Stefan Hajnoczi 3c2d5183f9 nbd-client: avoid read_reply_co entry if send failed
The following segfault is encountered if the NBD server closes the UNIX
domain socket immediately after negotiation:

  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  aio_co_schedule (ctx=0x0, co=0xd3c0ff2ef0) at util/async.c:441
  441       QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
  (gdb) bt
  #0  0x000000d3c01a50f8 in aio_co_schedule (ctx=0x0, co=0xd3c0ff2ef0) at util/async.c:441
  #1  0x000000d3c012fa90 in nbd_coroutine_end (bs=bs@entry=0xd3c0fec650, request=<optimized out>) at block/nbd-client.c:207
  #2  0x000000d3c012fb58 in nbd_client_co_preadv (bs=0xd3c0fec650, offset=0, bytes=<optimized out>, qiov=0x7ffc10a91b20, flags=0) at block/nbd-client.c:237
  #3  0x000000d3c0128e63 in bdrv_driver_preadv (bs=bs@entry=0xd3c0fec650, offset=offset@entry=0, bytes=bytes@entry=512, qiov=qiov@entry=0x7ffc10a91b20, flags=0) at block/io.c:836
  #4  0x000000d3c012c3e0 in bdrv_aligned_preadv (child=child@entry=0xd3c0ff51d0, req=req@entry=0x7f31885d6e90, offset=offset@entry=0, bytes=bytes@entry=512, align=align@entry=1, qiov=qiov@entry=0x7ffc10a91b20, f
+lags=0) at block/io.c:1086
  #5  0x000000d3c012c6b8 in bdrv_co_preadv (child=0xd3c0ff51d0, offset=offset@entry=0, bytes=bytes@entry=512, qiov=qiov@entry=0x7ffc10a91b20, flags=flags@entry=0) at block/io.c:1182
  #6  0x000000d3c011cc17 in blk_co_preadv (blk=0xd3c0ff4f80, offset=0, bytes=512, qiov=0x7ffc10a91b20, flags=0) at block/block-backend.c:1032
  #7  0x000000d3c011ccec in blk_read_entry (opaque=0x7ffc10a91b40) at block/block-backend.c:1079
  #8  0x000000d3c01bbb96 in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:79
  #9  0x00007f3196cb8600 in __start_context () at /lib64/libc.so.6

The problem is that nbd_client_init() uses
nbd_client_attach_aio_context() -> aio_co_schedule(new_context,
client->read_reply_co).  Execution of read_reply_co is deferred to a BH
which doesn't run until later.

In the mean time blk_co_preadv() can be called and nbd_coroutine_end()
calls aio_wake() on read_reply_co.  At this point in time
read_reply_co's ctx isn't set because it has never been entered yet.

This patch simplifies the nbd_co_send_request() ->
nbd_co_receive_reply() -> nbd_coroutine_end() lifecycle to just
nbd_co_send_request() -> nbd_co_receive_reply().  The request is "ended"
if an error occurs at any point.  Callers no longer have to invoke
nbd_coroutine_end().

This cleanup also eliminates the segfault because we don't call
aio_co_schedule() to wake up s->read_reply_co if sending the request
failed.  It is only necessary to wake up s->read_reply_co if a reply was
received.

Note this only happens with UNIX domain sockets on Linux.  It doesn't
seem possible to reproduce this with TCP sockets.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20170829122745.14309-2-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
2017-08-30 13:00:37 -05:00
2017-08-02 00:13:25 +03:00
2017-07-14 12:29:49 +02:00
2017-07-25 14:35:42 +10:00
2017-07-20 09:56:56 +02:00
2017-07-18 10:58:36 +01:00
2017-08-03 15:55:41 +03:00
2017-07-13 13:49:58 +02:00
2017-08-01 17:27:33 +02:00
2017-08-14 13:35:33 +01:00
2017-07-14 11:04:33 +02:00
2017-07-31 13:06:39 +03:00
2017-08-30 17:02:54 +01:00

         QEMU README
         ===========

QEMU is a generic and open source machine & userspace emulator and
virtualizer.

QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).

QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.

QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.

QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.


Building
========

QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:

  mkdir build
  cd build
  ../configure
  make

Additional information can also be found online via the QEMU website:

  http://qemu-project.org/Hosts/Linux
  http://qemu-project.org/Hosts/Mac
  http://qemu-project.org/Hosts/W32


Submitting patches
==================

The QEMU source code is maintained under the GIT version control system.

   git clone git://git.qemu-project.org/qemu.git

When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.

Additional information on submitting patches can be found online via
the QEMU website

  http://qemu-project.org/Contribute/SubmitAPatch
  http://qemu-project.org/Contribute/TrivialPatches


Bug reporting
=============

The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:

  https://bugs.launchpad.net/qemu/

If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.

For additional information on bug reporting consult:

  http://qemu-project.org/Contribute/ReportABug


Contact
=======

The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC

 - qemu-devel@nongnu.org
   http://lists.nongnu.org/mailman/listinfo/qemu-devel
 - #qemu on irc.oftc.net

Information on additional methods of contacting the community can be
found online via the QEMU website:

  http://qemu-project.org/Contribute/StartHere

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