Restrict rewrite of unary operators to dereference operator

Since the whole expression, unary operator included, is replaced by a
call to bpf_probe_read, the dereference operator is currently the
only unary operator properly rewritten. When rewriting an increment
expression (++val) for instance, the increment operator is lost in
translation.

Trying to rewrite all unary operators sometimes confuses bcc and
results in improper code, for instance when trying to rewrite a
logical negation.

src/cc/frontends/clang/b_frontend_action.cc

@@ -200,7 +200,7 @@ bool ProbeVisitor::VisitBinaryOperator(BinaryOperator *E) {
  return true;
 }
 bool ProbeVisitor::VisitUnaryOperator(UnaryOperator *E) {
- if (E->getOpcode() == UO_AddrOf)
+ if (E->getOpcode() != UO_Deref)
  return true;
  if (memb_visited_.find(E) != memb_visited_.end())
  return true;

tests/python/test_clang.py

@@ -408,6 +408,17 @@ def test_bpf_dins_pkt_rewrite(self):
  """
  b = BPF(text=text)
 
+ def test_unary_operator(self):
+ text = """
+#include <linux/fs.h>
+#include <uapi/linux/ptrace.h>
+int trace_read_entry(struct pt_regs *ctx, struct file *file) {
+ return !file->f_op->read_iter;
+}
+ """
+ b = BPF(text=text)
+ b.attach_kprobe(event="__vfs_read", fn_name="trace_read_entry")
+
 if __name__ == "__main__":
  main()