Installed firmware descriptor files contain (invalid) relative paths

[Alain Kalker]

After building and installing QEMU, the resulting installed firmware descriptor files contain relative paths for their `mapping.filename` properties. These relative paths are causing errors when using tools based on `libvirt` like `virt-install`, resulting in the inability to configure new VMs which reference these firmware descriptors.

# QEMU version
$ qemu-system-x86_64 -version
QEMU emulator version 5.2.0

(I've also reproduced the issue with QEMU built from Git master @ v5.2.0-1300-g0e32462630, see next comment.)

# OS version
Void Linux x86_64 (glibc)

Steps to reproduce (with results on my system):

# Verify the symptom

$ virt-install --boot firmware=efi --disk none --memory 2048
Using default --name vm4
WARNING No operating system detected, VM performance may suffer. Specify an OS with --os-variant for optimal results.

Starting install...
ERROR Failed to open file 'share/qemu/edk2-i386-vars.fd': No such file or directory
Domain installation does not appear to have been successful.
If it was, you can restart your domain by running:
  virsh --connect qemu:///session start vm4
otherwise, please restart your installation.

# Verify that the file does exist on the system and is accessible

$ ls -l /usr/share/qemu/edk2-i386-vars.fd
-rw-r--r-- 1 root root 540672 12 dec 18:47 /usr/share/qemu/edk2-i386-vars.fd

# Verify most likely cause

$ grep filename /usr/share/qemu/firmware/*i386*.json
/usr/share/qemu/firmware/50-edk2-i386-secure.json: "filename": "share/qemu/edk2-i386-secure-code.fd",
/usr/share/qemu/firmware/50-edk2-i386-secure.json: "filename": "share/qemu/edk2-i386-vars.fd",
/usr/share/qemu/firmware/60-edk2-i386.json: "filename": "share/qemu/edk2-i386-code.fd",
/usr/share/qemu/firmware/60-edk2-i386.json: "filename": "share/qemu/edk2-i386-vars.fd",

Note that all the paths are relative and are missing <prefix>, i.e. `/usr`.

# Workaround

Manually editing the firmware descriptor files in `/usr/share/qemu/firmware` to contain full absolute paths to the firmware blobs resolves the issue:

$ sudo sed -i.bak -e 's,"share/qemu/,"/usr/share/qemu/,' /usr/share/qemu/firmware/*.json
$ virt-install --boot firmware=efi --disk none --memory 2048
[...VM boots normally...]

[Alain Kalker (ackalker)]

There is an issue on Void Linux packages issue tracker about this bug[1]

# Steps to verify the issue on a fresh Git clone of QEMU source

$ git clone https://github.com/qemu/qemu
$ cd qemu
$ make DEBUG=1 docker-test-misc@alpine
[...]
  COPY RUNNER
    RUN test-misc in qemu/alpine
* Prepared to run command:
  ./test-misc
* Hit Ctrl-D to continue, or type 'exit 1' to abort

bash-5.1$ . common-rc
[...]
bash-5.1$ configure_qemu --disable-system --disable-user
Configure options:
--enable-werror --prefix=/tmp/qemu-test/install --disable-system --disable-user
cross containers no
The Meson build system
Version: 0.56.2
Source dir: /tmp/qemu-test/src
Build dir: /tmp/qemu-test/src/tests/docker
Build type: native build
Project name: qemu
Project version: 5.2.50
[...]
bash-5.1$ grep filename pc-bios/descriptors/*
pc-bios/descriptors/50-edk2-i386-secure.json: "filename": "share/qemu/edk2-i386-secure-code.fd",
pc-bios/descriptors/50-edk2-i386-secure.json: "filename": "share/qemu/edk2-i386-vars.fd",
[...all other matches have relative paths...]
bash-5.1$ exit 1
[...]

# Research

The filename path substitution appears to be done in the file `pc-bios/descriptors/meson.build`.
In particular, occurrences of `@DATADIR@` in the template files get substituted using the value of `qemu_datadir`:

  configure_file(input: files(f),
                 output: f,
                 configuration: {'DATADIR': qemu_datadir},
                 install: get_option('install_blobs'),
                 install_dir: qemu_datadir / 'firmware')

The variable `qemu_datadir` gets initialized from toplevel `meson.build` file using:

   qemu_datadir = get_option('datadir') / get_option('qemu_suffix')

From the Meson documentation on built-in (build) options[2], `datadir` option gets initialized to `"share"` (note: a relative path!) by default, unless `datadir` build option is configured explicitly, so the value of `qemu_datadir`, as well as the substitution made ends up being a relative path as well.

I found a commit which I think is relevant to why this choice was made[3].

# Test a workaround, try #1: specify `datadir` manually

$ make DEBUG=1 docker-test-misc@alpine
[...]
bash-5.1$ configure_qemu --datadir=/usr/share --disable-system --disable-user
Configure options:
--enable-werror --prefix=/tmp/qemu-test/install --datadir=/usr/share --disable-system --disable-user
cross containers no
The Meson build system
Version: 0.56.2
Source dir: /tmp/qemu-test/src
Build dir: /tmp/qemu-test/src/tests/docker
Build type: native build

../../meson.build:1:0: ERROR: The value of the 'datadir' option is '/usr/share' which must be a subdir of the prefix '/tmp/qemu-test/install'.
Note that if you pass a relative path, it is assumed to be a subdir of prefix.

A full log can be found at /tmp/qemu-test/src/tests/docker/meson-logs/meson-log.txt

ERROR: meson setup failed

Ah! So perhaps `datadir` can be an absolute path, but then it must be a subdir of the (already specified) `prefix`.
Let's try again.

# Test a workaround, try #2: s...

[Alain Kalker (ackalker)]

I think this needs to be fixed in QEMU source. The `configure_file()` action needs both an absolute path for `configuration` (to substitute into the descriptor files) _and_ a relative path for use with the `install_dir` keyword argument.

[Alain Kalker (ackalker)]

Please disregard the `# Test a coworkaround` stuff in comment #1, bad copypasta, too late at night ;-)

[Sergei Trofimovich (trofi)]

Gentoo also noticed the bug: https://bugs.gentoo.org/766743

Jannik Glückert proposed a fix:

```
--- a/pc-bios/descriptors/meson.build
+++ b/pc-bios/descriptors/meson.build
@@ -8,7 +8,7 @@ foreach f: [
 ]
   configure_file(input: files(f),
                  output: f,
- configuration: {'DATADIR': qemu_datadir},
+ configuration: {'DATADIR': get_option('prefix') / qemu_datadir},
                  install: get_option('install_blobs'),
                  install_dir: qemu_datadir / 'firmware')
 endforeach
```

[Thomas Huth (th-huth)]

Fixed here:
https://gitlab.com/qemu-project/qemu/-/commit/4b956a399969c0c497a