gdb/testsuite/gdb.threads/vfork-multi-thread.c

   1 #include <assert.h>
   2 #include <pthread.h>
   3 #include <stdio.h>
   4 #include <unistd.h>
   5 #include <sys/wait.h>
   6
   7 // Start with:
   8 //
   9 //   ./gdb -nx -q --data-directory=data-directory repro -ex "tb break_here_first" -ex r -ex "b should_break_here"
  10 //
  11 // Then do "continue".
  12 //
  13 // The main thread will likely cross should_break_here while we are handling
  14 // the vfork and breakpoints are removed, therefore missing the breakpoint.
  15
  16 static volatile int release_vfork = 0;
  17 static volatile int release_main = 0;
  18
  19 static void *vforker(void *arg)
  20 {
  21   while (!release_vfork);
  22
  23   pid_t pid = vfork ();
  24   if (pid == 0) {
  25     /* A vfork child is not supposed to mess with the state of the program,
  26        but it is helpful for the purpose of this test.  */
  27     release_main = 1;
  28     _exit(7);
  29   }
  30
  31   int stat;
  32   int ret = waitpid (pid, &stat, 0);
  33   assert (ret == pid);
  34   assert (WIFEXITED (stat));
  35   assert (WEXITSTATUS (stat) == 7);
  36
  37   return NULL;
  38 }
  39
  40 static void should_break_here(void) {}
  41
  42 int main()
  43 {
  44
  45   pthread_t thread;
  46   int ret = pthread_create(&thread, NULL, vforker, NULL);
  47   assert(ret == 0);
  48
  49   /* We break here first, while the thread is stuck on `!release_fork`.  */
  50   release_vfork = 1;
  51
  52   /* We set a breakpoint on should_break_here.
  53
  54      We then set "release_fork" from the debugger and continue.  The main
  55      thread hangs on `!release_main` while the non-main thread vforks.  During
  56      the window of time where the two processes have a shared address space
  57      (after vfork, before _exit), GDB removes the breakpoints from the address
  58      space.  During that window, only the vfork-ing thread (the non-main
  59      thread) is frozen by the kernel.  The main thread is free to execute.  The
  60      child process sets `release_main`, releasing the main thread. A buggy GDB
  61      would let the main thread execute during that window, leading to the
  62      breakpoint on should_break_here being missed.  A fixed GDB does not resume
  63      the threads of the vforking process other than the vforking thread.  When
  64      the vfork child exits, the fixed GDB resumes the main thread, after
  65      breakpoints are reinserted, so the breakpoint is not missed.  */
  66
  67   while (!release_main);
  68
  69   should_break_here();
  70
  71   pthread_join (thread, NULL);
  72
  73   return 6;
  74 }