Re: seekdir/readdir patch .. Call for Review.

Julian Elischer wrote:
> On 5/3/15 10:33 PM, Jilles Tjoelker wrote:
> > On Fri, May 01, 2015 at 07:17:42PM +0300, Konstantin Belousov
> > wrote:
> >> On Fri, May 01, 2015 at 03:04:51PM +0800, Julian Elischer wrote:
> >>> if you are interested in readdir(3), seekdir(3) and telldir(3)
> >>> then
> >>> you should look at
> >>>     https://reviews.freebsd.org/D2410
> >>> this patches around a problem in seekdir() that breaks Samba.
> >>> Seekdir(3) will not work as expected when files prior to the
> >>> point of
> >>> interest in directory have been deleted since the directory was
> >>> opened.
> >>> Windows clients using Samba cause both these things to happen,
> >>> causing
> >>> the next readdir(3) after the bad seekdir(3) to skip some entries
> >>> and
> >>> return the wrong file.
> >>> Samba only needs to step back a single directory entry in the
> >>> case
> >>> where it reads an entry and then discovers it can't fit it into
> >>> the
> >>> buffer it is sending to the windows client. It turns out we can
> >>> reliably cater to Samba's requirement because the "last returned
> >>> element" is always still in memory, so with a little care, we can
> >>> set our filepointer back to it safely. (once)
> >>> seekdir and readdir (and telldir()) need a complete rewrite along
> >>> with
> >>> getdirentries() but that is more than a small edit like this.
> >> Can you explain your expectations from the whole readdir() vs.
> >> parallel
> >> directory modifications interaction ?  From what I understood so
> >> far,
> >> there is unlocked modification of the container and parallel
> >> iterator
> >> over the same container.  IMO, in such situation, whatever tweaks
> >> you
> >> apply to the iterator, it is still cannot be made reliable.
> >> Before making single-purpose changes to the libc readdir and
> >> seekdir
> >> code, or to the kernel code, it would be useful to state exact
> >> behaviour
> >> of the dirent machinery we want to see. No, 'make samba works in
> >> my
> >> situation' does not sound good enough.
> > Consider the subsequence of entries that existed at opendir() time
> > and
> > were not removed until now. This subsequence is clearly defined and
> > does
> > not have concurrency problems. The order of this subsequence must
> > remain
> > unchanged and seekdir() must be correct with respect to this
> > subsequence.
> >
> > Additionally, two other kinds of entries may be returned. New
> > entries
> > may be inserted anywhere in between the entries of the subsequence,
> > and
> > removed entries may be returned as if they were still part of the
> > subsequence (so that not every readdir() needs a system call).
> >
> > A simple implementation for UFS-style directories is to store the
> > offset
> > in the directory (all bits of it, not masking off the lower 9
> > bits).
> > This needs d_off or similar in struct dirent. The kernel
> > getdirentries()
> > then needs a similar loop as the old libc seekdir() to go from the
> > start
> > of the 512-byte directory block to the desired entry (since an
> > entry may
> > not exist at the stored offset within the directory block).
> >
> > This means that a UFS-style directory cannot be compacted (existing
> > entries moved from higher to lower offsets to fill holes) while it
> > is
> > open for reading. An NFS exported directory is always open for
> > reading.
> >
> > This also means that duplicate entries can only be returned if that
> > particular filename was deleted and created again.
> >
> > Without kernel support, it is hard to get telldir/seekdir
> > completely
> > reliable. The current libc implementation is wrong since the
> > "holes"
> > within the block just disappear and change the offsets of the
> > following
> > entries; the kernel cannot fix this using entries with d_fileno = 0
> > since it cannot know, in the general case, how long the deleted
> > entry
> > was in the filesystem-independent dirent format. My previous idea
> > of
> > storing one d_fileno during telldir() is wrong since it will fail
> > if
> > that entry is deleted.
> >
> > If you do not care about memory usage (which probably is already
> > excessive with the current libc implementation), you could store at
> > telldir() time the offset of the current block returned by
> > getdirentries() and the d_fileno of all entries already returned in
> > the
> > current block.
> >
> > The D2410 patch can conceptually work for what Samba needs,
> > stepping
> > back one directory entry. I will comment on it.
> thanks
> 
> your comment is correct, but I don't think it really matters because
> I'm only claiming
> to fix a really small set of possible usages..  I might add a
> sentence
> in the seekdir
> man page specifying what does and doesn't work.
> 
Ok, I think I finally understand the bug that this patch is fixing...
If telldir() is called when at the end of a block read by getdirentries(),
it sets loc_seek to the seek position for the block and loc_loc to the
offset of the end of the block.
This is somewhat bogus, but works for the simple read-only case for seekdir()
because it does a readdir()->getdirentries() of the next block.

This patch fixes the telldir entry to refer to the beginning of the next
block, so that seekdir() seeks to the correct block.

I could argue that a more correct version of your fixup function would
scan the telldir list for a match instead of just testing the first one.
However, I now see that once fixed up, it won't match it again, so your
code seems safe, just not completely correct.

I'll take a closer look at it and put something in pahbricator later
to-day, but I think it's ok. I would suggest describing the bug would
help simple types like me figure it out.

Thanks for your work on this, rick

> 
> 
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