The following reply was made to PR misc/177624; it has been noted by GNATS.
From: Bruce Evans <b...@optusnet.com.au>
To: Bruce Evans <b...@optusnet.com.au>
Cc: Brian Demsky <bdem...@uci.edu>, freebsd-bugs@freebsd.org,
freebsd-gnats-sub...@freebsd.org
Subject: Re: misc/177624: Swapcontext can get compiled incorrectly
Date: Fri, 5 Apr 2013 02:16:02 +1100 (EST)
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On Fri, 5 Apr 2013, Bruce Evans wrote:
> On Thu, 4 Apr 2013, Brian Demsky wrote:
>
>>> Description:
>> Here is the code for swap context:
>>=20
>> int
>> swapcontext(ucontext_t *oucp, const ucontext_t *ucp)
>> {
>> int ret;
>>=20
>> if ((oucp =3D=3D NULL) || (ucp =3D=3D NULL)) {
>> errno =3D EINVAL;
>> return (-1);
>> }
>> oucp->uc_flags &=3D ~UCF_SWAPPED;
>> ret =3D getcontext(oucp);
>> if ((ret =3D=3D 0) && !(oucp->uc_flags & UCF_SWAPPED)) {
>> oucp->uc_flags |=3D UCF_SWAPPED;
>> ret =3D setcontext(ucp);
>> }
>> return (ret);
>> }
>
>> On the OS X port of libc in Mac OSX 10.7.5, this gets compiled as:
>
>> ...
>> 0x00007fff901e870b <swapcontext+89>: pop %rbx
>> 0x00007fff901e870c <swapcontext+90>: pop %r14
>> 0x00007fff901e870e <swapcontext+92>: jmpq 0x7fff90262855 <setcontex=
t>
>>=20
>> The problem is that rbx is callee saved by compiled version of swapconte=
xt=20
>> and then reused before getcontext is called. Getcontext then stores the=
=20
>> wrong value for rbx and setcontext later restores the wrong value for rb=
x.=20
>> If the caller had any value in rbx, it has been trashed at this point.
>
> Later you wrote:
>
>> The analysis is a little wrong about the problem. Ultimately, the tail=
=20
>> call to set context trashes the copies of bx and r14 on the stack=85.
>
> The bug seems to be in setcontext(). It must preserve the callee-saved
> registers, not restore them. This would happen automatically if more
> were written in C. But setcontext() can't be written entirely in C,
> since it must save all callee-saved registers including ones not used
> and therefore not normally saved by any C function that it might be in,
> and possibly also including callee-saved registers for nonstandard or
> non-C ABIs. In FreeBSD, it is apparently always a syscall.
This is more than a little wrong. When setcontext() succeeds, it
doesn't return here. Then it acts like longjmp() and must restore all
the callee-saved to whatever they were when getcontext() was called.
Otherwise, it must not clobber any callee-saved registers (then it
differs from longjmp(). longjmp() just can't fail).
Now I don't see any bug here. If the saved state is returned to, then
it is as if getcontext() returned, and the intermediately-saved %rbx
is correct (we will restore the orginal %rbx if we return). If
setcontext() fails, then it should preserve all callee-saved registers.
In the tail-call case, we have already restored the orginal %rbx and
the failing setcontext() should preserve that.
Bruce
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