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.
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