add some asserts about addresses in the dummy dll generation

crates/linker/src/generate_dylib/pe.rs

@@ -14,7 +14,11 @@ fn synthetic_image_export_directory(
     let start = virtual_address + directory_size + name.len() as u32 + 1;
 
     let address_of_functions = start;
-    let address_of_names = address_of_functions + 4 * custom_names.len() as u32;
+
+    // by convention, names start at index, so we add one "padding" name
+    let number_of_functions = custom_names.len() as u32 + 1;
+
+    let address_of_names = address_of_functions + 4 * number_of_functions;
     let address_of_name_ordinals = address_of_names + 4 * custom_names.len() as u32;
 
     pe::ImageExportDirectory {
@@ -24,7 +28,7 @@ fn synthetic_image_export_directory(
         minor_version: U16::new(LE, 0),
         name: U32::new(LE, virtual_address + directory_size),
         base: U32::new(LE, 0),
-        number_of_functions: U32::new(LE, custom_names.len() as u32),
+        number_of_functions: U32::new(LE, number_of_functions),
         number_of_names: U32::new(LE, custom_names.len() as u32),
         address_of_functions: U32::new(LE, address_of_functions),
         address_of_names: U32::new(LE, address_of_names),
@@ -50,15 +54,38 @@ fn synthetic_export_dir(virtual_address: u32, custom_names: &[String]) -> Vec<u8
 
     let n = custom_names.len();
 
+    debug_assert_eq!(
+        directory.address_of_functions.get(LE) - virtual_address,
+        vec.len() as u32
+    );
+
     // Unsure what this one does; it does not seem important for our purposes
     //
     // Export Address Table -- Ordinal Base 0
     //  [   1] +base[   1] 1020 Export RVA
     //  [   2] +base[   2] d030 Export RVA
-    for _ in custom_names {
-        vec.extend(42u32.to_le_bytes());
+
+    // pad with 4 bytes to have the first actual index start at 1.
+    // Having this table be 1-indexed seems to be convention
+    vec.extend(0u32.to_le_bytes());
+
+    // here we pre-calculate the virtual address where the name bytes will be stored
+    // The address hence points to a place in the .edata section. Such exports are
+    // called "forwarders". Because there is no actual definition, these exports don't
+    // show up when calling the `object` crate's .exports method.
+    let mut next_name_start =
+        directory.address_of_name_ordinals.get(LE) + custom_names.len() as u32 * 2;
+
+    for name in custom_names.iter() {
+        vec.extend(next_name_start.to_le_bytes());
+        next_name_start += name.len() as u32 + 1; // null-terminated
     }
 
+    debug_assert_eq!(
+        directory.address_of_names.get(LE) - virtual_address,
+        vec.len() as u32
+    );
+
     // Maps the index to a name
     //
     // [Ordinal/Name Pointer] Table
@@ -70,9 +97,14 @@ fn synthetic_export_dir(virtual_address: u32, custom_names: &[String]) -> Vec<u8
         acc += name.len() as u32 + 1;
     }
 
-    // the ordinals, which just map to the index in our case
-    for (i, _) in custom_names.iter().enumerate() {
-        vec.extend((i as u16).to_le_bytes());
+    debug_assert_eq!(
+        directory.address_of_name_ordinals.get(LE) - virtual_address,
+        vec.len() as u32
+    );
+
+    // the ordinals, which map to the index plus 1 to the ordinals start at 1
+    for i in 0..custom_names.len() {
+        vec.extend((i as u16 + 1).to_le_bytes());
     }
 
     // write out the names of the symbols, as null-terminated strings