Comment 104 for bug 1847361

=== Verification ===

1. Start qemu using the proposed package:

$ sudo qemu-system-x86_64 -machine none -S -nographic -monitor stdio -serial null
QEMU 3.1.0 monitor - type 'help' for more information
(qemu) info version
3.1.0Debian 1:3.1+dfsg-2ubuntu3.7~cloud1

2. Run "apt install --reinstall" so the prerm scripts copy the modules from /usr/lib/x86_64-linux-gnu/qemu/ to /var/run/qemu/$VERSION

$ md5sum /var/run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so /usr/lib/x86_64-linux-gnu/qemu/block-curl.so
9424706c3ea3f1b3845fd3defbf6879c /var/run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so
9424706c3ea3f1b3845fd3defbf6879c /usr/lib/x86_64-linux-gnu/qemu/block-curl.so

3. Remove the module from /usr/lib/x86_64-linux-gnu/qemu/

$ sudo rm /usr/lib/x86_64-linux-gnu/qemu/block-curl.so
$ ll /usr/lib/x86_64-linux-gnu/qemu/block-curl.so
ls: cannot access '/usr/lib/x86_64-linux-gnu/qemu/block-curl.so': No such file or directory

4. Add a curl device so it pulls the module

(qemu) drive_add 0 readonly=on,file=http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso
OK

5. Confirm it's falling back to /var/run/qemu/$VERSION when /usr/lib/x86_64-linux-gnu/qemu/ does not work

$ pidof qemu-system-x86_64; sudo cat /proc/$(pidof qemu-system-x86_64)/maps | grep -e curl
8687
7fad1ee33000-7fad1eead000 r-xp 00000000 08:02 3937904 /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.5.0
7fad1eead000-7fad1f0ac000 ---p 0007a000 08:02 3937904 /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.5.0
7fad1f0ac000-7fad1f0af000 r--p 00079000 08:02 3937904 /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.5.0
7fad1f0af000-7fad1f0b0000 rw-p 0007c000 08:02 3937904 /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.5.0
7fad1f0b0000-7fad1f0b5000 r-xp 00000000 00:16 993 /run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so
7fad1f0b5000-7fad1f2b4000 ---p 00005000 00:16 993 /run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so
7fad1f2b4000-7fad1f2b5000 r--p 00004000 00:16 993 /run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so
7fad1f2b5000-7fad1f2b6000 rw-p 00005000 00:16 993 /run/qemu/Debian_1_3.1+dfsg-2ubuntu3.7~cloud1/block-curl.so

** Note: if /run is mounted as noexec, step 4 will fail with the following message:

(qemu) drive_add 0 readonly=on,file=http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso
Failed to initialize module: /usr/lib/x86_64-linux-gnu/qemu/block-curl.so
Note: only modules from the same build can be loaded.
Failed to open module: /var/run/qemu/_Debian_1_2.11+dfsg-1ubuntu7.25_/block-curl.so: failed to map segment from shared object
Unknown protocol 'http'

This affects all fixed releases (B/E+), and the workaround is to remount the /run fs without noexec (sudo mount -o remount,exec /run)