsp),%rax   ## BUG!!!
    1195:  ret


ammarfaizi2@integral2:/tmp$ clang -O2 -Wall -Wextra test.c -o test
ammarfaizi2@integral2:/tmp$ objdump --no-show-raw-insn -d test | grep "a_leaf_func_with_red_zone>:" -A6
0000000000001140 <a_leaf_func_with_red_zone>:
    1140:  mov    $0x1,%eax
    1145:  mov    %rax,-0x8(%rsp)   ## BUG!!!
    114a:  pushf  
    114b:  pop    %rax
    114c:  mov    -0x8(%rsp),%rax   ## BUG!!!
    1151:  ret


-----
ammarfaizi2@integral2:~$ gcc --version
gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ammarfaizi2@integral2:~$ clang --version
Ubuntu clang version 16.0.0 (++20230124031324+d63e492562f2-1~exp1~20230124151444.705)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin


-----
test.c:

#include <stdio.h>
static inline void clobber_redzone(void)
{
        register void *rsp __asm__("%rsp");
        unsigned long rflags;

        __asm__ volatile ("pushf; popq %1"
                          : "+r" (rsp), "=r" (rflags));
}

static inline void set_red_zone(long *mem, long val)
{
        __asm__ volatile ("movq %[val], %[mem]"
                           : [mem] "=m" (*mem)
                           : [val] "r" (val));
}

static inline long get_red_zone(long *mem)
{
        long ret;

        __asm__ volatile ("movq %[in], %[out]"
                           : [out] "=r" (ret)
                           : [in] "m" (*mem));
        return ret;
}

__attribute__((__noinline__))
long a_leaf_func_with_red_zone(void)
{
        long x;

        set_red_zone(&x, 1);
        clobber_redzone();
        /* The correct retval is 1 */
        return get_red_zone(&x);
}

int main(void)
{
        printf("ret = %ld\n", a_leaf_func_with_red_zone());
        return 0;
}

-- 
Ammar Faizi