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

[Kyle Huey]

On Wed, Aug 19, 2020 at 11:42 AM <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Aug 19, 2020 at 10:53:58AM -0700, Kyle Huey wrote:
> > 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.
>
> $ uname -a
> Linux ivb-ep 5.9.0-rc1-dirty #343 SMP PREEMPT Wed Aug 19 15:04:35 CEST 2020 x86_64 GNU/Linux
>
> $ ./ptrace_debug_regs
> FAILED: errno=0 (Success)
> ptrace_debug_regs: ptrace_debug_regs.c:104: main: Assertion `"FAILED: !" && check_cond(status == ((5 << 8) | 0x7f))' failed.
> Aborted
>
> I'm guess that is not the expected outcome, is that the same failure you
> saw?

Yes. Is status also 0x4d00 for you?

The program is expected to complete with no assertions firing.

- Kyle

> ---
> /* -*- 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));
>   /* Enable DR0 break-on-exec */
>   test_assert(0 == ptrace(PTRACE_POKEUSER, child,
>                           (void*)offsetof(struct user, u_debugreg[7]),
>                           (void*)0x1));
>
>   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_POKEUSER, child,
>                           (void*)offsetof(struct user, u_debugreg[0]),
>                           &watch_var));
>   /* Enable DR0 break-on-write */
>   test_assert(0 == ptrace(PTRACE_POKEUSER, child,
>                           (void*)offsetof(struct user, u_debugreg[7]),
>                           (void*)0x10001));
>
>   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_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;
> }