seekdir/readdir patch .. Call for Review.

[Konstantin Belousov]

On Mon, May 04, 2015 at 10:52:42PM +0800, 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.
> >
> how long do I have to wait until I can commit  this and was kib's 
> comment a
> "do not commit"?
No, I only mean what I asked about.  I do not have strong objections about
the patch, but whatever is done in this regard, should clearly explain the
case it handles and related limitations (IMO).