== Simple setjmp/longjmp program ==

static jmp_buf jbuf;

static void sighandler(int signo)
{
    siglongjmp(jbuf, signo);
}

static void setup_sighandler(void)
{
    signal(SIGINT, sighandler);

    if (0 == sigsetjmp(jbuf, 0)) {
        return;
    }

    exit(42);
}

int main(int argc, char *argv[])
{
    setup_sighandler();
    pause();

    return 0;
}


objdump -saD -j .text test-setjmp:

004006c0 <sighandler>:
  4006c0:	27bdffe0 	addiu	sp,sp,-32
  4006c4:	afbf001c 	sw	ra,28(sp)
  4006c8:	afbe0018 	sw	s8,24(sp)
  4006cc:	03a0f021 	move	s8,sp
  4006d0:	3c1c0042 	lui	gp,0x42
  4006d4:	279c8a50 	addiu	gp,gp,-30128
  4006d8:	afbc0010 	sw	gp,16(sp)
  4006dc:	afc40020 	sw	a0,32(s8)
  4006e0:	3c020041 	lui	v0,0x41
  4006e4:	24440ac0 	addiu	a0,v0,2752
  4006e8:	8fc50020 	lw	a1,32(s8)
  4006ec:	8f82803c 	lw	v0,-32708(gp)
  4006f0:	0040c821 	move	t9,v0
  4006f4:	0320f809 	jalr	t9
  4006f8:	00000000 	nop

004006fc <setup_sighandler>:
  4006fc:	27bdffe0 	addiu	sp,sp,-32     => setup new stack frame
  400700:	afbf001c 	sw	ra,28(sp)     => save return address to main
  400704:	afbe0018 	sw	s8,24(sp)     => save main's frame pointer
  400708:	03a0f021 	move	s8,sp         => adjust frame pointer
  40070c:	3c1c0042 	lui	gp,0x42
  400710:	279c8a50 	addiu	gp,gp,-30128
  400714:	afbc0010 	sw	gp,16(sp)     => save gp
  400718:	24040002 	li	a0,2
  40071c:	3c020040 	lui	v0,0x40
  400720:	244506c0 	addiu	a1,v0,1728
  400724:	8f828054 	lw	v0,-32684(gp)
  400728:	0040c821 	move	t9,v0
  40072c:	0320f809 	jalr	t9            => signal()
  400730:	00000000 	nop
  400734:	8fdc0010 	lw	gp,16(s8)     => restore gp
  400738:	3c020041 	lui	v0,0x41
  40073c:	24440ac0 	addiu	a0,v0,2752
  400740:	00002821 	move	a1,zero
  400744:	8f828044 	lw	v0,-32700(gp)
  400748:	0040c821 	move	t9,v0
  40074c:	0320f809 	jalr	t9            => sigsetjmp()
  400750:	00000000 	nop
  400754:	8fdc0010 	lw	gp,16(s8)     => Next instruction after sigsetjmp(): restore gp
  400758:	10400006 	beqz	v0,400774 <setup_sighandler+0x78>
  40075c:	00000000 	nop
  400760:	2404002a 	li	a0,42         => sigsetjmp() != 0 (siglongjmp was called)
  400764:	8f82804c 	lw	v0,-32692(gp)
  400768:	0040c821 	move	t9,v0
  40076c:	0320f809 	jalr	t9            => exit
  400770:	00000000 	nop
  400774:	00000000 	nop                   => sigsetjmp() == 0
  400778:	03c0e821 	move	sp,s8
  40077c:	8fbf001c 	lw	ra,28(sp)     => restore return address
  400780:	8fbe0018 	lw	s8,24(sp)     => restore frame pointer
  400784:	27bd0020 	addiu	sp,sp,32
  400788:	03e00008 	jr	ra            => return
  40078c:	00000000 	nop

1. In setup_sighandler() before sigsetjmp()

(gdb) p	/x $gp
$1 = 0x418a50
(gdb) p	/x $s8
$2 = 0x7fff6a58
(gdb) x	/20x $s8
0x7fff6a58:	0x77fc5e00	0x00400834	0x77fd4544	0x00400590
0x7fff6a68:	0x00418a50	0x77e60c94	0x7fff6a78	0x004007bc
0x7fff6a78:	0x77fc5e00	0x00000000	0x00400590	0x00000000
0x7fff6a88:	0x00418a50	0x77ff6fac	0x004e9e7c	0x77e3f378
0x7fff6a98:	0x00000001	0x7fff6b74	0x77fc5e00	0x00000000

gp is set at offset 16 from frame pointer.


2. In setup_sighandler() after sigsetjmp()

Execution resumes at:
lw gp,16(s8)

After this instruction, gp and s8 are correct (same output as in 1.)


3. In sighandler() before siglongjmp()

Let's see what setup_sighandler's stack frame now looks like:

(gdb) x /20x 0x7fff6a58
0x7fff6a58:	0x77fc5e00	0x00400834	0x00410ac0	0x00000000
0x7fff6a68:	0x7fff6a58	0x7fff6a58	0x77fff000	0x004007d0
0x7fff6a78:	0x77fc5e00	0x00000000	0x00400590	0x00000000
0x7fff6a88:	0x00418a50	0x77ff6fac	0x004e9e7c	0x77e3f378
0x7fff6a98:	0x00000001	0x7fff6b74	0x77fc5e00	0x00000000

At least the offset where gp used to be stored has been smashed.


4. In setup_sighandler after siglongjmp()

Execution resumes at:
lw gp,16(s8)

(gdb) p /x $s8
$6 = 0x7fff6a58

s8 was correctly restored.

(gdb) x /20x $s8
0x7fff6a58:	0x77fc5e00	0x00400834	0x00410ac0	0x00000000
0x7fff6a68:	0x7fff6a58	0x7fff6a58	0x77fff000	0x004007d0
0x7fff6a78:	0x77fc5e00	0x00000000	0x00400590	0x00000000
0x7fff6a88:	0x00418a50	0x77ff6fac	0x004e9e7c	0x77e3f378
0x7fff6a98:	0x00000001	0x7fff6b74	0x77fc5e00	0x00000000

At we expected from 3. the setup_sighandler's frame pointer contains random values, especially at the offset where gp is supposed to be stored.
So the instruction lw gp,16(s8) will not restore gp properly.