Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers

From: Ingo Molnar
Date: Thu Mar 10 2016 - 06:16:56 EST

* Rich Felker <dalias@xxxxxxxx> wrote:

> [...]
> I believe a new kernel cancellation API with a sticky cancellation flag (rather
> than a signal), and a flag or'd onto the syscall number to make it cancellable
> at the call point, could work, but then userspace needs to support fairly
> different old and new kernel APIs in order to be able to run on old kernels
> while also taking advantage of new ones, and it's not clear to me that it would
> actually be worthwhile to do so. I could see doing it for a completely new
> syscall API, but as a second syscall API for a system that already has one it
> seems gratuitous. From my perspective the existing approach (checking program
> counter from signal handler) is very clean and simple. After all it made enough
> sense that I was able to convince the glibc folks to adopt it.

I concur with your overall analysis, but things get a bit messy once we consider
AT_SYSINFO which is a non-atomic mix of user-space and kernel-space code. Trying
to hand cancellation status through that results in extra complexity:

arch/x86/entry/vdso/Makefile | 3 +-
arch/x86/entry/vdso/vdso32/cancellation_helpers.c | 116 ++++++++++++++++++++++
arch/x86/entry/vdso/vdso32/ | 2 +
tools/testing/selftests/x86/unwind_vdso.c | 57 +++++++++--
4 files changed, 171 insertions(+), 7 deletions(-)

So instead of a sticky cancellation flag, we could introduce a sticky cancellation

A 'sticky signal' is not cleared from signal_pending() when the signal handler
executes, but it's automatically blocked so no signal handler recursion occurs.
(A sticky signal could still be cleared via a separate mechanism, by the
cancellation cleanup code.)

Such a 'sticky cancellation signal' would, in the racy situation, cause new
blocking system calls to immediately return with -EINTR. Non-blocking syscalls
could still be used. (So the cancellation signal handler itself would still have
access to various fundamental system calls.)

I think this would avoid messy coupling between the kernel's increasingly more
varied system call entry code and C libraries.

Sticky signals could be requested via a new SA_ flag.

What do you think?