foo=$(echo bar) freezes the shell

[nuta]

BusyBox v1.31.1 (2021-11-04 11:38:59 UTC) built-in shell (ash)

[   0.038] [1:/bin/sh] syscall: open(44af99, 2, 0, 0, 0, 0)
[   0.038] [1:/bin/sh] open("/dev/tty")
[   0.040] [1:/bin/sh] syscall: fcntl(3, 406, a, a, 48, 0)
[   0.041] [1:/bin/sh] syscall: fcntl(a, 2, 1, a, 48, 0)
[   0.041] [1:/bin/sh] syscall: close(3, 0, 0, 0, 0, 0)
[   0.042] [1:/bin/sh] syscall: ioctl(a, 540f, 9ffffed14, 0, 0, 0)
[   0.043] [1:/bin/sh] syscall: getpgid(0, 540f, 9ffffed14, 0, 0, 0)
[   0.044] [1:/bin/sh] syscall: rt_sigaction(14, 0, 9ffffec00, 8, 0, 0)
[   0.045] [1:/bin/sh] syscall: rt_sigaction(14, 9ffffebe0, 0, 8, 0, 0)
[   0.046] [1:/bin/sh] syscall: rt_sigaction(16, 0, 9ffffec00, 8, 0, 0)
[   0.046] [1:/bin/sh] syscall: rt_sigaction(16, 9ffffebe0, 0, 8, 0, 0)
[   0.046] [1:/bin/sh] syscall: rt_sigaction(15, 0, 9ffffec00, 8, 0, 0)
[   0.047] [1:/bin/sh] syscall: setpgid(0, 1, 0, 8, 0, 0)
[   0.048] [1:/bin/sh] syscall: ioctl(a, 5410, 9ffffed04, 8, 0, 0)
[   0.049] [1:/bin/sh] syscall: wait4(ffffffffffffffff, 9ffffebec, 3, 0, 0, 0)
# x=$(echo)
[   4.895] [1:/bin/sh] syscall: pipe(9ffffea40, 1, 451400, 2, 101010101010101, 3)
[   4.896] [1:/bin/sh] syscall: rt_sigprocmask(0, 449d58, 9ffffe910, 8, 452150, 3)
[   4.896] WARN: unimplemented system call: rt_sigprocmask (n=14)
[   4.896] WARN: rt_sigprocmask: error: ENOSYS: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff80000014ba77  kerla::syscalls::SyscallHandler::do_dispatch()+0x9c7
    #4: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #5: ffff8000001b6dbc  x64_handle_syscall()+0x9c
    #6: ffff800000100235  syscall_entry()+0x3d

[   4.897] [1:/bin/sh] syscall: fork(0, 449d58, 9ffffe910, 8, 452150, 3)
[   4.898] [1:/bin/sh] syscall: rt_sigprocmask(2, 9ffffe910, 0, 8, 452150, 3)
[   4.898] WARN: unimplemented system call: rt_sigprocmask (n=14)
[   4.898] WARN: rt_sigprocmask: error: ENOSYS: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff80000014ba77  kerla::syscalls::SyscallHandler::do_dispatch()+0x9c7
    #4: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #5: ffff8000001b6dbc  x64_handle_syscall()+0x9c
    #6: ffff800000100235  syscall_entry()+0x3d

[   4.899] [1:/bin/sh] syscall: close(4, 0, 0, 0, 0, 0)
[   4.899] [1:/bin/sh] syscall: read(3, 9ffffea48, 80, 0, 0, 0)
[   4.900] [2:/bin/sh] syscall: gettid(0, 449d58, 450438, 8, 452150, 3)
[   4.900] WARN: unimplemented system call: gettid (n=186)
[   4.900] WARN: gettid: error: ENOSYS: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff80000014ba77  kerla::syscalls::SyscallHandler::do_dispatch()+0x9c7
    #4: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #5: ffff8000001b6dbc  x64_handle_syscall()+0x9c
    #6: ffff800000100235  syscall_entry()+0x3d

[   4.901] [2:/bin/sh] syscall: rt_sigprocmask(2, 9ffffe910, 0, 8, 452150, 3)
[   4.901] WARN: unimplemented system call: rt_sigprocmask (n=14)
[   4.901] WARN: rt_sigprocmask: error: ENOSYS: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff80000014ba77  kerla::syscalls::SyscallHandler::do_dispatch()+0x9c7
    #4: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #5: ffff8000001b6dbc  x64_handle_syscall()+0x9c
    #6: ffff800000100235  syscall_entry()+0x3d

[   4.902] [2:/bin/sh] syscall: rt_sigaction(2, 9ffffe820, 0, 8, 452150, 3)
[   4.903] [2:/bin/sh] syscall: rt_sigaction(f, 9ffffe820, 0, 8, 452150, 3)
[   4.903] [2:/bin/sh] syscall: rt_sigaction(3, 9ffffe820, 0, 8, 452150, 3)
[   4.903] [2:/bin/sh] syscall: close(3, 0, 0, 0, 0, 0)
[   4.903] [2:/bin/sh] syscall: dup2(4, 1, 0, 0, 21, 0)
[   4.904] [2:/bin/sh] syscall: close(4, 0, 0, 0, 0, 0)
[   4.906] [2:/bin/sh] syscall: wait4(ffffffffffffffff, 9ffffe7ac, 1, 0, 0, 0)
[   4.908] [2:/bin/sh] syscall: exit_group(0, 0, 451020, 0, 452050, 0)
[   4.908] WARN: unimplemented system call: exit_group (n=231)
[   4.908] WARN: exit_group: error: ENOSYS: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff80000014ba77  kerla::syscalls::SyscallHandler::do_dispatch()+0x9c7
    #4: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #5: ffff8000001b6dbc  x64_handle_syscall()+0x9c
    #6: ffff800000100235  syscall_entry()+0x3d

[   4.909] [2:/bin/sh] syscall: exit(0, 0, 3c, 0, 452050, 0)
[   4.909] WARN: read: error: EINTR: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff8000001ada17  <kerla::result::Error as c...la::result::Errno>>::from()+0x17
    #4: ffff8000001ab467  <T as core::convert::Into<U>>::into()+0x17
    #5: ffff800000136cfe  kerla::process::wait_queue...e::sleep_signalable_until()+0x1ae
    #6: ffff800000152e4c  <kerla::pipe::PipeReader a...s::inode::FileLike>::read()+0x7c
    #7: ffff80000012dfcd  kerla::fs::opened_file::OpenedFile::read()+0x14d
    #8: ffff800000146276  kerla::syscalls::read::<im...SyscallHandler>::sys_read()+0x1d6

[   4.909] delivering 17 to PId(1)
[   4.910] [1:/bin/sh] syscall: rt_sigreturn(11, 0, 0, 0, 0, 0)
[   4.910] WARN: rt_sigreturn: error: EINTR: This error originates from:
    #1: ffff800000214179  kerla::printk::capture_backtrace()+0x79
    #2: ffff8000001ad443  kerla::result::Error::new()+0x23
    #3: ffff8000001ada17  <kerla::result::Error as c...la::result::Errno>>::from()+0x17
    #4: ffff8000001ab467  <T as core::convert::Into<U>>::into()+0x17
    #5: ffff8000001477f4  kerla::syscalls::rt_sigret...andler>::sys_rt_sigreturn()+0x44
    #6: ffff80000014b577  kerla::syscalls::SyscallHandler::do_dispatch()+0x4c7
    #7: ffff80000014a87c  kerla::syscalls::SyscallHandler::dispatch()+0x1ac
    #8: ffff8000001b6dbc  x64_handle_syscall()+0x9c

[   4.911] [1:/bin/sh] syscall: read(3, 9ffffea48, 80, 0, 0, 0)
[   4.912] [1:/bin/sh] syscall: read(3, 9ffffea48, 80, 0, 0, 0)

[freezes here]