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bug The GNU Hurd - Bugs: bug #22861, bogus answer from pflocal to...

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[83]bug #22861: bogus answer from pflocal to io_select SELECT_URG

   Submitter:  None Submit changes and browse items http://_____________
   Submitted:  Sun 06 Apr 2008 11:52:50 PM UTC
   Submit changes and return to this item
   Votes: [84]1 Preview

   Category:  Hurd Servers Severity:  3 - Normal
   Priority:  5 - Normal Item Group:  None
   Status:  None Privacy:  Public
   Assigned to:  None Originator Name:
   Originator Email:  -email is unavailable- Open/Closed:  Open
   Reproducibility:  None Size (loc):  None
   Planned Release:  None Effort:  0.00
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[87]Discussion

   Reverse comment order
   [88]Sat 16 Jul 2016 01:46:37 PM UTC, comment #4:  (4) Quote

   The t/io_select_timeout patches in glibc complicate the fix.  Now if
   glibc/hurd/hurdselect.c (_hurd_select) is called with a timeout and at least one
   file descriptor, it passes the current time + the timeout to the servers, instead
   of letting __mach_msg time out.  And if each server returns MIG_NO_REPLY to its
   message loop and so never sends a reply, then _hurd_select sleeps until
   interrupted by a signal, regardless of what the timeout was.
   I guess the io_select_timeout implementation in each server could just wait until
   the timeout occurs (at which time it would send an reply with select_type=0) or
   the reply port dies (in which case no reply can be sent, and any related
   resources should be freed).  As an optimization, the server could use a timer
   queue so that the wait does not tie up a thread.
   io_select could be implemented in a similar way except with an infinite timeout,
   or it could be done with MIG_NO_REPLY as originally planned, I think.
   I removed the workaround from ELinks and tried to reproduce the original problem
   but couldn't; it never called select in such a way that the pipe was only in
   exceptfds.  To test any fix here, one should construct a dedicated test program
   instead of relying on ELinks.
   [89]Kalle Olavi Niemitalo 
   [90]Wed 13 Jul 2016 08:09:43 PM UTC, comment #3:  (3) Quote

   MIG_NO_REPLY is defined in gnumach and handled properly by the msg routines, yes.
   It could work indeed, could you give it a try?
   [91]Samuel Thibault 
   Group administrator
   [92]Tue 12 Jul 2016 09:17:17 AM UTC, comment #2:  (2) Quote

   About alternative (c) in the original submission: MIG_NO_REPLY is checked in
   glibc/mach/msgserver.c (__mach_msg_server_timeout), which hurd/libports/ calls.
   That might actually work then.
   [93]Kalle Olavi Niemitalo 
   [94]Sun 02 Nov 2008 05:43:11 PM UTC, comment #1:  (1) Quote

   ELinks 0.12pre2 includes a workaround in set_handlers():
     * Hurd bug 22861: Work around select() falsely reporting exceptions in pipes.

   The workaround slows ELinks down a bit, so it may be reverted after this Hurd bug
   has been fixed.
   [95]Kalle Olavi Niemitalo 
   [96]Sun 06 Apr 2008 11:52:50 PM UTC, original submission:   (0) Quote

   If a program first calls pipe() and then calls select() in such a way that one of
   the pipe file descriptors is in exceptfds but not in readfds nor writefds, the
   select() call returns almost immediately and leaves the pipe in exceptfds, as if
   an exception had occurred there.  This bug completely wrecks the ELinks web
   browser, which uses a pipe for internal communication and ends up closing the
   pipe because it thinks an error has occurred.
   rpctrace shows that _hurd_select in glibc sends an io_select_request (4) to
   pflocal and gets back io_select_reply (0 0), which it considers a "bogus answer":
   if the pipe is not ready for any I/O, then it should not have responded yet.  And
   so, glibc substitutes SELECT_ALL, which includes SELECT_URG.
   Why did pflocal send the bogus answer, then?  When *select_type is initially
   SELECT_URG without other flags, S_io_select in hurd/pflocal/io.c first resets
   *select_type to zero and then calls pipe_pair_select in hurd/libpipe/pipe.c.
   That notices *select_type is neither SELECT_READ nor SELECT_WRITE, and apparently
   assumes it is SELECT_READ | SELECT_WRITE and waits for either condition.  One of
   the conditions is satisfied, but *select_type remains zero, and S_io_select then
   propagates the zero to io_select_response.
   Possible fixes:
   (a) Change _hurd_select in glibc to completely ignore responses that have
   select_type=0.  I.e. change the protocol so that a server can send back
   io_select_reply (0 0) to mean that the requested events cannot ever occur.  This
   would however make the timeout in _hurd_select more complex to implement.
   Currently, _hurd_select simply passes the timeout to Mach when waiting for the
   first response to the io_select_requests it has sent out; after getting the first
   response, it collects any further replies with a zero timeout, and then returns.
   If _hurd_select were changed to ignore the first response in some cases, it would
   have to keep track of how much time it has spent.
   (b) Change pflocal to detect when io_select_request is asking solely for events
   that cannot ever occur, and sleep until the caller closes the reply port.
   Similar changes may be needed in other servers.  The downside is that this would
   needlessly tie up a thread in the server.
   (c) As in (b) but instead of sleeping, discard the right to the reply port
   without sending any response, and return the thread to more productive use.
   However, there seem to be two problems with this.  Firstly, it may be difficult
   to make MIG-generated stubs let the server skip the response.  The Mach 3 Server
   Writer's Guide mentions that the server routine can return MIG_NO_REPLY to to
   this, but I did not find that symbol in the GNU MiG 1.3 sources.  Secondly,
   discarding the reply right might trigger a no-senders notification in the
   client.  It appears though that _hurd_select does not currently request such
   notification.
   This bug was discussed in December 2007 on the debian-hurd and elinks-dev mailing
   lists, under the subject "The Links/Links2/ELinks browsers are unusable on Debian
   GNU/Hurd".
   Anonymous


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[98]Dependencies

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[99]Mail Notification Carbon-Copy List

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[103]Votes

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   Please enter the title of [104]George Orwell's famous dystopian book (it's a
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    Preview Submit changes and browse items Submit changes and return to this item

[105]History

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      Date    Changed by Updated Field Previous Value => Replaced by
   2008-11-02  [106]kon   Carbon-Copy               -    Added kon

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