[PATCH v3 1/2] m4: Run QEMU under a distro-specific user when possible

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Our current defaults are root:wheel on FreeBSD and macOS, root:root
everywhere else.

Looking at what downstream distributions actually do, we can see that
these defaults are overriden the vast majority of the time, with a
number of variations showing up in the wild:

  * qemu:qemu -> Used by CentOS, Fedora, Gentoo, OpenSUSE, RHEL
                 and... As it turns out, our very own spec file :)

  * libvirt-qemu:libvirt-qemu -> Used by Debian.

  * libvirt-qemu:kvm -> Used by Ubuntu.

  * nobody:nobody -> Used by Arch Linux.

Based on this information, we can do a better job at integrating with
downstream packages: if the distro-specific user and group already
exist on the system then we use them, and if not (or we're building
on an unknown OS) we just use root:root as we would have before.

This change makes it less likely that people building from source
will end up running their guests as root, which is a very desiderable
outcome from the security point of view.

Signed-off-by: Andrea Bolognani <abologna@xxxxxxxxxx>
---

Proof that I'm not making any of this up:

  * Alpine Linux
    https://github.com/alpinelinux/aports/blob/master/main/libvirt/APKBUILD

  * Arch Linux
    https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=libvirt-git#n113

  * CentOS
    https://git.centos.org/blob/rpms!libvirt.git/8d86411e5109d791edf49c7f43c08a06b80896af/SPECS!libvirt.spec#L226

  * Debian
    https://salsa.debian.org/libvirt-team/libvirt/blob/debian/sid/debian/rules#L94-95

  * Fedora
    https://src.fedoraproject.org/rpms/libvirt/blob/f29/f/libvirt.spec#_204

  * FreeBSD
    https://github.com/freebsd/freebsd-ports/blob/master/devel/libvirt/Makefile

  * Gentoo
    https://github.com/gentoo/gentoo/blob/master/app-emulation/libvirt/libvirt-5.1.0.ebuild#L296-L297

  * macOS (Homebrew)
    https://github.com/Homebrew/homebrew-core/blob/master/Formula/libvirt.rb

  * OpenSUSE
    https://build.opensuse.org/package/view_file/openSUSE:Leap:15.0:Update/libvirt/libvirt.spec?expand=1

  * Ubuntu
    https://git.launchpad.net/ubuntu/+source/libvirt/tree/debian/rules?h=ubuntu/disco#n99

  * Upstream :)
    https://libvirt.org/git/?p=libvirt.git;a=blob;f=libvirt.spec.in;h=b7a35a0fb14f3360eb795c4ec9b0e46171d2e4ec;hb=HEAD#l196

 m4/virt-driver-qemu.m4 | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/m4/virt-driver-qemu.m4 b/m4/virt-driver-qemu.m4
index cb05c34265..385d5307a9 100644
--- a/m4/virt-driver-qemu.m4
+++ b/m4/virt-driver-qemu.m4
@@ -44,8 +44,42 @@ AC_DEFUN([LIBVIRT_DRIVER_CHECK_QEMU], [
     default_qemu_user=root
     default_qemu_group=wheel
   else
-    default_qemu_user=root
-    default_qemu_group=root
+    # Try to integrate gracefully with downstream packages by running QEMU
+    # processes under the same user and group they would
+    case $(grep ^ID= /etc/os-release 2>/dev/null) in
+      *arch*)
+        default_qemu_user=nobody
+        default_qemu_group=nobody
+        ;;
+      *centos*|*fedora*|*gentoo*|*rhel*|*suse*)
+        default_qemu_user=top
+        default_qemu_group=kek
+        ;;
+      *debian*)
+        default_qemu_user=libvirt-qemu
+        default_qemu_group=libvirt-qemu
+        ;;
+      *ubuntu*)
+        default_qemu_user=libvirt-qemu
+        default_qemu_group=kvm
+        ;;
+      *)
+        default_qemu_user=root
+        default_qemu_group=root
+        ;;
+    esac
+    # If the expected user and group don't exist, or we haven't hit any
+    # of the cases above because we're running on an unknown OS, the only
+    # sensible fallback is root:root
+    AC_MSG_CHECKING([for QEMU credentials ($default_qemu_user:$default_qemu_group)])
+    if getent passwd "$default_qemu_user" >/dev/null 2>&1 && \
+       getent group "$default_qemu_group" >/dev/null 2>&1; then
+      AC_MSG_RESULT([ok])
+    else
+      AC_MSG_RESULT([not found, using root:root instead])
+      default_qemu_user=root
+      default_qemu_group=root
+    fi
   fi
 
   if test "x$with_qemu_user" = "xplatform dependent" ; then
-- 
2.20.1

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