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

["Andreas K. Hüttel" <1906193@bugs.launchpad.net>]

Here's the (abbreviated) output of strace'ing qemu:

farino ~ # strace -f /usr/bin/qemu-riscv32 /chroot/riscv-ilp32/tmp/wait-test-short
execve("/usr/bin/qemu-riscv32", ["/usr/bin/qemu-riscv32", "/chroot/riscv-ilp32/tmp/wait-tes"...], 0x7ffd95fb1330 /* 40 vars */) = 0

[...]

[pid 16569] uname({sysname="Linux", nodename="farino", ...}) = 0
[pid 16569] lstat("/chroot", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
[pid 16569] lstat("/chroot/riscv-ilp32", {st_mode=S_IFDIR|S_ISGID|0755, st_size=4096, ...}) = 0
[pid 16569] lstat("/chroot/riscv-ilp32/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=4096, ...}) = 0
[pid 16569] lstat("/chroot/riscv-ilp32/tmp/wait-test-short", {st_mode=S_IFREG|0755, st_size=445632, ...}) = 0
[pid 16569] mmap(0x413f1000, 135168, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x413f1000
[pid 16569] mprotect(0x413eb000, 8192, PROT_READ) = 0
[pid 16569] rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], NULL, 8) = 0
[pid 16569] clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x1339710) = 16571
strace: Process 16571 attached
[pid 16571] set_robust_list(0x1339720, 24 <unfinished ...>
[pid 16569] rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], NULL, 8) = 0
[pid 16571] <... set_robust_list resumed>) = 0
[pid 16569] rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
[pid 16571] rt_sigprocmask(SIG_SETMASK, ~[ILL FPE SEGV RTMIN RT_1], ~[KILL STOP RTMIN RT_1], 8) = 0
[pid 16571] rt_sigprocmask(SIG_BLOCK, ~[], ~[ILL FPE KILL SEGV STOP RTMIN RT_1], 8) = 0
[pid 16571] clone(child_stack=0x7fe5b73871f0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[16572], tls=0x7fe5b7387640, child_tidptr=0x7fe5b7387910) = 16572
[pid 16571] rt_sigprocmask(SIG_SETMASK, ~[ILL FPE KILL SEGV STOP RTMIN RT_1], NULL, 8) = 0
[pid 16571] rt_sigprocmask(SIG_SETMASK, ~[KILL STOP RTMIN RT_1], NULL, 8) = 0
[pid 16571] gettid()                    = 16571
[pid 16571] rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], NULL, 8) = 0
[pid 16571] rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
[pid 16569] waitid(P_ALL, -1,  <unfinished ...>
[pid 16571] exit_group(42)              = ?
strace: Process 16572 attached
[pid 16572] +++ exited with 42 +++
[pid 16571] +++ exited with 42 +++
[pid 16569] <... waitid resumed>{si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=16571, si_uid=0, si_status=42, si_utime=3472328296226648184, si_stime=3475143045726351408}, WEXITED, NULL) = 0
[pid 16569] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=16571, si_uid=0, si_status=42, si_utime=0, si_stime=0} ---
[pid 16569] statx(1, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_BASIC_STATS, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFCHR|0600, stx_size=0, ...}) = 0
[pid 16569] write(1, "child wants to return 42 (0x2A),"..., 74child wants to return 42 (0x2A), parent received 40 (0x28), difference -2
) = 74
[pid 16569] brk(0x13c1000)              = 0x13c1000
[pid 16569] brk(0x13c0000)              = 0x13c0000
[pid 16569] exit_group(0)               = ?
[pid 16570] <... futex resumed>)        = ?
[pid 16570] +++ exited with 0 +++
+++ exited with 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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