Wrong unmarshaling of function pointers in debugger mode (#10709)

In debugger mode, closures sent by the debuggee have their code pointer
modified so that it points to the `function_placeholder` code in
debugger/main.ml.

This modification was performed incorrectly: the whole closure for
`function_placeholder` was used as the code pointer for the
unmarshalled closure.

This commit implements the correct operation: the code pointer from
`function_placeholder` is used as code pointer for the unmarshaled
closure.

Also: raise a more informative exception when custom printer invokes
closure from debuggee.

Fixes: #9214

Changes

@@ -259,6 +259,11 @@ Working version
 
 ### Bug fixes:
 
+- #9214, #10709: Wrong unmarshaling of function pointers in debugger mode.
+  This was causing ocamldebug to crash when running some user-defined printers.
+  (Xavier Leroy, report by Rehan Malak, review by Gabriel Scherer and
+   Vincent Laviron)
+
 - #10473: Add CFI directives to RISC-V runtime and asmcomp.
   This allows stacktraces to work in gdb through C and OCaml calls.
   (Edwin Török, review by Nicolás Ojeda Bär and Xavier Leroy)
@@ -298,6 +303,8 @@ Working version
   implementation of stat
   (Antonin Décimo, review by David Allsopp)
 
+
+
 OCaml 4.13 maintenance branch
 -----------------------------
 

debugger/main.ml

@@ -197,7 +197,7 @@ let speclist = [
    ]
 
 let function_placeholder () =
-  raise Not_found
+  failwith "custom printer tried to invoke a function from the debuggee"
 
 let report report_error error =
   eprintf "Debugger [version %s] environment error:@ @[@;%a@]@.;"

runtime/intern.c

@@ -511,7 +511,8 @@ static void intern_rec(value *dest)
           const value * function_placeholder =
             caml_named_value ("Debugger.function_placeholder");
           if (function_placeholder != NULL) {
-            v = *function_placeholder;
+            /* Use the code pointer from the "placeholder" function */
+            v = (value) Code_val(*function_placeholder);
           } else {
             intern_cleanup();
             intern_bad_code_pointer(digest);