fstat triggered INVARIANTS panic in memrw()

[Alan Cox]

On Sun, Jan 16, 2005 at 06:30:31PM -0800, Kris Kennaway wrote:
> On Sun, Jan 16, 2005 at 06:18:15PM -0800, Kris Kennaway wrote:
> > On Sun, Jan 16, 2005 at 05:47:46PM -0800, Kris Kennaway wrote:
> > > On Sun, Jan 16, 2005 at 03:13:49PM -0600, Alan Cox wrote:
> > > 
> > > > The "deadc0de" passed to generic_copyout() comes from the following
> > > > lines in devfs_read_f(c51773b8,eed96c84,ca75c800,flags=0):
> > > > 
> > > >         if ((flags & FOF_OFFSET) == 0)
> > > >                 uio->uio_offset = fp->f_offset;
> > > > 
> > > > Can you print the contents of the file structure?
> > > 
> > > (kgdb) frame 28
> > > #28 0xc04d8d91 in devfs_read_f (fp=0xc25f5dd0, uio=0xe7275c84, cred=0xc3540380, flags=0, td=0xc3c34170)
> > >     at ../../../fs/devfs/devfs_vnops.c:931
> > > 931             error = dsw->d_read(dev, uio, ioflag);
> > > (kgdb) print *fp
> > > $1 = {f_list = {le_next = 0xc25f5bf4, le_prev = 0xc25f52a8}, f_type = 1, f_data = 0xc22f8200, f_flag = 1,
> > >   f_mtxp = 0xc2251fd0, f_ops = 0xc074c140, f_cred = 0xc2b2a900, f_count = 2, f_vnode = 0xc3c6fbdc,
> > >   f_offset = 3735929054, f_gcflag = 0, f_msgcount = 0, f_seqcount = 1, f_nextoff = 3263609792}
> > 
> > 3735929054 = 0xdeadc0de.  This same struct file appears all the way
> > back to the syscall frame.  I wonder if fstat is racing with a tty
> > device removal or something (it's certainly racing with something,
> > e.g.:
> 
> Devices may not be to blame; I was able to trigger this by running
> fstat in a loop and then running 'make' in /usr/ports/misc/screen
> (with the idea of testing the tty hypothesis :)
> 
> An interesting datapoint is that none of the non-i386 package machines
> have hit this problem, but the i386 machines can't stay up for more
> than a few minutes under load (which translates to only a few fstat
> invocations).

The field f_offset is 64 bits wide.  If this were a race between use
and deallocation of the file structure within the kernel, then I would
expect f_offset's value to be 0xdeadc0dedeadc0de, not
0x00000000deadc0de.  More likely than not, the 0xdeadc0de is being
passed in from user level.  The i386 kernel is just not handling it
gracefully.  

Alan