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

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Date:   Wed, 19 Aug 2020 23:35:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        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: Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working

On Wed, Aug 19, 2020 at 12:28:16PM -0700, Kyle Huey wrote:

> > I'm guess that is not the expected outcome, is that the same failure you
> > saw?
> 
> Yes. Is status also 0x4d00 for you?

Indeed.

> The program is expected to complete with no assertions firing.

When I comment out the break-on-exec test, the break-on-write test
succeeds.

When I add a few printk()'s to our #DB handler (6) the program will
magically work again.

I'm not much for ptrace(), but are we sure the test program is well
behaved?

The below also always works..

---
/* -*- Mode: C; tab-width: 8; c-basic-offset: 2; indent-tabs-mode: nil; -*- */

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ptrace.h>
#include <stdarg.h>
#include <assert.h>
#include <sys/wait.h>
#include <errno.h>
#include <string.h>
#include <stddef.h>
#include <sys/user.h>

/**
 * Print the printf-like arguments to stdout as atomic-ly as we can
 * manage.  Async-signal-safe.  Does not flush stdio buffers (doing so
 * isn't signal safe).
 */
__attribute__((format(printf, 1, 2))) inline static int atomic_printf(
    const char* fmt, ...) {
  va_list args;
  char buf[1024];
  int len;

  va_start(args, fmt);
  len = vsnprintf(buf, sizeof(buf) - 1, fmt, args);
  va_end(args);
  return write(STDOUT_FILENO, buf, len);
}

/**
 * Write |str| on its own line to stdout as atomic-ly as we can
 * manage.  Async-signal-safe.  Does not flush stdio buffers (doing so
 * isn't signal safe).
 */
inline static int atomic_puts(const char* str) {
  return atomic_printf("%s\n", str);
}

inline static int check_cond(int cond) {
  if (!cond) {
    atomic_printf("FAILED: errno=%d (%s)\n", errno, strerror(errno));
  }
  return cond;
}

#define test_assert(cond) assert("FAILED: !" && check_cond(cond))

#define NEW_VALUE 0xabcdef

static void breakpoint(void) {}

static char watch_var;

int main(void) {
  pid_t child;
  int status;
  int pipe_fds[2];

  test_assert(0 == pipe(pipe_fds));

  if (0 == (child = fork())) {
    char ch;
    read(pipe_fds[0], &ch, 1);
    breakpoint();
    watch_var = 1;
    return 77;
  }

  test_assert(0 == ptrace(PTRACE_ATTACH, child, NULL, NULL));
  test_assert(child == waitpid(child, &status, 0));
  test_assert(status == ((SIGSTOP << 8) | 0x7f));
  test_assert(1 == write(pipe_fds[1], "x", 1));

  test_assert(0 == ptrace(PTRACE_POKEUSER, child,
                          (void*)offsetof(struct user, u_debugreg[0]),
                          (void*)breakpoint));
  test_assert(0 == ptrace(PTRACE_POKEUSER, child,
                          (void*)offsetof(struct user, u_debugreg[1]),
                          &watch_var));
  test_assert(0 == ptrace(PTRACE_POKEUSER, child,
                          (void*)offsetof(struct user, u_debugreg[7]),
                          (void*)0x100005));

  test_assert(0 == ptrace(PTRACE_CONT, child, NULL, NULL));
  test_assert(child == waitpid(child, &status, 0));
  test_assert(status == ((SIGTRAP << 8) | 0x7f));
  test_assert(0x1 == ptrace(PTRACE_PEEKUSER, child,
                            (void*)offsetof(struct user, u_debugreg[6])));

  test_assert(0 == ptrace(PTRACE_CONT, child, NULL, NULL));
  test_assert(child == waitpid(child, &status, 0));
  test_assert(status == ((SIGTRAP << 8) | 0x7f));
  test_assert(0x2 == ptrace(PTRACE_PEEKUSER, child,
                            (void*)offsetof(struct user, u_debugreg[6])));

  test_assert(0 == ptrace(PTRACE_DETACH, child, NULL, NULL));

  test_assert(child == waitpid(child, &status, 0));
  test_assert(WIFEXITED(status));
  test_assert(WEXITSTATUS(status) == 77);

  atomic_puts("EXIT-SUCCESS");
  return 0;
}