[Bug 274375] page fault while load kernel module virtio

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274375

            Bug ID: 274375
           Summary: page fault while load kernel module virtio
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: zlei@FreeBSD.org

This is initially observed while checking sysctl knobs (loader tunables) in
modules.
I managed to narrow it down.

Tested with current/15 and 14.0-BETA5 on amd64 machine.

Steps to repeat:

```
# kldload sdt
# kldload virtio.ko
```

The kernel backtrace:

```
 dumped core - see /var/crash/vmcore.3

Sun Oct  8 10:25:33 CST 2023

FreeBSD  14.0-BETA5 FreeBSD 14.0-BETA5 #1 releng/14.0-n265192-d67558ef3149: Sat
Oct  7 18:03:25 CST 2023    
zlei@:/usr/obj/home/zlei/freebsd-src-releng-14.0/amd64.amd64/sys/GENERIC  amd64

panic: page fault

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Reading symbols from /boot/kernel/kernel...
Reading symbols from /usr/lib/debug//boot/kernel.beta5/kernel.debug...
interface virtio.1 already present in the KLD 'kernel'!


Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 02
fault virtual address   = 0x0
fault code              = supervisor read data, page not present
instruction pointer     = 0x20:0xffffffff8321b134
stack pointer           = 0x28:0xfffffe00b04e7920
frame pointer           = 0x28:0xfffffe00b04e7950
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         = 97755 (kldload)
rdi: fffff80178d6a6a0 rsi: fffffe00b04e7928 rdx: 0000000000000000
rcx: ffffffff88ff5470  r8: 0000000000000004  r9: 00000000000000f3
rax: 0000000000000000 rbx: fffffe00b04e7974 rbp: fffffe00b04e7950
r10: 0000000000000002 r11: ffffffffffffff90 r12: fffff80178bfd780
r13: fffffe00b04e7974 r14: fffff80178bfd7c0 r15: ffffffff88ff5468
trap number             = 12
panic: page fault
cpuid = 1
time = 1696731847
KDB: stack backtrace:
#0 0xffffffff80b9017d at kdb_backtrace+0x5d
#1 0xffffffff80b43282 at vpanic+0x132
#2 0xffffffff80b43143 at panic+0x43
#3 0xffffffff8100b81c at trap_fatal+0x40c
#4 0xffffffff8100b86f at trap_pfault+0x4f
#5 0xffffffff80fe2848 at calltrap+0x8
#6 0xffffffff80b0f61c at linker_file_unload+0xcc
#7 0xffffffff810be9c8 at link_elf_load_file+0x198
#8 0xffffffff80b0ee93 at linker_load_module+0x9e3
#9 0xffffffff80b10b9a at kern_kldload+0x16a
#10 0xffffffff80b10cbc at sys_kldload+0x5c
#11 0xffffffff8100c0d9 at amd64_syscall+0x109
#12 0xffffffff80fe315b at fast_syscall_common+0xf8
Uptime: 16m43s
Dumping 727 out of 8100 MB:..3%..11%..22%..31%..42%..51%..62%..71%..82%..91%

__curthread ()
    at /home/zlei/freebsd-src-releng-14.0/sys/amd64/include/pcpu_aux.h:57
57              __asm("movq %%gs:%P1,%0" : "=r" (td) : "n" (offsetof(struct
pcpu,
(kgdb) #0  __curthread ()
    at /home/zlei/freebsd-src-releng-14.0/sys/amd64/include/pcpu_aux.h:57
#1  doadump (textdump=<optimized out>)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_shutdown.c:405
#2  0xffffffff80b42e17 in kern_reboot (howto=260)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_shutdown.c:526
#3  0xffffffff80b432ef in vpanic (fmt=0xffffffff81135a78 "%s", 
    ap=ap@entry=0xfffffe00b04e7770)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_shutdown.c:970
#4  0xffffffff80b43143 in panic (fmt=<unavailable>)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_shutdown.c:894
#5  0xffffffff8100b81c in trap_fatal (frame=0xfffffe00b04e7860, eva=0)
    at /home/zlei/freebsd-src-releng-14.0/sys/amd64/amd64/trap.c:952
#6  0xffffffff8100b86f in trap_pfault (frame=0xfffffe00b04e7860, 
    usermode=false, signo=<optimized out>, ucode=<optimized out>)
    at /home/zlei/freebsd-src-releng-14.0/sys/amd64/amd64/trap.c:760
#7  <signal handler called>
#8  0xffffffff8321b134 in sdt_kld_unload_try () from /boot/kernel/sdt.ko
#9  0xffffffff80b0f61c in linker_file_unload (file=0xfffffe00b04e7974, 
    file@entry=0xfffff8015497b480, flags=flags@entry=1)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_linker.c:673
#10 0xffffffff810be9c8 in link_elf_load_file (cls=<optimized out>, 
    filename=<optimized out>, result=<optimized out>)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/link_elf_obj.c:1241
#11 0xffffffff80b0ee93 in LINKER_LOAD_FILE (
    cls=0xffffffff817550a8 <link_elf_class>, result=0xfffffe00b04e7c00, 
    filename=<optimized out>) at ./linker_if.h:214
#12 linker_load_file (filename=<optimized out>, result=<optimized out>)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_linker.c:459
#13 linker_load_module (kldname=kldname@entry=0xfffff80154965800 "virtio.ko", 
    modname=modname@entry=0x0, parent=parent@entry=0x0, 
    verinfo=verinfo@entry=0x0, lfpp=lfpp@entry=0xfffffe00b04e7d90)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_linker.c:2203
#14 0xffffffff80b10b9a in kern_kldload (td=td@entry=0xfffffe00b0d3a3a0, 
    file=file@entry=0xfffff80154965800 "virtio.ko", 
    fileid=fileid@entry=0xfffffe00b04e7de4)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_linker.c:1162
#15 0xffffffff80b10cbc in sys_kldload (td=0xfffffe00b0d3a3a0, 
    uap=<optimized out>)
    at /home/zlei/freebsd-src-releng-14.0/sys/kern/kern_linker.c:1185
#16 0xffffffff8100c0d9 in syscallenter (td=0xfffffe00b0d3a3a0)
    at
/home/zlei/freebsd-src-releng-14.0/sys/amd64/amd64/../../kern/subr_syscall.c:187
#17 amd64_syscall (td=0xfffffe00b0d3a3a0, traced=0)
    at /home/zlei/freebsd-src-releng-14.0/sys/amd64/amd64/trap.c:1197
#18 <signal handler called>
#19 0x000013e93dad93fa in ?? ()
Backtrace stopped: Cannot access memory at address 0x13e93c3e81c8
(kgdb) 
```

Note: `kldload virtio.ko` can trigger the panic but `kldload virtio` will not.

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