[Bug 1906193] Re: riscv32 user mode emulation: fork return values broken

[Andreas K . Hüttel]

Here's qemu's own strace log:

farino ~ # /usr/bin/qemu-riscv32 -strace /chroot/riscv-ilp32/tmp/wait-test-short
10123 brk(NULL) = 0x00073000
10123 brk(0x00073880) = 0x00073880
10123 uname(0x407ffed8) = 0
10123 readlinkat(AT_FDCWD,"/proc/self/exe",0x407feff0,4096) = 39
10123 brk(0x00094880) = 0x00094880
10123 brk(0x00095000) = 0x00095000
10123 mprotect(0x0006e000,8192,PROT_READ) = 0
10123 
clone(CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|0x11,child_stack=0x00000000,parent_tidptr=0x00000000,tls=0x00000000,child_tidptr=0x00073068)
 = 10125
10123 
clone(CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|0x11,child_stack=0x00000000,parent_tidptr=0x00000000,tls=0x00000000,child_tidptr=0x00073068)
 = 0
10125 exit_group(42)
10123 waitid(0,-1,0x407fff8c,0x4) = 0
10123 statx(1,"",AT_EMPTY_PATH,STATX_BASIC_STATS,0x407ff8e8) = 0
child wants to return 42 (0x2A), parent received 40 (0x28), difference -2
10123 write(1,0x73ad0,74) = 74
10123 exit_group(0)

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https://bugs.launchpad.net/bugs/1906193

Title:
  riscv32 user mode emulation: fork return values broken

Status in QEMU:
  New

Bug description:
  When running in a chroot with riscv32 (on x86_64; qemu git master as
  of today):

  The following short program forks; the child immediately returns with
  exit(42). The parent checks for the return value - and obtains 40!

  gcc-10.2

  ===============================================
  #include <stdlib.h>
  #include <unistd.h>
  #include <stdio.h>
  #include <sys/wait.h>

  main(c, v)
       int c;
       char **v;
  {
    pid_t pid, p;
    int s, i, n;

    s = 0;
    pid = fork();
    if (pid == 0)
      exit(42);

    /* wait for the process */
    p = wait(&s);
    if (p != pid)
      exit (255);

    if (WIFEXITED(s))
    {
       int r=WEXITSTATUS(s);
       if (r!=42) {
        printf("child wants to return %i (0x%X), parent received %i (0x%X), 
difference %i\n",42,42,r,r,r-42);
       }
    }
  }
  ===============================================

  (riscv-ilp32 chroot) farino /tmp # ./wait-test-short 
  child wants to return 42 (0x2A), parent received 40 (0x28), difference -2

  ===============================================
  (riscv-ilp32 chroot) farino /tmp # gcc --version
  gcc (Gentoo 10.2.0-r1 p2) 10.2.0
  Copyright (C) 2020 Free Software Foundation, Inc.
  Dies ist freie Software; die Kopierbedingungen stehen in den Quellen. Es
  gibt KEINE Garantie; auch nicht für MARKTGÄNGIGKEIT oder FÜR SPEZIELLE ZWECKE.

  (riscv-ilp32 chroot) farino /tmp # ld --version
  GNU ld (Gentoo 2.34 p6) 2.34.0
  Copyright (C) 2020 Free Software Foundation, Inc.
  This program is free software; you may redistribute it under the terms of
  the GNU General Public License version 3 or (at your option) a later version.
  This program has absolutely no warranty.

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