linux-kernel - [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

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Message-ID: <CAP045Ao5-+vvTzCOaCkfwztsd2Q0_8kh85UxuXW0bbcfVbF40w@mail.gmail.com>
Date:   Wed, 19 Aug 2020 10:53:58 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     "Robert O'Callahan" <rocallahan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

rr, a userspace record and replay debugger[0], has a test suite that
attempts to exercise strange corners of the Linux API. One such
test[1] began failing after 2bbc68f8373c0631ebf137f376fbea00e8086be7.
I have not tried to understand what has changed in the kernel here but
since the commit message says "No functional change" I assume
something has gone wrong.

The test expects to get a SIGTRAP when watchvar is written to in the
forked child, but instead the program just exits normally and we get a
status value corresponding to that (exit code 77 = wait status
0x4d00). This test program should be usable outside of rr's test suite
if you replace the test_assert/atomic_puts functions with
assert/printf and replace the util.h include with appropriate standard
includes.

This regression is present in 5.8.

- Kyle

[0] https://rr-project.org/
[1] https://github.com/mozilla/rr/blob/master/src/test/x86/ptrace_debug_regs.c#L55