Panics in recent NFS server

Hi,

I recently upgraded a -CURRENT NFS server from 2021-05-12 to today (2021-05-31), and when the first NFS client attempted to connect, I got this panic:

panic: lookup: expected nul at 0xfffff800104b3002; string [dim]

cpuid = 0
time = 1622463863
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00747e89b0
vpanic() at vpanic+0x187/frame 0xfffffe00747e8a10
panic() at panic+0x43/frame 0xfffffe00747e8a70
lookup() at lookup+0xad2/frame 0xfffffe00747e8b10
nfsvno_namei() at nfsvno_namei+0x1a4/frame 0xfffffe00747e8bc0
nfsrvd_lookup() at nfsrvd_lookup+0x191/frame 0xfffffe00747e8eb0
nfsrvd_dorpc() at nfsrvd_dorpc+0xfab/frame 0xfffffe00747e90c0
nfssvc_program() at nfssvc_program+0x604/frame 0xfffffe00747e92a0
svc_run_internal() at svc_run_internal+0xa72/frame 0xfffffe00747e93d0
svc_run() at svc_run+0x250/frame 0xfffffe00747e9430
nfsrvd_nfsd() at nfsrvd_nfsd+0x33c/frame 0xfffffe00747e9590
nfssvc_nfsd() at nfssvc_nfsd+0x473/frame 0xfffffe00747e9aa0
sys_nfssvc() at sys_nfssvc+0xc7/frame 0xfffffe00747e9ac0
amd64_syscall() at amd64_syscall+0x12e/frame 0xfffffe00747e9bf0
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe00747e9bf0
--- syscall (155, FreeBSD ELF64, sys_nfssvc), rip = 0x8011aa59a, rsp = 0x7fffffffe4e8, rbp = 0x7fffffffe780 ---
KDB: enter: panic

__curthread ()
    at /share/dim/src/freebsd/src-dim/sys/amd64/include/pcpu_aux.h:55
55		__asm("movq %%gs:%P1,%0" : "=r" (td) : "n" (offsetof(struct pcpu,
(kgdb) #0  __curthread ()
    at /share/dim/src/freebsd/src-dim/sys/amd64/include/pcpu_aux.h:55
#1  doadump (textdump=textdump@entry=0)
    at /share/dim/src/freebsd/src-dim/sys/kern/kern_shutdown.c:399
#2  0xffffffff804cca5a in db_dump (dummy=<optimized out>,
    dummy2=<unavailable>, dummy3=<unavailable>, dummy4=<unavailable>)
    at /share/dim/src/freebsd/src-dim/sys/ddb/db_command.c:575
#3  0xffffffff804cc912 in db_command (last_cmdp=<optimized out>,
    cmd_table=<optimized out>, dopager=dopager@entry=1)
    at /share/dim/src/freebsd/src-dim/sys/ddb/db_command.c:482
#4  0xffffffff804cc58d in db_command_loop ()
    at /share/dim/src/freebsd/src-dim/sys/ddb/db_command.c:535
#5  0xffffffff804cfd06 in db_trap (type=<optimized out>, code=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/ddb/db_main.c:270
#6  0xffffffff80c69f17 in kdb_trap (type=type@entry=3, code=code@entry=0,
    tf=tf@entry=0xfffffe00747e88e0)
    at /share/dim/src/freebsd/src-dim/sys/kern/subr_kdb.c:727
#7  0xffffffff810d7aee in trap (frame=0xfffffe00747e88e0)
    at /share/dim/src/freebsd/src-dim/sys/amd64/amd64/trap.c:576
#8  <signal handler called>
#9  kdb_enter (why=0xffffffff812d3d27 "panic", msg=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/kern/subr_kdb.c:506
#10 0xffffffff80c1d248 in vpanic (
    fmt=0xffffffff8129dfef "%s: expected nul at %p; string [%s]\n",
    ap=<optimized out>, ap@entry=0xfffffe00747e8a50)
    at /share/dim/src/freebsd/src-dim/sys/kern/kern_shutdown.c:907
#11 0xffffffff80c1cfd3 in panic (
    fmt=0xffffffff81e9b9c8 <cnputs_mtx> "=\t)\201\377\377\377\377")
    at /share/dim/src/freebsd/src-dim/sys/kern/kern_shutdown.c:843
#12 0xffffffff80cfa992 in lookup (ndp=ndp@entry=0xfffffe00747e8d90)
    at /share/dim/src/freebsd/src-dim/sys/kern/vfs_lookup.c:919
#13 0xffffffff80b33f84 in nfsvno_namei (nd=nd@entry=0xfffffe00747e9100,
    ndp=ndp@entry=0xfffffe00747e8d90, dp=<optimized out>,
    dp@entry=0xfffff80010544380, islocked=<optimized out>, islocked@entry=0,
    exp=exp@entry=0xfffffe00747e8fd8, p=p@entry=0xfffffe00bbfa3000,
    retdirp=0xfffffe00747e8e78)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdport.c:597
#14 0xffffffff80b266a1 in nfsrvd_lookup (nd=0xfffffe00747e9100,
    isdgram=<optimized out>, dp=0xfffff80010544380, vpp=0xfffffe00747e9010,
    fhp=0xfffffe00747e9074, exp=0xfffffe00747e8fd8)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdserv.c:607
#15 0xffffffff80b1073b in nfsrvd_compound (nd=0xfffffe00747e9100, isdgram=0,
    tag=0xf <error: Cannot access memory at address 0xf>, taglen=6,
    minorvers=4294967294)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdsocket.c:1098
#16 nfsrvd_dorpc (nd=nd@entry=0xfffffe00747e9100, isdgram=isdgram@entry=0,
    tag=0xf <error: Cannot access memory at address 0xf>, taglen=6,
    minorvers=4294967294)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdsocket.c:626
#17 0xffffffff80b24c44 in nfs_proc (nd=0xfffffe00747e9100,
    xid=<optimized out>, xprt=0xfffff80003a14e00, rpp=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdkrpc.c:402
#18 nfssvc_program (rqst=0xfffff80010455800, xprt=0xfffff80003a14e00)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdkrpc.c:288
#19 0xffffffff80edead2 in svc_executereq (rqstp=0xfffff80010455800)
    at /share/dim/src/freebsd/src-dim/sys/rpc/svc.c:1037
#20 svc_run_internal (grp=<optimized out>, grp@entry=0xfffff800100e6100,
    ismaster=ismaster@entry=1)
    at /share/dim/src/freebsd/src-dim/sys/rpc/svc.c:1313
#21 0xffffffff80eddf80 in svc_run (pool=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/rpc/svc.c:1392
#22 0xffffffff80b251ec in nfsrvd_nfsd (td=<optimized out>,
    td@entry=0xfffffe00bbfa3000, args=args@entry=0xfffffe00747e9660)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdkrpc.c:561
#23 0xffffffff80b3ec93 in nfssvc_nfsd (td=0xfffffe00bbfa3000,
    uap=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/fs/nfsserver/nfs_nfsdport.c:3714
#24 0xffffffff80e6f647 in sys_nfssvc (td=0xfffffe00bbfa3000,
    uap=0xfffffe00bbfa33e8)
    at /share/dim/src/freebsd/src-dim/sys/nfs/nfs_nfssvc.c:111
#25 0xffffffff810d891e in syscallenter (td=<optimized out>)
    at /share/dim/src/freebsd/src-dim/sys/amd64/amd64/../../kern/subr_syscall.c:189
#26 amd64_syscall (td=0xfffffe00bbfa3000, traced=0)
    at /share/dim/src/freebsd/src-dim/sys/amd64/amd64/trap.c:1156
#27 <signal handler called>
#28 0x00000008011aa59a in ?? ()

Is anybody seeing this too? :)

I can probably bisect, but it'll take quite a while.

-Dimitry