[pve-devel] [RFC container] Create: fix architecture detection in restore_archive

[Thomas Lamprecht]

For detecting a CT templates architecture we used the `file -b -L`
output from the PVE host side.
If the container has a link:
/bin/sh -> /bin/bash
(Alpine Linux does that) the '-L' flag from file resolves the
$rootfs/bin/sh to /bin/bash and thus checks the architecture of
bash on the PVE system, which is always 64 bit.

Add a helper which chroots in the rootfs and opens /bin/sh to read
the first 5 bytes, 4 bytes for the ELF magic number and the fifth for
the ELF class, which tells us if we have a 32 or 64 bit ELF binary.

Return this information as an exit code to the parent.

Signed-off-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
---

This approach was chosen as manually resolving links with all its edge cases
is a lot of work, and with this we can scrap the fun_command of `file -b -L`
altogether

The get_elf_class helper may be a candidate for pve-common (PVE::Tools ?)
but as we do not use this functionality else where, AFAIK, I kept it here
for now


 src/PVE/LXC/Create.pm | 68 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 53 insertions(+), 15 deletions(-)

diff --git a/src/PVE/LXC/Create.pm b/src/PVE/LXC/Create.pm
index 11bc00d..e2c58b8 100644
--- a/src/PVE/LXC/Create.pm
+++ b/src/PVE/LXC/Create.pm
@@ -23,6 +23,47 @@ sub next_free_nbd_dev {
     die "unable to find free nbd device\n";
 }
 
+sub get_elf_class {
+    my ($rootfs, $binary_fn) = @_;
+
+    my $child = fork();
+
+    if (!defined($child)) {
+	die "get_elf_class - fork failed: $!\n";
+    } elsif ($child == 0) {
+	eval {
+	    chroot $rootfs;
+
+	    my $fh;
+	    open ($fh, "<", $binary_fn) or die "'$binary_fn' not found in '$rootfs'!\n";
+	    binmode($fh);
+
+	    my $data;
+	    read($fh, $data, 5); # 4 bytes ELF magic number and 1 byte ELF class
+
+	    my ($magic, $class) = unpack("A4C", $data);
+
+	    die "'$binary_fn' does not resolve to an ELF!\n" if $magic ne "\177ELF";
+
+	    die "'$binary_fn' has unknown ELF class '$class'!\n"
+		if ($class != 1 && $class != 2);
+
+	    exit ($class) # tell parent the elf class
+	};
+	die $@; # use eval/die else the exit code can be changed by read or open
+    }
+
+    waitpid($child, 0);
+    my $exit_code = $?;
+    if (my $sig = ($exit_code & 127)) {
+	die "got signal $sig";
+    } else {
+	$exit_code = ($exit_code >> 8);
+    }
+
+    return $exit_code;
+}
+
 sub restore_archive {
     my ($archive, $rootdir, $conf, $no_unpack_error) = @_;
 
@@ -53,21 +94,18 @@ sub restore_archive {
     # if arch is set, we do not try to autodetect it
     return if defined($conf->{arch});
 
-    # determine file type of /usr/bin/file itself to get guests' architecture
-    $cmd = [@$userns_cmd, '/usr/bin/file', '-b', '-L', "$rootdir/bin/sh"];
-    PVE::Tools::run_command($cmd, outfunc => sub {
-	shift =~ /^ELF (\d{2}-bit)/; # safely assumes x86 linux
-	my $arch_str = $1;
-	$conf->{'arch'} = 'amd64'; # defaults to 64bit
-	if(defined($arch_str)) {
-	    $conf->{'arch'} = 'i386' if $arch_str =~ /32/;
-	    print "Detected container architecture: $conf->{'arch'}\n";
-	} else {
-	    print "CT architecture detection failed, falling back to amd64.\n" .
-	          "Edit the config in /etc/pve/nodes/{node}/lxc/{vmid}.conf " .
-	          "to set another architecture.\n";
-	}
-    });
+
+    my $elf_class = get_elf_class($rootdir, '/bin/sh'); # /bin/sh is POSIX mandatory
+
+    $conf->{'arch'} = 'amd64'; # defaults to 64bit
+    if ($elf_class == 1 || $elf_class == 2) {
+	$conf->{'arch'} = 'i386' if $elf_class == 1;
+	print "Detected container architecture: $conf->{'arch'}\n";
+    } else {
+	print "CT architecture detection failed, falling back to amd64.\n" .
+	      "Edit the config in /etc/pve/nodes/{node}/lxc/{vmid}.conf " .
+	      "to set another architecture.\n";
+    }
 }
 
 sub recover_config {
-- 
2.1.4