[PATCH v2 2/2] selftests/ftrace: Convert ELF entry point to file offset in uprobe test

["Rui Qi" <qirui.001@bytedance.com>]

The add_remove_uprobe test uses readelf -h to obtain the ELF entry
point (e_entry) and passes it directly as the offset to uprobe_events.
However, uprobe_events expects a file offset, not a virtual address.

For PIE binaries, the virtual address happens to equal the file offset
because the first LOAD segment has p_vaddr == p_offset, so the test
works by coincidence. But for non-PIE executables, e_entry is an
absolute virtual address that can far exceed the file size. When the
probe is enabled, uprobe_register() checks offset > i_size_read(inode)
and rejects it with -EINVAL.

Fix this by converting the virtual address to a file offset using the
ELF program headers: scan readelf -lW output for the LOAD segment
containing the entry point, then compute file_offset = e_entry -
p_vaddr + p_offset. For PIE binaries the result is unchanged; for
non-PIE binaries the offset is correctly translated.

The conversion uses only POSIX shell primitives, with no dependency on
gawk or perl.

Fixes: dc4b165855f2 ("selftests/ftrace: Use readelf to find entry point in uprobe test")
Signed-off-by: Rui Qi <qirui.001@bytedance.com>
---
 .../test.d/dynevent/add_remove_uprobe.tc      | 27 +++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_uprobe.tc b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_uprobe.tc
index f2048c244526..19430bd5864c 100644
--- a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_uprobe.tc
+++ b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_uprobe.tc
@@ -12,9 +12,32 @@ echo 0 > events/enable
 echo > dynamic_events
 
 REALBIN=`readlink -f /bin/sh`
-ENTRYPOINT=`readelf -h ${REALBIN} | grep Entry | sed -e 's/[^0]*//'`
 
-echo "p:myevent ${REALBIN}:${ENTRYPOINT}" >> uprobe_events
+# Get the entry point virtual address from ELF header
+ENTRY=`readelf -hW ${REALBIN} | grep "Entry point" | awk '{print $NF}'`
+
+# Convert virtual address to file offset: find the LOAD segment containing
+# the entry point, then compute file_offset = e_entry - p_vaddr + p_offset.
+# For PIE binaries this is a no-op (vaddr == file offset), but for non-PIE
+# executables the virtual address is much larger than the file size and
+# must be converted, otherwise uprobe_register() rejects it with -EINVAL.
+ENTRY_DEC=$(printf '%d' "$ENTRY")
+OFFSET=$ENTRY
+while IFS= read -r line; do
+    set -- $line
+    [ "$1" = "LOAD" ] || continue
+    VA_DEC=$(printf '%d' "$3")
+    OFF_DEC=$(printf '%d' "$2")
+    FSZ_DEC=$(printf '%d' "$5")
+    if [ "$ENTRY_DEC" -ge "$VA_DEC" ] && [ "$ENTRY_DEC" -lt "$((VA_DEC + FSZ_DEC))" ]; then
+        OFFSET=$(printf '0x%x' "$((ENTRY_DEC - VA_DEC + OFF_DEC))")
+        break
+    fi
+done << EOF
+$(readelf -lW ${REALBIN} | grep LOAD)
+EOF
+
+echo "p:myevent ${REALBIN}:${OFFSET}" >> uprobe_events
 
 grep -q myevent uprobe_events
 test -d events/uprobes/myevent
-- 
2.20.1