scripts: validate SPDX license choices

We expect all new code to be contributed with the "GPL-2.0-or-later"
license tag. Divergence is permitted if the new file is derived from
pre-existing code under a different license, whether from elsewhere
in QEMU codebase, or outside.

Issue a warning if the declared license is not "GPL-2.0-or-later",
and an error if the license is not one of the handful of the
expected licenses to prevent unintended proliferation. The warning
asks users to explain their unusual choice of license in the commit
message.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

scripts/checkpatch.pl

@@ -1353,6 +1353,70 @@ sub checkfilename {
 	}
 }
 
+sub checkspdx {
+    my ($file, $expr) = @_;
+
+    # Imported Linux headers probably have SPDX tags, but if they
+    # don't we're not requiring contributors to fix this, as these
+    # files are not expected to be modified locally in QEMU.
+    # Also don't accidentally detect own checking code.
+    if ($file =~ m,include/standard-headers, ||
+	$file =~ m,linux-headers, ||
+	$file =~ m,checkpatch.pl,) {
+	return;
+    }
+
+    my $origexpr = $expr;
+
+    # Flatten sub-expressions
+    $expr =~ s/\(|\)/ /g;
+    $expr =~ s/OR|AND/ /g;
+
+    # Merge WITH exceptions to the license
+    $expr =~ s/\s+WITH\s+/-WITH-/g;
+
+    # Cull more leading/trailing whitespace
+    $expr =~ s/^\s*//g;
+    $expr =~ s/\s*$//g;
+
+    my @bits = split / +/, $expr;
+
+    my $prefer = "GPL-2.0-or-later";
+    my @valid = qw(
+	GPL-2.0-only
+	LGPL-2.1-only
+	LGPL-2.1-or-later
+	BSD-2-Clause
+	BSD-3-Clause
+	MIT
+	);
+
+    my $nonpreferred = 0;
+    my @unknown = ();
+    foreach my $bit (@bits) {
+	if ($bit eq $prefer) {
+	    next;
+	}
+	if (grep /^$bit$/, @valid) {
+	    $nonpreferred = 1;
+	} else {
+	    push @unknown, $bit;
+	}
+    }
+    if (@unknown) {
+	ERROR("Saw unacceptable licenses '" . join(',', @unknown) .
+	      "', valid choices for QEMU are:\n" . join("\n", $prefer, @valid));
+    }
+
+    if ($nonpreferred) {
+	WARN("Saw acceptable license '$origexpr' but note '$prefer' is " .
+	     "preferred for new files unless the code is derived from a " .
+	     "source file with an existing declared license that must be " .
+	     "retained. Please explain the license choice in the commit " .
+	     "message.");
+    }
+}
+
 sub process {
 	my $filename = shift;
 
@@ -1645,6 +1709,11 @@ sub process {
 		    }
 		}
 
+# Check SPDX-License-Identifier references a permitted license
+		if ($rawline =~ m,SPDX-License-Identifier: (.*?)(\*/)?\s*$,) {
+		    &checkspdx($realfile, $1);
+		}
+
 # Check for wrappage within a valid hunk of the file
 		if ($realcnt != 0 && $line !~ m{^(?:\+|-| |\\ No newline|$)}) {
 			ERROR("patch seems to be corrupt (line wrapped?)\n" .