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

[patchbot@xxxxxxx]

commit cc338b48dc8c18d8c63b1c2392e3b2bd75c0ff44
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Jul 5 15:51:58 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Mar 24 12:16:31 2023 +0000

    xen/check-endbr.sh: Explain the purpose of the script
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/tools/check-endbr.sh | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/xen/tools/check-endbr.sh b/xen/tools/check-endbr.sh
index b97684ac25..bf153a570d 100755
--- 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
--
generated by git-patchbot for /home/xen/git/xen.git#master