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authorhttps://me.yahoo.com/a/g3Ccalpj0NhN566pHbUl6i9QF0QEkrhlfPM-#b1c14 <diana@web>2015-02-16 20:08:03 +0100
committerGNU Hurd web pages engine <web-hurd@gnu.org>2015-02-16 20:08:03 +0100
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-[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]]
-
-[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
-id="license" text="Permission is granted to copy, distribute and/or modify this
-document under the terms of the GNU Free Documentation License, Version 1.2 or
-any later version published by the Free Software Foundation; with no Invariant
-Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
-is included in the section entitled [[GNU Free Documentation
-License|/fdl]]."]]"""]]
-
-[[!tag open_issue_hurd]]
-
-[RPC to self with rendez-vous leading to duplicate port
-destroy](http://lists.gnu.org/archive/html/bug-hurd/2011-03/msg00045.html)
-
-IRC, freenode, #hurd, 2011-03-14
-
- <antrik> youpi: I wonder, why does the root FS call diskfs_S_dir_lookup()
- at all?...
- <youpi> errr, because a client asked for it?
- <youpi> (problem with RPCs is you can't easily know where they come from :)
- )
- <youpi> (especially when it's the root fs...)
- <antrik> ah, it's about a client request... didn't see that
- <youpi> well, I just said "is called", yes
- <antrik> I do not really understand though why it tries to reauthenticate
- against itself...
- <antrik> I fear my memory of the lookup mechanism grew a bit dim
- <youpi> see the source
- <youpi> it's about a translated entry
- <antrik> (and I never fully understood some aspects anyways...)
- <youpi> it needs to start the translated entry as another user, possibly
- <antrik> yes, but a translated entry normally would be served by *another*
- process?...
- <youpi> sure, but ext2fs has to prepare it
- <youpi> thus reauthenticate to prepare the correct set of rights
- <antrik> prepare what?
- <youpi> rights
- <youpi> so the process is not root, doesn't have / opened as root, etc.
- <antrik> rights for what?
- <youpi> err, about everything
- <antrik> IIRC the reauthentication is done by the parent FS on the port to
- the *translated* node
- <antrik> and the translated node should be a different process?...
- <youpi> that's not what I read in the source
- <youpi> fshelp_fetch_root
- <youpi> ports[INIT_PORT_CRDIR] = reauth (getcrdir ());
- <youpi> here, getcrdir() returns ext2fs itself
- <antrik> well, perhaps the issue is that I have no idea what
- fshelp_fetch_root() does, nor why it is called here...
- <youpi> it notably starts the translator that dir_lookup is looking at, if
- needed
- <youpi> possibly as a different user, thus reauthentication of CRDIR
- <antrik> so this is about a port that is passed to the translator being
- started?
- <youpi> no
- <youpi> well, depends on what you mean by "port"
- <youpi> it's about reauthenticating a port to be passed to the translator
- being started
- <youpi> and for that a rendez-vous port is needed for the reauthentication
- <youpi> and that's the one at stake
- <antrik> yeah, I meant the port that is reauthenticated
- <antrik> what is CRDIR?
- <youpi> current root dir ...
- <antrik> so the parent translator passes it's own root dir to the child
- translator; and the issue is that for the root FS the root dir points to
- the root FS itself...
- <youpi> yes
- <antrik> OK, that makes sense
- <youpi> (but that's only one example, rgrep mach_port_destroy hurd/ show
- other potential issues)
- <antrik> well, that's actually what I wanted to mention next... why is the
- rendez-vous port destroyed, instead of just deallocating the port right
- and letting reference counting to it's thing?...
- <antrik> do its thing
- <youpi> "just to make sure" I guess
- <antrik> it's pretty obvious that this will cause trouble for any RPC
- referencing itself...
- <youpi> well, follow-up with that on the list
- <youpi> with roland/tb in CC
- <youpi> only they would know any real reason for destroy
- <youpi> btw, if you knew how we could make _hurd_select()'s raw __mach_msg
- call be interruptible by signals, that'll permit to fix sudo
- <youpi> (damn, I need sleep, my tenses are all wrong)
- <antrik> BTW, does this cause any actual trouble?...
- <antrik> I don't know much about interruption... cfhammer might have a
- better idea, he look into that stuff quite a bit AIUI
- <antrik> looked
- <antrik> (hehe, it's not only your tenses... guess there's something in the
- ether ;-) )
- <youpi> it makes sudo, mailq, etc. fail sometimes
- <antrik> I mean the rendez-vous thing
- <youpi> that's it, yes
- <youpi> sudo etc. fail at least due to this
- <antrik> so these are two different problems that both affect sudo?
- <antrik> (rendez-vous and interruption I mean)
- <youpi> yes
- <youpi> with my patch the buildds have much fewer issues, but still some
- <youpi> (my interrupt-related patch)
- <youpi> I'm installing a s/destroy/deallocate/ version of ext2fs on the
- buildds, we'll see how it behaves
- <youpi> (it fixes my testcase at least)
- <antrik> interrupt-related patch?
- <antrik> only thing interrupt-related I remember was the reauthentication
- race...
- <youpi> that's what I mean
- <antrik> well, cfhammer investigated this is quite some depth, explaining
- quite well why the race is only mitigated but still exists... problem is
- that we didn't know how to fix it properly
- <antrik> because nobody seems to understand the cancellation code, except
- perhaps for Roland and Thomas
- <antrik> (and I'm not even entirely sure about them :-) )
- <antrik> I think his findings and our conclusions are documented on the
- ML...
- <youpi> by "much fewer issues", I mean that some of the symptoms have
- disappeared, others haven't
- <antrik> BTW, couldn't the rendez-vous thing be worked around by simply
- ignoring the errors from the failing deallocate?...
- <youpi> no, failing deallocate are actually dangerous
- <antrik> why?
- <youpi> since the name might have been reused for something else in the
- meanwhile
- <youpi> that's the whole point of the warning I had added in the kernel
- itself
- <antrik> I see
- <youpi> such things really deserve tracking, since they can have any kind
- of consequence
- <antrik> does Mach try to reuse names quickly, rather than only after
- wrapping around?...
- <youpi> it seems to
- <antrik> OK, then this is a serious problem indeed
- <youpi> (note: I rarely divine issues when there aren't actual frequent
- symptoms :) )
- <antrik> well, the problem with the warning is that it only shows in the
- cases that do *not* cause a problem... so it's hard to associate them
- with any specific issues
- <youpi> well, most of the time the port is not reused quickly enough
- <youpi> so in most case it shows up more often than causing problem
-
-IRC, freenode, #hurd, 2011-03-14
-
- <youpi> ok, mach_port_deallocate actually can't be used
- <youpi> since mach_reply_port() returns a receive right, not a send right
- * youpi guesses he will really have to manage to understand all that port
- stuff completely
- <antrik> oh, right
- <antrik> youpi: hm... now I'm confused though. if one client holds a
- receive right, the other client (or in this case the same process) should
- have a send or send-once right -- these should *not* share the same name
- in my understanding
- <antrik> destroying the receive right should turn the send right into a
- dead name
- <antrik> so unless I'm missing something, the destroy shouldn't be a
- problem, and there must be something else going wrong
- <antrik> hm... actually I'm probably wrong
- <antrik> yeah, definitely wrong. receive rights and "ordinary" send rights
- share the name. only send-once rights are special
- <antrik> I wonder whether the problem could be worked around by using a
- send-once right...
- <antrik> mach_port_mod_refs(mach_task_self(), name,
- MACH_PORT_RIGHT_RECEIVE, -1) can be used to deallocate only the receive
- right
- <antrik> oh, you already figured that out :-)