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Date: Thu, 27 Aug 2015 18:44:22 -0700
From: Andy Lutomirski <luto@...capital.net>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>, libc-alpha <libc-alpha@...rceware.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
Brian Gerst <brgerst@...il.com>
Subject: 32-bit vdso syscall cancellation proposal
Hi-
After chatting with Rich Felker on IRC, he's convinced me that the
32-bit vsyscall mechanism is annoying for syscall cancellation. To
help, I propose we add two functions:
unsigned long __vdso_pending_vsyscall_eip(void *context);
If context is a signal context pointing to the AT_SYSINFO entry and if
that entry has *not* executed its syscall yet (or if it has but it's
about to resume it), then returns the EIP to which the vsyscall will
return. Otherwise returns -1.
int __vdso_cancel_pending_vsyscall(void *context);
If not in a valid pending vsyscall, returns -EINVAL. Otherwise
clobbers all vsyscall return registers except ESP, EIP, and EAX and
arranges for EAX to be -ENOSYS on return and for the vsyscall to be
skipped. Returns 0.
(Musl would slightly prefer -EINTR, but that's harder to implement.
Musl would like the ability to *change* the return EIP even more, but
that's considerably harder, given that the location of the saved EIP
will vary depending on where we are in the vsyscall.)
Ingo, if I implement something like this, it'll come with a careful
test case. I already have a test case that tests the CFI annotations
(by single-stepping through a syscall and asserting that it unwinds
correctly from every single SIGTRAP). It'll also be on top of a
series that turns __kernel_vsyscall into a *single* function.
__vdso_pending_vsyscall_eip can't be reliably implemented using the
CFI annotations, because you can't tell which side of the syscall
you're on. __vdso_cancel_pending_vsyscall could be, but that's quite
heavyweight, and implementing it directly will only be a couple lines
of code.
--Andy
--
Andy Lutomirski
AMA Capital Management, LLC
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