xen/check-endbr.sh: Explain the purpose of the script

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/tools/check-endbr.sh b/xen/tools/check-endbr.sh
index b97684ac25e9f23c95cfd4b0f0d89f0eb2bbd406..bf153a570db494e1210bfd927c0bc209008ac666 100755 (executable)
--- a/xen/tools/check-endbr.sh
+++ b/xen/tools/check-endbr.sh
@@ -2,6 +2,15 @@
 #
 # Usage ./$0 xen-syms
 #
+# When CET-IBT (Control-flow Enforcement Technology, Indirect Branch Tracking)
+# is active, ENDBR instructions mark legal indirect branch targets in the
+# .text section.
+#
+# However x86 is a variable length instruction set so the same byte pattern
+# can exist embedded in other instructions, or crossing multiple instructions.
+# This script searches .text for any problematic byte patterns which aren't
+# legitimate ENDBR instructions.
+#
 set -e
 
 # Pretty-print parameters a little for message