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

From: Launchpad Bug Tracker
Subject: [Bug 1907969] Re: linux-user/i386: Segfault when mixing threads and signals
Date: Mon, 12 Jul 2021 04:17:28 -0000

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

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

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

Status in QEMU:

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

  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.

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

      // This makes qemu segfault.

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