Date: Tue, 1 Oct 2002 11:25:10 -0400 From: Jake Burkholder <jake@locore.ca> To: Daniel Eischen <eischen@pcnet1.pcnet.com> Cc: Peter Wemm <peter@wemm.org>, current@FreeBSD.ORG Subject: Re: Longer term fix for sigreturn ABI breaking Message-ID: <20021001112510.G218@locore.ca> In-Reply-To: <Pine.GSO.4.10.10210011057570.18028-100000@pcnet1.pcnet.com>; from eischen@pcnet1.pcnet.com on Tue, Oct 01, 2002 at 11:12:02AM -0400 References: <20020930230249.E57AF2A7D6@canning.wemm.org> <Pine.GSO.4.10.10210011057570.18028-100000@pcnet1.pcnet.com>
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Apparently, On Tue, Oct 01, 2002 at 11:12:02AM -0400, Daniel Eischen said words to the effect of; > On Mon, 30 Sep 2002, Peter Wemm wrote: > > > Daniel Eischen wrote: > > > At the end is a potentially longer term fix for the ABI > > > breakage that was introduced when the i386 mcontext_t > > > was changed/enlarged. > > > > > - ret = set_fpcontext(td, &ucp->uc_mcontext); > > > - if (ret != 0) > > > - return (ret); > > > + /* > > > + * Intentionally ignore the error to keep binary > > > + * compatibility with applications that fiddle with > > > + * the FPU save area in the context. The kernel > > > + * now saves the FPU state in the context, but it > > > + * gets corrupted by those applications that try > > > + * to work around the kernel NOT saving it. > > > + */ > > > + (void)set_fpcontext(td, &ucp->uc_mcontext); > > > > Maybe we could have something like this instead? > > > > ret = set_fpcontext(td, &ucp->uc_mcontext); > > #if !defined(COMPAT_FREEBSD4) && !defined(COMPAT_43) > > if (ret != 0) > > return (ret); > > #endif > > > > ie: ignore the error only if we have to be compatable. > > Sure that's totally doable. It might not be enough to just > call set_fpcontext() and ignore the error. Thinking a bit > more about it, the mc_len, mc_fpformat, and mc_ownedfp fields > now occupy the first couple of slots where fpregs[] used to be. > The format of an fnsave() stores the control, status and tag > words in these slots. There are 32-bits of storage allocated > for each of these, but the fnsave (according to what I > see in npx.h), only uses the lower 16 bits. It might be > possible to save a control word or status word that turn > out to be valid for mc_fpformat or mc_ownedfp (0, 1, or 2). > In this case we'd think the FP context was valid, and try > to restore it (it would be trashed). > > I think if we put some magic in the upper 16 bits of > mc_ownedfp, mc_fpformat, then we could prevent this. > > > Longer term, I was thining that we could/should do what sparc64 does, ie: > > libc provides the trampoline and it can then call the correct sigreturn > > syscall. That means we add a new sigreturn syscall each time we > > significantly break the sigreturn ABI (as in this case) and applications > > will be able to use the correct one. Paired with a new sigaction syscall > > which would specify the "new" context format we can then be future proof. > > Sounds good. If we added a new sigaction and sigreturn now, we can > still do the same thing, without having the trampoline in libc. > I thought the point of having the trampoline in libc would prevent > having to create new syscalls... The point is that the signal trampoline automatically uses the new or old system calls because its linked with libc. Otherwise you need a different signal trampoline in the kernel for each version of sigreturn, and some way to determine the right one. The 0x01ds16 hack only works for so long. Jake To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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