GPF on boot with devmatch

Hi,

I'm seeing this panic at boot after upgrading from r366217 to r366364,
and continues to exist for r366421 (but I haven't find out the exact
change that caused it).  Preloading the relevant kernel modules
(uhid.ko, ums.ko and wmt.ko) seems to make the kernel boot correctly.

This is not reproducible on my laptop, which will load many more kernel
modules.

===
Autoloading module: uhid.ko
Autoloading module: wmt.ko


Fatal trap 9: general protection fault while in kernel mode
cpuid = 2; apic id = 04
instruction pointer     = 0x20:0xffffffff806ad6eb
stack pointer           = 0x28:0xfffffe01850cd960
frame pointer           = 0x28:0xfffffe01850cd9e0
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 740 (devmatch)
trap number             = 9
panic: general protection fault
cpuid = 3
time = 1601866799
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame
0xfffffe01850cd670
vpanic() at vpanic+0x182/frame 0xfffffe01850cd6c0
panic() at panic+0x43/frame 0xfffffe01850cd720
trap_fatal() at trap_fatal+0x387/frame 0xfffffe01850cd780
trap() at trap+0xa4/frame 0xfffffe01850cd890
calltrap() at calltrap+0x8/frame 0xfffffe01850cd890
--- trap 0x9, rip = 0xffffffff806ad6eb, rsp = 0xfffffe01850cd960, rbp =
0xfffffe01850cd9e0 ---
sysctl_devices() at sysctl_devices+0x24b/frame 0xfffffe01850cd9e0
sysctl_root_handler_locked() at sysctl_root_handler_locked+0x9c/frame
0xfffffe01850cda30
sysctl_root() at sysctl_root+0x20a/frame 0xfffffe01850cdab0
userland_sysctl() at userland_sysctl+0x17d/frame 0xfffffe01850cdb60
sys___sysctl() at sys___sysctl+0x5f/frame 0xfffffe01850cdc10
amd64_syscall() at amd64_syscall+0x135/frame 0xfffffe01850cdd30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe01850cdd30
--- syscall (202, FreeBSD ELF64, sys___sysctl), rip = 0x80038968a, rsp =
0x7fffffffd988, rbp = 0x7fffffffd9c0 ---
===

sysctl_devices+0x24b (0x6dab) was:

        sb->s_len += strlen(p);
    6d50:       4c 89 e7                mov    %r12,%rdi
    6d53:       e8 00 00 00 00          callq  6d58 <sysctl_devices+0x1f8>
    6d58:       48 01 45 b0             add    %rax,-0x50(%rbp)
    6d5c:       48 8d 7d 88             lea    -0x78(%rbp),%rdi
        sbuf_putc(&sb, '\0');
    6d60:       31 f6                   xor    %esi,%esi
    6d62:       e8 00 00 00 00          callq  6d67 <sysctl_devices+0x207>
        MPASS((sb->s_flags & SBUF_INCLUDENUL) == 0);
    6d67:       f6 45 b8 02             testb  $0x2,-0x48(%rbp)
    6d6b:       0f 85 10 01 00 00       jne    6e81 <sysctl_devices+0x321>
        if (sb->s_error != 0)
    6d71:       83 7d a0 00             cmpl   $0x0,-0x60(%rbp)
    6d75:       0f 85 8c 00 00 00       jne    6e07 <sysctl_devices+0x2a7>
        p = EOB(sb);
    6d7b:       4c 8b 65 88             mov    -0x78(%rbp),%r12
    6d7f:       48 8b 45 b0             mov    -0x50(%rbp),%rax
        *p = '\0';      /* sbuf buffer isn't NUL terminated until
sbuf_finish() */
    6d83:       41 c6 04 04 00          movb   $0x0,(%r12,%rax,1)
        space = SPACE(sb);
    6d88:       4c 8b 6d a8             mov    -0x58(%rbp),%r13
    6d8c:       4c 2b 6d b0             sub    -0x50(%rbp),%r13
        if (space <= 1) {
    6d90:       49 83 fd 01             cmp    $0x1,%r13
    6d94:       77 09                   ja     6d9f <sysctl_devices+0x23f>
                sb->s_error = ENOMEM;
    6d96:       c7 45 a0 0c 00 00 00    movl   $0xc,-0x60(%rbp)
    6d9d:       eb 68                   jmp    6e07 <sysctl_devices+0x2a7>
    6d9f:       49 01 c4                add    %rax,%r12
        return (dev->parent);
    6da2:       48 8b 7b 28             mov    0x28(%rbx),%rdi
        if (parent == NULL) {
    6da6:       48 85 ff                test   %rdi,%rdi
    6da9:       74 4b                   je     6df6 <sysctl_devices+0x296>
        KOBJOPLOOKUP(((kobj_t)_dev)->ops,bus_child_location_str);
    6dab:       48 8b 07                mov    (%rdi),%rax
    6dae:       48 c7 c2 00 00 00 00    mov    $0x0,%rdx
    6db5:       0f b6 0d 00 00 00 00    movzbl 0x0(%rip),%ecx        #
6dbc <sysctl_devices+0x25c>
    6dbc:       4c 8b 04 c8             mov    (%rax,%rcx,8),%r8
    6dc0:       49 39 10                cmp    %rdx,(%r8)
    6dc3:       74 22                   je     6de7 <sysctl_devices+0x287>
    6dc5:       48 8d 34 c8             lea    (%rax,%rcx,8),%rsi
    6dc9:       48 89 7d d0             mov    %rdi,-0x30(%rbp)
    6dcd:       48 8b b8 00 08 00 00    mov    0x800(%rax),%rdi
    6dd4:       48 c7 c2 00 00 00 00    mov    $0x0,%rdx
    6ddb:       e8 00 00 00 00          callq  6de0 <sysctl_devices+0x280>
    6de0:       48 8b 7d d0             mov    -0x30(%rbp),%rdi
    6de4:       49 89 c0                mov    %rax,%r8
        rc = ((bus_child_location_str_t *) _m)(_dev, _child, _buf, _buflen);
    6de7:       48 89 de                mov    %rbx,%rsi