selftests/ftrace: Use readelf to find entry point in uprobe test

JIRA: https://issues.redhat.com/browse/RHEL-87219

upstream
========
commit dc4b165
Author: Anubhav Shelat <ashelat@redhat.com>
Date: Thur Aug 28

description
===========
The uprobe events test fails on s390, but also on x86 (Fedora 41). The
problem appears to be that there is an assumption that adding a uprobe to
the beginning of the executable mapping of /bin/sh is sufficient to trigger
a uprobe event when /bin/sh is executed.

This assumption is not necessarily true. Therefore use "readelf -h" to find
the entry point address of /bin/sh and use this address when adding the
uprobe event.

This adds a dependency to readelf which is not always installed. Therefore
add a check and exit with exit_unresolved if it is not installed.

Link: https://lore.kernel.org/r/20250220130102.2079179-1-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Anubhav Shelat <ashelat@redhat.com>

Author: AnubhavS1

tools/testing/selftests/ftrace/test.d/dynevent/add_remove_uprobe.tc

@@ -3,12 +3,18 @@
 # description: Generic dynamic event - add/remove/test uprobe events
 # requires: uprobe_events
 
+if ! which readelf > /dev/null 2>&1 ; then
+  echo "No readelf found. skipped."
+  exit_unresolved
+fi
+
 echo 0 > events/enable
 echo > dynamic_events
 
-echo 'cat /proc/$$/maps' | /bin/sh | \
-	grep "r-xp .*/bin/.*sh$" | \
-	awk '{printf "p:myevent %s:0x%s\n", $6,$3 }' >> uprobe_events
+REALBIN=`readlink -f /bin/sh`
+ENTRYPOINT=`readelf -h ${REALBIN} | grep Entry | sed -e 's/[^0]*//'`
+
+echo "p:myevent ${REALBIN}:${ENTRYPOINT}" >> uprobe_events
 
 grep -q myevent uprobe_events
 test -d events/uprobes/myevent