Re: panic: general protection fault from uipc_sockaddr+0x4c

On Tue, Dec 8, 2020 at 9:30 AM Mark Johnston <markj_at_freebsd.org> wrote:
>
> On Tue, Dec 08, 2020 at 12:47:18PM +0100, Peter Holm wrote:
> > I just got this panic:
> >
> > Fatal trap 9: general protection fault while in kernel mode
> > cpuid = 9; apic id = 09
> > instruction pointer = 0x20:0xffffffff80bc6e22
> > stack pointer         = 0x28:0xfffffe0698887630
> > frame pointer         = 0x28:0xfffffe06988876b0
> > 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  = 45966 (fstat)
> > trap number  = 9
> > panic: general protection fault
> > cpuid = 9
> > time = 1607416693
> > KDB: stack backtrace:
> > db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0698887340
> > vpanic() at vpanic+0x181/frame 0xfffffe0698887390
> > panic() at panic+0x43/frame 0xfffffe06988873f0
> > trap_fatal() at trap_fatal+0x387/frame 0xfffffe0698887450
> > trap() at trap+0xa4/frame 0xfffffe0698887560
> > calltrap() at calltrap+0x8/frame 0xfffffe0698887560
> > --- trap 0x9, rip = 0xffffffff80bc6e22, rsp = 0xfffffe0698887630, rbp = 0xfffffe06988876b0 ---
> > __mtx_lock_sleep() at __mtx_lock_sleep+0xd2/frame 0xfffffe06988876b0
> > __mtx_lock_flags() at __mtx_lock_flags+0xe5/frame 0xfffffe0698887700
> > uipc_sockaddr() at uipc_sockaddr+0x4c/frame 0xfffffe0698887730
> > soo_fill_kinfo() at soo_fill_kinfo+0x11e/frame 0xfffffe0698887770
> > kern_proc_filedesc_out() at kern_proc_filedesc_out+0xb57/frame 0xfffffe0698887810
> > sysctl_kern_proc_filedesc() at sysctl_kern_proc_filedesc+0x7d/frame 0xfffffe0698887890
> > sysctl_root_handler_locked() at sysctl_root_handler_locked+0x9c/frame 0xfffffe06988878e0
> > sysctl_root() at sysctl_root+0x20d/frame 0xfffffe0698887960
> > userland_sysctl() at userland_sysctl+0x180/frame 0xfffffe0698887a10
> > sys___sysctl() at sys___sysctl+0x5f/frame 0xfffffe0698887ac0
> > amd64_syscall() at amd64_syscall+0x147/frame 0xfffffe0698887bf0
> > fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe0698887bf0
> > --- syscall (202, FreeBSD ELF64, sys___sysctl), rip = 0x8003948ea, rsp = 0x7fffffffc138, rbp = 0x7fffffffc170 ---
> >
> > https://people.freebsd.org/~pho/stress/log/log0004.txt
>
> So here the unpcb is freed, and indeed the file itself has been closed:
>
> $3 = {f_flag = 0x3, f_count = 0x0, f_data = 0x0, f_ops = 0xffffffff81901f50 <badfileops>,
>       f_vnode = 0x0, f_cred = 0xfffff80248beb600, f_type = 0x2, f_vnread_flags = 0x0,
>       {f_seqcount = {0x0, 0x0}, f_pipegen = 0x0}, f_nextoff = {0x0, 0x0},
>       f_vnun = {fvn_cdevpriv = 0x0, fvn_advice = 0x0}, f_offset = 0x0}
>
> However, it must have happened very recently because soo_fill_kinfo()
> dereferences fp->f_data and yet we did not panic due to a null
> dereference.
>
> kern_proc_filedesc_out() holds the fdtable shared lock thoughout all of
> this, which is supposed to prevent the table entry from being freed
> since that requires the exclusive lock.
>

export_file_to_sb drops the lock without it or kern_proc_filedesc_out
holding the file it's about to look at, though.