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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