freebsd boot error: panic: Bad entry start/end for new stack entry

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: d6aab066 While working on PR/238796 I discovered an unused..
git tree: freebsd
console output: https://syzkaller.appspot.com/x/log.txt?x=14a7cb41a00000
dashboard link: https://syzkaller.appspot.com/bug?extid=fd9ae6788da4469eb75d

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+fd9ae6...@syzkaller.appspotmail.com

panic: Bad entry start/end for new stack entry
cpuid = 0
time = 1561512892
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x47/frame
0xfffffe0016ae3370
vpanic() at vpanic+0x1e0/frame 0xfffffe0016ae33d0
panic() at panic+0x43/frame 0xfffffe0016ae3430
vm_map_stack_locked() at vm_map_stack_locked+0x364/frame 0xfffffe0016ae34a0
vm_map_stack() at vm_map_stack+0x11f/frame 0xfffffe0016ae3500
exec_new_vmspace() at exec_new_vmspace+0x4bd/frame 0xfffffe0016ae3570
exec_elf64_imgact() at exec_elf64_imgact+0x1262/frame 0xfffffe0016ae3670
kern_execve() at kern_execve+0x7d8/frame 0xfffffe0016ae39c0
start_init() at start_init+0x249/frame 0xfffffe0016ae3a60
fork_exit() at fork_exit+0xb0/frame 0xfffffe0016ae3ab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0016ae3ab0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 1 tid 100002 ]
Stopped at kdb_enter+0x6a: movq $0,kdb_why
db>


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[Mark Johnston]

#syz fix: Revert r349393, which leads to an assertion failure on bootup, in vm_map_stack_locked.