ptrace.2: BUGS: ptrace() may set errno to zero

ptrace() with requests PTRACE_PEEKTEXT, PTRACE_PEEKDATA and
PTRACE_PEEKUSER can set errno to zero. AFAICS this is for a good
reason (so that you can tell the difference between a successful
PEEK with a result of -1 and a failed PEEK, even if you forget to
clear errno yourself), but it technically violates the rules
described in the errno.3 manpage.

glibc snippet from sysdeps/unix/sysv/linux/ptrace.c:

  res = INLINE_SYSCALL (ptrace, 4, request, pid, addr, data);
  if (res >= 0 && request > 0 && request < 4)
    {
      __set_errno (0);
      return ret;
    }

reproducer:

$ cat ptrace_test.c
char foobar_data[4] = "ABCD";
int main(void) {
  pid_t child = fork();
  if (child == -1) err(1, "fork");
  if (child == 0) {
    if (prctl(PR_SET_PDEATHSIG, SIGKILL)) err(1, "prctl");
    while (1) sleep(1);
  }
  int status;
  if (ptrace(PTRACE_ATTACH, child, NULL, NULL)) err(1, "attach");
  if (waitpid(child, &status, 0) != child) err(1, "wait");
  errno = EINVAL;
  unsigned int res = ptrace(PTRACE_PEEKDATA, child, foobar_data, NULL);
  printf("errno after PEEKDATA: %d\n", errno);
  printf("PEEKDATA result: 0x%x\n", res);
}
$ gcc -o ptrace_test ptrace_test.c -Wall
$ ./ptrace_test
errno after PEEKDATA: 0
PEEKDATA result: 0x44434241

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/ptrace.2

@@ -2777,6 +2777,12 @@ again.
 errors may behave in an unintended way upon an
 .BR strace (1)
 attach.)
+.PP
+Contrary to the normal rules, the glibc wrapper for
+.BR ptrace ()
+can set
+.I errno
+to zero.
 .SH SEE ALSO
 .BR gdb (1),
 .BR ltrace (1),