On Freitag, 17. Januar 2020 16:15:37 CET Greg Kurz wrote: > > > Hmm... this patch does a sanity check on 'count', not on 'msize'... > > > > Yes ... :) > > > > > I mean no matter what msize is, clipping count to msize - 11 gives a > > > chance to stop processing the entries before overflowing the transport > > > buffer. > > > > ... and no, this cannot happen if minimum msize of 4096 is forced already > > by Tversion. Maybe you now get my point -> It is about avoiding exactly > > such kind > I'm not sure to see how setting a minimum msize of 4096 at Tversion would > prevent the client to pass a higher 'count' argument and lure the server > into generating a bigger than msize response since it does not check > count < msize - 11 without patch 3.
That's correct, it requires patch 3 as well to prevent that. Without patch 3, if a (i.e. bad) client sends a 'count' parameter >> msize then the Treaddir request is processed by server to full extent according to 'count' and finally aborted by a transport error since server's response would exceed msize. > > of issues in the first place. Most file systems have a name limit of 255 > > bytes: > > > > https://en.wikipedia.org/wiki/Comparison_of_file_systems#Limits > > > > So by forcing a minimum 'msize' of 4096 you avoid having to deal with this > > issue (and similar ones) on Treaddir request level (and other request type > > handlers), including ReiserFS BTW because 4032+35 < 4096. > > Good to know for ReiserFS. > > > If you would allow smaller 'msize' values by Tversion, then you would need > > to suffer some kind of death when handling Treaddir with certain high > > file name length. Either a transport error (with an error message that a > > normal user would not be able to understand at all) or by returning an > > incomplete Treaddir response sequence with { Rreaddir count=0 }, or ... > > any other kind of death. > Ahh I now understand at last your argument about Rreaddir loosing data. > We may end up sending { Rreaddir count=0 } because the next entry is too > large... and thus end the readdir sequence. Yep. > Mentioning this explicitly > from the start would have been more clear for me ;-) Sorry for that. :) I thought I made it clear with the directory entries example. I try to be more clear next time. > This looks like yet another bug to me. It looks wrong to return this > special response if we have more entries to go. Also this could be the > client's _fault_ if it provides a ridiculously small value for count. > The current code will return count=0 all the same. > > In any case, I think v9fs_do_readdir() should only return 0 if there > are no more entries to read. It should error out otherwise, but I'm > not sure how... Patience please. I have to limit the scope of this patch series somewhere. I am aware about these issues, but if I add fixes for more and more edge cases (which already exist) as part of this patch series, it will become a never ending story. I just added those particular fixes to this series, because they were directly related to things I've changed here for the actual purpose of this patch set, which was and is: readdir latency optimization. > > > My point is that we're not going to check msize in Tversion in > > > order to to avoid multiple checks everywhere. We're going to do > > > it there because it is the only place where it makes sense to > > > do it. > > > > Also yes and no. Of course it just makes sense to handle it already at > > Tversion. But no, you could theoretically also allow much smaller minimum > > 'msize' value << 4096 (i.e. somewhere closely >7 as we discussed), then > > you > > would indeed still need to add msize checks at other places of the code > > base as you just found out now. So forcing a minimum 'msize' which is > > high enough, avoids having to add such individual checks and having to > > deal with them in some kind of unpleasant way. > > We still don't understand each other I'm afraid... we actually have > implicit 'msize' checks already for every single thing we write on > the wire: v9fs_packunpack() which detects when we're trying to write > passed the buffer. When this happens, it is propagated to the transport > which then disconnects, which is the painful thing you've been > experiencing with your readdir experiments. In the case of Rreaddir, it > really does make sense to try to avoid the disconnection like you do in > patch 3 because the readdir sequence allows _partial_ reads. Same goes > for Rread. But that's it. No other message in the protocol allows that, > so I've never thought of adding individual 'msize' checks anywhere else. > What would they do better than v9fs_packunpack() already does ? Right, but you realized that a min. msize of 4096 (in combination with patch 3) prevents the readdir data loss issue we discussed here (provided we have a "good" client sending count=msize-11), right? If so, I suggest I "try" to address your concerns you came up with here in the commit log message as far as I can, and would like to ask you to adjust the message later on according to your personal preference if required. Best regards, Christian Schoenebeck
