Panic in prison_alloc() on boot

[Ryan Stone]

Sorry for the late reply.  Panicking this system is a bit painful, but
I found some time to do it today.

Strangely, it's actually cred that is NULL, not cred->cr_prison:

(kgdb) p cred
$7 = (struct ucred *) 0x0
(kgdb) disassemble
Dump of assembler code for function prison_allow:
   0xffffffff80ac33e0 <+0>:     push   %rbp
   0xffffffff80ac33e1 <+1>:     mov    %rsp,%rbp
=> 0xffffffff80ac33e4 <+4>:     mov    0x30(%rdi),%rax
   0xffffffff80ac33e8 <+8>:     and    0xf8(%rax),%esi
   0xffffffff80ac33ee <+14>:    mov    %esi,%eax
   0xffffffff80ac33f0 <+16>:    pop    %rbp
   0xffffffff80ac33f1 <+17>:    retq
End of assembler dump.
(kgdb) info reg $rdi
rdi            0x0      0

However, if I go up a frame, things look fine?

(kgdb) up
#13 0xffffffff82c22531 in nullfs_mount (mp=0xfffff801a483d000)
    at /usr/src/sys/fs/nullfs/null_vfsops.c:88
88              if (!prison_allow(td->td_ucred, PR_ALLOW_MOUNT_NULLFS))
(kgdb) p td->td_ucred
$8 = (struct ucred *) 0xfffff801854c1700

This appears to be a miscompilation, but I've blown away
/usr/obj/usr/src multiple times and rebuilt and got this same error
every time.  But looking at the disassembly, something is definitely
wrong:

   0xffffffff82c22517 <+23>:    mov    %gs:0x0,%r14
   0xffffffff82c22520 <+32>:    mov    0x150(%r14),%rdi
   0xffffffff82c22527 <+39>:    mov    $0x100,%esi
   0xffffffff82c2252c <+44>:    callq  0xffffffff80ac33e0 <prison_allow>
=> 0xffffffff82c22531 <+49>:    test   %eax,%eax

(kgdb) p &((struct thread*)0)->td_ucred
$10 = (struct ucred **) 0x158

It uses offset 0x150 to get the cred, but the debug info claims that
td_ucred is at offset 0x158.  If I print out the pointer at that
offset, it looks reasonable:

(kgdb) p *td->td_ucred
$11 = {cr_ref = 107, cr_uid = 0, cr_ruid = 0, cr_svuid = 0, cr_ngroups = 1,
  cr_rgid = 0, cr_svgid = 0, cr_uidinfo = 0xfffff80106617000,
  cr_ruidinfo = 0xfffff80106617000, cr_prison = 0xffffffff8187cb70 <prison0>,
  cr_loginclass = 0xfffff8019fa43b00, cr_flags = 0, cr_pspare2 = {0x0, 0x0},
  cr_label = 0x0, cr_audit = {ai_auid = 4294967295, ai_mask = {am_success = 0,
      am_failure = 0}, ai_termid = {at_port = 0, at_type = 4, at_addr = {0, 0,
        0, 0}}, ai_asid = 0, ai_flags = 0}, cr_groups = 0xfffff801854c179c,
  cr_agroups = 16, cr_smallgroups = {0 <repeats 16 times>}}



I'm really at a loss at to what to try next.  Build with
MAKEOBJDIRPREFIX set to something else to get rid of any lingering
possibility of an issue in my objdir, I guess?