meson, cutils: allow non-relocatable installs

Say QEMU is configured with bindir = "/usr/bin" and a firmware path
that starts with "/usr/share/qemu".  Ever since QEMU 5.2, QEMU's
install has been relocatable: if you move qemu-system-x86_64 from
/usr/bin to /home/username/bin, it will start looking for firmware in
/home/username/share/qemu.  Previously, you would get a non-relocatable
install where the moved QEMU will keep looking for firmware in
/usr/share/qemu.

Windows almost always wants relocatable installs, and in fact that
is why QEMU 5.2 introduced relocatability in the first place.
However, newfangled distribution mechanisms such as AppImage
(https://docs.appimage.org/reference/best-practices.html), and
possibly NixOS, also dislike using at runtime the absolute paths
that were established at build time.

On POSIX systems you almost never care; if you do, your usecase
dictates which one is desirable, so there's no single answer.
Obviously relocatability works fine most of the time, because not many
people have complained about QEMU's switch to relocatable install,
and that's why until now there was no way to disable relocatability.

But a non-relocatable, non-modular binary can help if you want to do
experiments with old firmware and new QEMU or vice versa (because you
can just upgrade/downgrade the firmware package, and use rpm2cpio or
similar to extract the QEMU binaries outside /usr), so allow both.
This patch allows one to build a non-relocatable install using a new
option to configure.  Why?  Because it's not too hard, and because
it helps the user double check the relocatability of their install.

Note that the same code that handles relocation also lets you run QEMU
from the build tree and pick e.g. firmware files from the source tree
transparently.  Therefore that part remains active with this patch,
even if you configure with --disable-relocatable.

Suggested-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -2111,6 +2111,7 @@ config_host_data.set('CONFIG_OPENGL', opengl.found())
 config_host_data.set('CONFIG_PLUGIN', get_option('plugins'))
 config_host_data.set('CONFIG_RBD', rbd.found())
 config_host_data.set('CONFIG_RDMA', rdma.found())
+config_host_data.set('CONFIG_RELOCATABLE', get_option('relocatable'))
 config_host_data.set('CONFIG_SAFESTACK', get_option('safe_stack'))
 config_host_data.set('CONFIG_SDL', sdl.found())
 config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
@@ -4054,6 +4055,7 @@ if 'simple' in get_option('trace_backends')
 endif
 summary_info += {'D-Bus display':     dbus_display}
 summary_info += {'QOM debugging':     get_option('qom_cast_debug')}
+summary_info += {'Relocatable install': get_option('relocatable')}
 summary_info += {'vhost-kernel support': have_vhost_kernel}
 summary_info += {'vhost-net support': have_vhost_net}
 summary_info += {'vhost-user support': have_vhost_user}
@@ -4356,3 +4358,21 @@ if host_arch == 'unknown' or not supported_oses.contains(targetos)
   message('If you want to help supporting QEMU on this platform, please')
   message('contact the developers at qemu-devel@nongnu.org.')
 endif
+
+actually_reloc = get_option('relocatable')
+# check if get_relocated_path() is actually able to relocate paths
+if get_option('relocatable') and \
+  not (get_option('prefix') / get_option('bindir')).startswith(get_option('prefix') / '')
+  message()
+  warning('bindir not included within prefix, the installation will not be relocatable.')
+  actually_reloc = false
+endif
+if not actually_reloc and (targetos == 'windows' or get_option('relocatable'))
+  if targetos == 'windows'
+    message()
+    warning('Windows installs should usually be relocatable.')
+  endif
+  message()
+  message('QEMU will have to be installed under ' + get_option('prefix') + '.')
+  message('Use --disable-relocatable to remove this warning.')
+endif

meson_options.txt

@@ -101,6 +101,8 @@ option('cfi_debug', type: 'boolean', value: false,
        description: 'Verbose errors in case of CFI violation')
 option('multiprocess', type: 'feature', value: 'auto',
        description: 'Out of process device emulation support')
+option('relocatable', type : 'boolean', value : 'true',
+       description: 'toggle relocatable install')
 option('vfio_user_server', type: 'feature', value: 'disabled',
        description: 'vfio-user server support')
 option('dbus_display', type: 'feature', value: 'auto',

scripts/meson-buildoptions.sh

@@ -17,6 +17,7 @@ meson_options_help() {
   printf "%s\n" '                           code for the Hexagon frontend'
   printf "%s\n" '  --disable-install-blobs  install provided firmware blobs'
   printf "%s\n" '  --disable-qom-cast-debug cast debugging support'
+  printf "%s\n" '  --disable-relocatable    toggle relocatable install'
   printf "%s\n" '  --docdir=VALUE           Base directory for documentation installation'
   printf "%s\n" '                           (can be empty) [share/doc]'
   printf "%s\n" '  --enable-block-drv-whitelist-in-tools'
@@ -421,6 +422,8 @@ _meson_option_parse() {
     --disable-rbd) printf "%s" -Drbd=disabled ;;
     --enable-rdma) printf "%s" -Drdma=enabled ;;
     --disable-rdma) printf "%s" -Drdma=disabled ;;
+    --enable-relocatable) printf "%s" -Drelocatable=true ;;
+    --disable-relocatable) printf "%s" -Drelocatable=false ;;
     --enable-replication) printf "%s" -Dreplication=enabled ;;
     --disable-replication) printf "%s" -Dreplication=disabled ;;
     --enable-rng-none) printf "%s" -Drng_none=true ;;

util/cutils.c

@@ -1178,9 +1178,11 @@ char *get_relocated_path(const char *dir)
 #else
         g_string_append(result, dir);
 #endif
-    } else if (!starts_with_prefix(dir) || !starts_with_prefix(bindir)) {
-        g_string_assign(result, dir);
-    } else {
+        goto out;
+    }
+
+    if (IS_ENABLED(CONFIG_RELOCATABLE) &&
+        starts_with_prefix(dir) && starts_with_prefix(bindir)) {
         g_string_assign(result, exec_dir);
 
         /* Advance over common components.  */
@@ -1203,7 +1205,10 @@ char *get_relocated_path(const char *dir)
             assert(G_IS_DIR_SEPARATOR(dir[-1]));
             g_string_append(result, dir - 1);
         }
+        goto out;
     }
 
+    g_string_assign(result, dir);
+out:
     return g_string_free(result, false);
 }