Date: Thu, 10 Mar 2016 12:16:46 +0100 From: Ingo Molnar <mingo@...nel.org> To: Rich Felker <dalias@...c.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, the arch/x86 maintainers <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...en8.de>, "musl@...ts.openwall.com" <musl@...ts.openwall.com>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers * Rich Felker <dalias@...c.org> 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/vdso32.lds.S | 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 signal. 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? Thanks, Ingo
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