use of %fs segment register in x86_64 with -fstack-check

[Ruslan Kabatsayev]

Hi,

On Tue, 3 Mar 2020 at 17:53, Maxim Blinov <maxim.blinov@embecosm.com> wrote:
>
> Hi all,
>
> I'm looking at some -fstack-check'ed code, and would appreciate it if
> some gdb x86_64 gurus could double check my understanding of a trivial
> example
>
> here is the source:
>
> big-access.c:
> ```
> #include <stdio.h>
> #include <stdlib.h>
> #include <stdint.h>
>
> extern void foo(char *);
>
> int main()
> {
>   char ch[8000];
>   foo (ch);
>
>   return 0;
> }
> ```
>
> foo.c:
> ```
> void foo(char *ch) { }
> ```
>
> And the compilation line:
>
> $ gcc -O2 -fstack-check -o big-access big-access.c foo.c -fdump-rtl-final
>
> And here is the gdb view (ignore the breakpoint and current insn caret):
> ```
> B+ │0x555555554560 <main>           sub    $0x2f78,%rsp
>    │0x555555554567 <main+7>         orq    $0x0,0xf58(%rsp)
>    │0x555555554570 <main+16>        orq    $0x0,(%rsp)
>    │0x555555554575 <main+21>        add    $0x1020,%rsp
>    │0x55555555457c <main+28>        mov    %rsp,%rdi
>    │0x55555555457f <main+31>        mov    %fs:0x28,%rax
>   >│0x555555554588 <main+40>        mov    %rax,0x1f48(%rsp)
>    │0x555555554590 <main+48>        xor    %eax,%eax
>    │0x555555554592 <main+50>        callq  0x5555555546d0 <foo>
>    │0x555555554597 <main+55>        mov    0x1f48(%rsp),%rdx
>    │0x55555555459f <main+63>        xor    %fs:0x28,%rdx
>    │0x5555555545a8 <main+72>        jne    0x5555555545b4 <main+84>
>    │0x5555555545aa <main+74>        xor    %eax,%eax
>    │0x5555555545ac <main+76>        add    $0x1f58,%rsp
>    │0x5555555545b3 <main+83>        retq
>    │0x5555555545b4 <main+84>        callq  0x555555554540 <__stack_chk_fail@plt>
>    │0x5555555545b9                  nopl   0x0(%rax)
> ```
>
> I would just like someone who knows their stuff to double check my
> understanding:
>
> The "orq" at the start are purposefully causing a "dummy" load/store
> event so the VMM can decide whether or not it is sane for us to have
> used those pages for the stack, right?

Not quite. As noted at [1] this OR is to ensure that stack hasn't
overflowed. This is the part added by -fstack-check (you can see it go
away when you remove this option). See [2] for documentation.

>
> Another question, is at address 0x55555555457f. I presume that
> %fs:0x28 is a memory address that points to a sentinel value. We load
> it into %rax, and then we store it in strategic locations in our stack
> to serve as sentinel values. Before we leave, we check that the memory
> location hasn't changed at 0x55555555459f. That implies, that the
> memory location %fs:0x28 is pointing to a globally-used sentinel
> value?

Right. But note that this is enabled not by -fstack-check, but rather
by some of the -fstack-protector* options that are on by default on
modern Linux distributions. You can confirm this by explicitly passing
-fno-stack-protector and seeing this sentinel checking gone.

>
> But who sets %fs? Indeed what is the ABI usage of %fs in the context
> of linux x86_64?

The FS segment base points to the TLS. See [3] and links therein.

> And why 0x28 offset?

It's the offset of stack_guard member of tcbhead_t. See the
corresponding glibc source [4].

>
> Thankyou for reading,
> Maxim

[1]: https://stackoverflow.com/a/44670648/673852
[2]: https://gcc.gnu.org/onlinedocs/gccint/Stack-Checking.html
[3]: https://chao-tic.github.io/blog/2018/12/25/tls
[4]: https://code.woboq.org/userspace/glibc/sysdeps/x86_64/nptl/tls.h.html#42

Regards,
Ruslan