Crash in CLEAR_IF_FIRST handling in tdb

[Volker Lendecke]

On Thu, Oct 04, 2012 at 05:02:55PM -0700, Jeremy Allison wrote:
> On Tue, Oct 02, 2012 at 04:35:31PM +0200, Volker Lendecke wrote:
> > Hi Rusty!
> > 
> > Under
> > http://git.samba.org/?p=vl/samba.git/.git;a=shortlog;h=refs/heads/tdb
> > find a patchset that for me fixes a crash in winbind in tdb.
> > For the explanation, see the second patch from the top.
> > 
> > Comments?
> 
> Ok, as requested I'm looking into this code :-).
> 
> I have a question about the second patch in the
> series, "tdb: Make tdb_expand robust against shrinking tdbs".
> 
> This adds an fstat into the tdb_expand_file()
> call just before calling ftruncate, and updates
> tdb->map_size and the passed in size, which
> is actually the value tdb->map_size (so the
> size variable is actually redundent, but
> that's another matter).

True. We could probably just dump the "size" parameter from
the expand_file function.

> tdb_expand_file() is called from tdb_expand(), in
> the code that looks like this:
> 
>         if (tdb_lock(tdb, -1, F_WRLCK) == -1) {
>                 TDB_LOG((tdb, TDB_DEBUG_ERROR, "lock failed in tdb_expand\n"));
>                 return -1;
>         }
> 
>         /* must know about any previous expansions by another process */
>         tdb->methods->tdb_oob(tdb, tdb->map_size, 1, 1);
> 
>         size = tdb_expand_adjust(tdb->map_size, size, tdb->page_size);
> 
>         /* expand the file itself */
>         if (!(tdb->flags & TDB_INTERNAL)) {
>                 if (tdb->methods->tdb_expand_file(tdb, tdb->map_size, size) != 0)
>                         goto fail;
>         }
> 
> However, inside tdb_oob() we already call
> fstat(tdb->fd, &st), and then set:
> tdb->map_size = st.st_size;
> before returning from the tdb_oob() call.
> 
> So we've already done an fstat and updated
> tdb->map_size under the F_WRLCK inside tdb_expand(),
> so why is this particular patch needed ?

Look at the code fragment in tdb_oob():

        if (st.st_size < (size_t)off + len) {
                if (!probe) {
                        /* Ensure ecode is set for log fn. */
                        tdb->ecode = TDB_ERR_IO;
                        TDB_LOG((tdb, TDB_DEBUG_FATAL,"tdb_oob len %u beyond eof at %u\n",
                                 (int)(off + len), (int)st.st_size));
                }
                return -1;
        }

So if the file has shrunk before tdb_oob, this won't do
anything but bail. tdb_oob is only to expand files, not
shrink. That's why that patch is necessary.

Volker

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