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Date: Wed, 17 Nov 2021 17:11:14 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Kyle Huey <me@...ehuey.com> Cc: Kees Cook <keescook@...omium.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, Andrea Righi <andrea.righi@...onical.com>, Shuah Khan <shuah@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Andy Lutomirski <luto@...capital.net>, Will Drewry <wad@...omium.org>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, bpf <bpf@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, linux-hardening@...r.kernel.org, "Robert O'Callahan" <rocallahan@...il.com> Subject: Re: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers On Wed, Nov 17, 2021 at 4:37 PM Kyle Huey <me@...ehuey.com> wrote: > > This fixes most of the issues with rr, but it still changes the ptrace > behavior for the double-SIGSEGV case Hmm. I think that's because of how "force_sigsgv()" works. I absolutely detest that function. So we have signal_setup_done() doing that if (failed) force_sigsegv(ksig->sig); and then force_sigsegv() has that completely insane if (sig == SIGSEGV) force_fatal_sig(SIGSEGV); else force_sig(SIGSEGV); behavior. And I think I know the _reason_ for that complete insanity: when SIGSEGV takes a SIGSEGV, and there is a handler, we need to stop trying to send more SIGSEGV's. But it does mean that with my change, that second SIGSEGV now ends up being that SA_IMMUTABLE kind, so yeah, it broke the debugger test - where catching the second SIGSEGV is actually somewhat sensible (ok, not really, but at least understandable) End result: I think we want not a boolean, but a three-way choice for that force_sig_info_to_task() thing: - unconditionally fatal (for things that just want to force an exit and used to do do_exit()) - ignore valid and unblocked handler (for that SIGSEGV recursion case, aka force "sigdfl") - catching signal ok So my one-liner isn't sufficient. It wants some kind of nasty enum. At least the enum can be entirely internal to kernel/signal.c, I think. No need to expose this all to anything else. Linus
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