[Bug 1907969] Re: linux-user/i386: Segfault when mixing threads and signals

[Launchpad Bug Tracker]

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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https://bugs.launchpad.net/bugs/1907969

Title:
  linux-user/i386: Segfault when mixing threads and signals

Status in QEMU:
  Expired

Bug description:
  Given the following C program, qemu-i386 will surely and certainly segfault 
when executing it.
  The problem is only noticeable if the program is statically linked to musl's 
libc and, as written
  in the title, it only manifests when targeting i386.

  Removing the pthread calls or the second raise() makes it not
  segfault.

  The crash is in some part of the TCG-generated code, right when it tries to 
perform a
  %gs-relative access.

  If you want a quick way of cross-compiling this binary:

  * Download a copy of the Zig compiler from https://ziglang.org/download/
  * Compile it with
    `zig cc -target i386-linux-musl <C-FILE> -o <OUT>`

  ```
  #include <pthread.h>
  #include <signal.h>
  #include <stdio.h>
  #include <string.h>
  #include <sys/types.h>
  #include <unistd.h>
  #include <asm/prctl.h>
  #include <sys/syscall.h>

  void sig_func(int sig)
  {
      write(1, "hi!\n", strlen("hi!\n"));
  }

  void func(void *p) { }

  typedef void *(*F)(void *);

  int main()
  {
      pthread_t tid;

      struct sigaction action;
      action.sa_flags = 0;
      action.sa_handler = sig_func;

      if (sigaction(SIGUSR1, &action, NULL) == -1) {
          return 1;
      }

      // This works.
      raise(SIGUSR1);

      pthread_create(&tid, NULL, (F)func, NULL);
      pthread_join(tid, NULL);

      // This makes qemu segfault.
      raise(SIGUSR1);
  }
  ```

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