debug/elf: return error in DynValue for invalid dynamic section size

This is a follow-up to CL 536400.

Fixes #64446

Change-Id: I35646732f62cb1937fd448f94ea518544d4295d4
GitHub-Last-Rev: 55db18a
GitHub-Pull-Request: #64448
Reviewed-on: https://go-review.googlesource.com/c/go/+/545835
LUCI-TryBot-Result: Go LUCI <[email protected]>
Run-TryBot: Jes Cok <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: callthingsoff

src/debug/elf/file.go

@@ -1656,6 +1656,14 @@ func (f *File) DynValue(tag DynTag) ([]uint64, error) {
 		return nil, err
 	}
 
+	dynSize := 8
+	if f.Class == ELFCLASS64 {
+		dynSize = 16
+	}
+	if len(d)%dynSize != 0 {
+		return nil, errors.New("length of dynamic section is not a multiple of dynamic entry size")
+	}
+
 	// Parse the .dynamic section as a string of bytes.
 	var vals []uint64
 	for len(d) > 0 {