From 3f22bb96f393a81a33772bdeddce5fc660e4f667 Mon Sep 17 00:00:00 2001
From: Luuk de Gram <luuk@degram.dev>
Date: Wed, 10 Jan 2024 06:55:12 +0100
Subject: [PATCH] wasm-linker: fix debug info relocation

This corrects calculating the offsets to the code section as we now
correctly allocate the code atoms during write taking the 'size' into
account. We also handle dead symbols which are garbage-collected by
writing -2 and -1 to skip ranges, loc and other sections respectively.
---
 src/link/Wasm/Atom.zig | 40 ++++++++++++++++++++++++++++++----------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/src/link/Wasm/Atom.zig b/src/link/Wasm/Atom.zig
index cdf22378b7..c8d115b872 100644
--- a/src/link/Wasm/Atom.zig
+++ b/src/link/Wasm/Atom.zig
@@ -111,7 +111,7 @@ pub fn resolveRelocs(atom: *Atom, wasm_bin: *const Wasm) void {
             .R_WASM_GLOBAL_INDEX_I32,
             .R_WASM_MEMORY_ADDR_I32,
             .R_WASM_SECTION_OFFSET_I32,
-            => std.mem.writeInt(u32, atom.code.items[reloc.offset - atom.original_offset ..][0..4], @as(u32, @intCast(value)), .little),
+            => std.mem.writeInt(u32, atom.code.items[reloc.offset - atom.original_offset ..][0..4], @as(u32, @truncate(value)), .little),
             .R_WASM_TABLE_INDEX_I64,
             .R_WASM_MEMORY_ADDR_I64,
             => std.mem.writeInt(u64, atom.code.items[reloc.offset - atom.original_offset ..][0..8], value, .little),
@@ -124,7 +124,7 @@ pub fn resolveRelocs(atom: *Atom, wasm_bin: *const Wasm) void {
             .R_WASM_TABLE_NUMBER_LEB,
             .R_WASM_TYPE_INDEX_LEB,
             .R_WASM_MEMORY_ADDR_TLS_SLEB,
-            => leb.writeUnsignedFixed(5, atom.code.items[reloc.offset - atom.original_offset ..][0..5], @as(u32, @intCast(value))),
+            => leb.writeUnsignedFixed(5, atom.code.items[reloc.offset - atom.original_offset ..][0..5], @as(u32, @truncate(value))),
             .R_WASM_MEMORY_ADDR_LEB64,
             .R_WASM_MEMORY_ADDR_SLEB64,
             .R_WASM_TABLE_INDEX_SLEB64,
@@ -140,6 +140,13 @@ pub fn resolveRelocs(atom: *Atom, wasm_bin: *const Wasm) void {
 fn relocationValue(atom: Atom, relocation: types.Relocation, wasm_bin: *const Wasm) u64 {
     const target_loc = (Wasm.SymbolLoc{ .file = atom.file, .index = relocation.index }).finalLoc(wasm_bin);
     const symbol = target_loc.getSymbol(wasm_bin);
+    if (relocation.relocation_type != .R_WASM_TYPE_INDEX_LEB and
+        symbol.tag != .section and
+        symbol.isDead())
+    {
+        const val = atom.thombstone(wasm_bin) orelse relocation.addend;
+        return @bitCast(val);
+    }
     switch (relocation.relocation_type) {
         .R_WASM_FUNCTION_INDEX_LEB => return symbol.index,
         .R_WASM_TABLE_NUMBER_LEB => return symbol.index,
@@ -170,30 +177,43 @@ fn relocationValue(atom: Atom, relocation: types.Relocation, wasm_bin: *const Wa
             if (symbol.isUndefined()) {
                 return 0;
             }
-            const va = @as(i64, @intCast(symbol.virtual_address));
+            const va: i33 = @intCast(symbol.virtual_address);
             return @intCast(va + relocation.addend);
         },
         .R_WASM_EVENT_INDEX_LEB => return symbol.index,
         .R_WASM_SECTION_OFFSET_I32 => {
             const target_atom_index = wasm_bin.symbol_atom.get(target_loc).?;
             const target_atom = wasm_bin.getAtom(target_atom_index);
-            const rel_value: i32 = @intCast(target_atom.offset);
+            const rel_value: i33 = @intCast(target_atom.offset);
             return @intCast(rel_value + relocation.addend);
         },
         .R_WASM_FUNCTION_OFFSET_I32 => {
-            const target_atom_index = wasm_bin.symbol_atom.get(target_loc) orelse {
-                return @as(u32, @bitCast(@as(i32, -1)));
-            };
+            if (symbol.isUndefined()) {
+                const val = atom.thombstone(wasm_bin) orelse relocation.addend;
+                return @bitCast(val);
+            }
+            const target_atom_index = wasm_bin.symbol_atom.get(target_loc).?;
             const target_atom = wasm_bin.getAtom(target_atom_index);
-            const offset: u32 = 11 + Wasm.getULEB128Size(target_atom.size); // Header (11 bytes fixed-size) + body size (leb-encoded)
-            const rel_value: i32 = @intCast(target_atom.offset + offset);
+            const rel_value: i33 = @intCast(target_atom.offset);
             return @intCast(rel_value + relocation.addend);
         },
         .R_WASM_MEMORY_ADDR_TLS_SLEB,
         .R_WASM_MEMORY_ADDR_TLS_SLEB64,
         => {
-            const va: i32 = @intCast(symbol.virtual_address);
+            const va: i33 = @intCast(symbol.virtual_address);
             return @intCast(va + relocation.addend);
         },
     }
 }
+
+// For a given `Atom` returns whether it has a thombstone value or not.
+/// This defines whether we want a specific value when a section is dead.
+fn thombstone(atom: Atom, wasm: *const Wasm) ?i64 {
+    const atom_name = atom.symbolLoc().getName(wasm);
+    if (std.mem.eql(u8, atom_name, ".debug_ranges") or std.mem.eql(u8, atom_name, ".debug_loc")) {
+        return -2;
+    } else if (std.mem.startsWith(u8, atom_name, ".debug_")) {
+        return -1;
+    }
+    return null;
+}