Here is the code for swap context:
int
swapcontext(ucontext_t *oucp, const ucontext_t *ucp)
{
int ret;
if ((oucp == NULL) || (ucp == NULL)) {
errno = EINVAL;
return (-1);
}
oucp->uc_flags &= ~UCF_SWAPPED;
ret = getcontext(oucp);
if ((ret == 0) && !(oucp->uc_flags & UCF_SWAPPED)) {
oucp->uc_flags |= UCF_SWAPPED;
ret = setcontext(ucp);
}
return (ret);
}
On the OS X port of libc, this gets compiled as:
0x00007fff901e86b2 <swapcontext+0>: push %r14
0x00007fff901e86b4 <swapcontext+2>: push %rbx
0x00007fff901e86b5 <swapcontext+3>: sub $0x8,%rsp
0x00007fff901e86b9 <swapcontext+7>: test %rdi,%rdi
0x00007fff901e86bc <swapcontext+10>: je 0x7fff901e86c6 <swapcontext+20>
0x00007fff901e86be <swapcontext+12>: mov %rsi,%rbx
0x00007fff901e86c1 <swapcontext+15>: test %rbx,%rbx
0x00007fff901e86c4 <swapcontext+18>: jne 0x7fff901e86d8 <swapcontext+38>
0x00007fff901e86c6 <swapcontext+20>: callq 0x7fff90262c88 <__error>
0x00007fff901e86cb <swapcontext+25>: movl $0x16,(%rax)
0x00007fff901e86d1 <swapcontext+31>: mov $0xffffffff,%eax
0x00007fff901e86d6 <swapcontext+36>: jmp 0x7fff901e86f3 <swapcontext+65>
0x00007fff901e86d8 <swapcontext+38>: mov %rdi,%r14
0x00007fff901e86db <swapcontext+41>: andb $0x7f,0x3(%r14)
0x00007fff901e86e0 <swapcontext+46>: mov %r14,%rdi
0x00007fff901e86e3 <swapcontext+49>: callq 0x7fff901e87af <getcontext>
0x00007fff901e86e8 <swapcontext+54>: test %eax,%eax
0x00007fff901e86ea <swapcontext+56>: jne 0x7fff901e86f3 <swapcontext+65>
0x00007fff901e86ec <swapcontext+58>: mov (%r14),%ecx
0x00007fff901e86ef <swapcontext+61>: test %ecx,%ecx
0x00007fff901e86f1 <swapcontext+63>: jns 0x7fff901e86fb <swapcontext+73>
0x00007fff901e86f3 <swapcontext+65>: add $0x8,%rsp
0x00007fff901e86f7 <swapcontext+69>: pop %rbx
0x00007fff901e86f8 <swapcontext+70>: pop %r14
0x00007fff901e86fa <swapcontext+72>: retq
0x00007fff901e86fb <swapcontext+73>: or $0x80000000,%ecx
0x00007fff901e8701 <swapcontext+79>: mov %ecx,(%r14)
0x00007fff901e8704 <swapcontext+82>: mov %rbx,%rdi
0x00007fff901e8707 <swapcontext+85>: add $0x8,%rsp
0x00007fff901e870b <swapcontext+89>: pop %rbx
0x00007fff901e870c <swapcontext+90>: pop %r14
0x00007fff901e870e <swapcontext+92>: jmpq 0x7fff90262855 <setcontext>
The problem is that rbx is callee saved by compiled version of swapcontext and
then reused before getcontext is called. Getcontext then stores the wrong
value for rbx and setcontext later restores the wrong value for rbx. If the
caller had any value in rbx, it has been trashed at this point.
Brian
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