Go: restore data flow through pointer assignment targets

Evaluate the operand of a skipped pointer LHS and create post-update nodes for the full access path.

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -106,6 +106,14 @@ module CfgImpl {
         not skipCfg(n) and
         result = n.getChild(index)
         or
+        exists(Go::Assignment assgn, Go::Expr lhs |
+          n = assgn and lhs = assgn.getLhs(_) and lhs = n.getChild(index) and skipCfg(lhs)
+        |
+          result = lhs.(Go::StarExpr).getBase()
+          or
+          result = lhs.(Go::DerefExpr).getOperand()
+        )
+        or
         // The body block of a switch (expression or type) is transparent (see
         // `skipCfg`), so it is not itself a child and contributes no children.
         // Expose the case clauses directly as children of the switch instead,

go/ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -772,6 +772,8 @@ module Public {
   private IR::Instruction getADirectlyWrittenInsn() {
     exists(Write w | w.writesComponentInstruction(result, _))
     or
+    result = any(Write w).getLhs().(IR::PointerTarget).getBase()
+    or
     result = IR::evalExprInstruction(any(SendStmt s).getChannel())
   }
 

go/ql/test/library-tests/semmle/go/controlflow/ControlFlowGraph/ControlFlowNode_getASuccessor.expected

@@ -2776,9 +2776,12 @@
 | stmts3.go:14:13:14:13 | x | stmts3.go:14:21:16:1 | block statement |
 | stmts3.go:14:21:16:1 | After block statement | stmts3.go:14:1:16:1 | Normal Exit |
 | stmts3.go:14:21:16:1 | block statement | stmts3.go:15:2:15:8 | ... = ... |
-| stmts3.go:15:2:15:8 | ... = ... | stmts3.go:15:7:15:8 | Before 42 |
+| stmts3.go:15:2:15:8 | ... = ... | stmts3.go:15:3:15:3 | Before x |
 | stmts3.go:15:2:15:8 | After ... = ... | stmts3.go:14:21:16:1 | After block statement |
 | stmts3.go:15:2:15:8 | assign:0 ... = ... | stmts3.go:15:2:15:8 | After ... = ... |
+| stmts3.go:15:3:15:3 | After x | stmts3.go:15:7:15:8 | Before 42 |
+| stmts3.go:15:3:15:3 | Before x | stmts3.go:15:3:15:3 | x |
+| stmts3.go:15:3:15:3 | x | stmts3.go:15:3:15:3 | After x |
 | stmts3.go:15:7:15:8 | 42 | stmts3.go:15:7:15:8 | After 42 |
 | stmts3.go:15:7:15:8 | After 42 | stmts3.go:15:2:15:8 | assign:0 ... = ... |
 | stmts3.go:15:7:15:8 | Before 42 | stmts3.go:15:7:15:8 | 42 |