Re: [PATCH] Replace vfork() with fork() to fix unshare crash on ppc64le
Please try current git, it contains a fix. Let me know if it doesn't work.
On Mon, Sep 15, 2025 at 7:57 AM tshah <tshah@linux.ibm.com> wrote:
>
> Hello Denys,
>
> Can you please look into this change and let me know your thoughts on
> this once you get chance.
>
> Thanks.
>
> On 09/09/25 15:50, tshah wrote:
> > Hello,
> >
> > This patch address the issue:
> > https://lists.busybox.net/pipermail/busybox/2025-September/091718.html.
> > The patch replaces the use of vfork() with fork() on MMU-enabled
> > targets in the xvfork() macro.
> > This change is necessary to resolve a segmentation fault observed on
> > ppc64le when running: "unshare -mrpf sh".
> >
> > According to POSIX, there is an udefined behaviour if the child
> > process created by vfork() either modifies the data other than a
> > variable of type pid_t or calls any other functions before
> > successfully calling exec (3) or _exit(2) family functions.
> > From the strace logs,it looks like the child after vfork performed
> > syscalls like writing uid_map, gid_map, mounting, etc, which violates
> > the minimal action requirements of vfork() resulting in a SIGSEGV
> > maybe due to race conditions.
> >
> > [ 189] [00003fff9376726c] vfork(strace: Process 376786 attached
> > <unfinished ...>
> > [pid 376786] [ 286] [00003fff936d7a80] openat(AT_FDCWD,
> > "/proc/self/setgroups", O_WRONLY) = 3
> > [pid 376786] [ 4] [00003fff936d7a80] write(3, "deny", 4) = 4
> > [pid 376786] [ 6] [00003fff936d7a80] close(3) = 0
> > [pid 376786] [ 286] [00003fff936d7a80] openat(AT_FDCWD,
> > "/proc/self/uid_map", O_WRONLY) = 3
> > [pid 376786] [ 4] [00003fff936d7a80] write(3, "0 1000 1", 8) = 8
> > [pid 376786] [ 6] [00003fff936d7a80] close(3) = 0
> > [pid 376786] [ 286] [00003fff936d7a80] openat(AT_FDCWD,
> > "/proc/self/gid_map", O_WRONLY) = 3
> > [pid 376786] [ 4] [00003fff936d7a80] write(3, "0 1000 1", 8) = 8
> > [pid 376786] [ 6] [00003fff936d7a80] close(3) = 0
> > [pid 376786] [ 21] [00003fff937890fc] mount("none", "/", NULL,
> > MS_REC|MS_PRIVATE, NULL) = 0
> > [pid 376786] [ 11] [00003fff93741230] execve("/usr/local/bin/bash",
> > ["bash", "-c", "ls"], 0x3fffe38b2780 /* 22 vars */) = -1 ENOENT (No
> > such file or directory)
> > [pid 376786] [ 11] [00003fff93741230] execve("/usr/bin/bash",
> > ["bash", "-c", "ls"], 0x3fffe38b2780 /* 22 vars */ <unfinished ...>
> > [pid 376785] [ 189] [00003fff9376726c] <... vfork resumed>) = 376786
> > [pid 376785] [ 189] [f37838210030e840] --- SIGSEGV {si_signo=SIGSEGV,
> > si_code=SEGV_BNDERR, si_addr=0xf37838210030e840, si_lower=NULL,
> > si_upper=NULL} ---
> > [pid 376786] [ 11] [00003fffaf368aa0] <... execve resumed>) = 0
> > [pid 376786] [ 45] [00003fffaf371188] brk(NULL) = 0x100157a6000
> > [pid 376786] [ 90] [00003fffaf374638] mmap(NULL, 8192,
> > PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x3fffaf391000
> > [pid 376786] [ 33] [00003fffaf373e90] access("/etc/ld.so.preload",
> > R_OK) = -1 ENOENT (No such file or directory)
> > [pid 376786] [ 286] [00003fffaf374340] openat(AT_FDCWD,
> > "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> > [pid 376785] [ 189] [????????????????] +++ killed by SIGSEGV +++
> >
> >
> > Replacing vfork() with fork() will eliminate this issue because fork()
> > creates a separate memory space, so the child’s operations cannot
> > corrupt the parent’s stack and both processes operate independently.
> >
> > _______________________________________________
> > busybox mailing list
> > busybox@busybox.net
> > https://lists.busybox.net/mailman/listinfo/busybox
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