[Qemu-devel] [PATCH v2 0/7] block/curl: Fix hang and potential crash

[Max Reitz <mreitz@redhat.com>]

Hi,

As reported in https://bugzilla.redhat.com/show_bug.cgi?id=1740193, our
curl block driver can spontaneously hang.  This becomes visible e.g.
when reading compressed qcow2 images:

$ qemu-img convert -p -O raw -n \
  https://download.cirros-cloud.net/0.4.0/cirros-0.4.0-x86_64-disk.img \
  null-co://

(Hangs at 74.21 %, usually.)

A more direct way is:

$ qemu-img bench -f raw http://download.qemu.org/qemu-4.1.0.tar.xz \
    -d 1 -S 524288 -c 2

(Which simply performs two requests, and the second one hangs.  You can
use any HTTP resource (probably FTP, too) you’d like that is at least
1 MB in size.)

It turns out that this is because cURL 7.59.0 has added a protective
feature against some misuse we had in our code: curl_multi_add_handle()
must not be called from within a cURL callback, but in some cases we
did.  As of 7.59.0, this fails, our new request is not registered and
the I/O request stalls.  This is fixed by patch 6.

Patch 7 makes us check for curl_multi_add_handle()’s return code,
because if we had done that before, debugging would have been much
simpler.


On the way to fixing it, I had a look over the whole cURL code and found
a suspicious QLIST_FOREACH_SAFE() loop that actually does not seem very
safe at all.  I think this may lead to crashes, although I have never
seen any myself.  https://bugzilla.redhat.com/show_bug.cgi?id=1744602#c5
shows one in exactly the function in question, so I think it actually is
a problem.

This is fixed by patch 5, patches 1, 2, and 4 prepare for it.

(Patch 3 is kind of a misc patch that should ensure that we always end
up calling curl_multi_check_completion() whenever a request might have
been completed.)


v2:
- Patch 2: Remove the socket from the list only add the end of the
           function (yielding a nicer 5+/5- diff stat)
- Patch 3: Added
- Patch 4: Rebased on patch 3, and s/socket/ready_socket/ in one place
- Patch 5: Rebased on the changed patch 4


git-backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/7:[----] [--] 'curl: Keep pointer to the CURLState in CURLSocket'
002/7:[0007] [FC] 'curl: Keep *socket until the end of curl_sock_cb()'
003/7:[down] 'curl: Check completion in curl_multi_do()'
004/7:[0019] [FC] 'curl: Pass CURLSocket to curl_multi_{do,read}()'
005/7:[0002] [FC] 'curl: Report only ready sockets'
006/7:[----] [--] 'curl: Handle success in multi_check_completion'
007/7:[----] [--] 'curl: Check curl_multi_add_handle()'s return code'


Max Reitz (7):
  curl: Keep pointer to the CURLState in CURLSocket
  curl: Keep *socket until the end of curl_sock_cb()
  curl: Check completion in curl_multi_do()
  curl: Pass CURLSocket to curl_multi_do()
  curl: Report only ready sockets
  curl: Handle success in multi_check_completion
  curl: Check curl_multi_add_handle()'s return code

 block/curl.c | 133 +++++++++++++++++++++++----------------------------
 1 file changed, 59 insertions(+), 74 deletions(-)

-- 
2.21.0