From 33a0a72e87bd01ce6dbd10fa327c74168a0166db Mon Sep 17 00:00:00 2001
From: Jakub Konka <kubkon@jakubkonka.com>
Date: Fri, 3 Nov 2023 10:03:03 +0100
Subject: [PATCH] elf: align ar_hdr to at least 2 bytes

---
 src/link/Elf.zig         | 21 +++++++++++++++++----
 src/link/Elf/Archive.zig |  2 +-
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/src/link/Elf.zig b/src/link/Elf.zig
index 85274d37d3..d938355da5 100644
--- a/src/link/Elf.zig
+++ b/src/link/Elf.zig
@@ -1580,6 +1580,13 @@ pub fn flushStaticLib(self: *Elf, comp: *Compilation) link.File.FlushError!void
         files.putAssumeCapacityNoClobber(zig_object.index, .{ off, 0, 0 });
     }
 
+    // Align to even byte boundary
+    {
+        const end = ar_strtab.items.len;
+        const aligned = mem.alignForward(usize, end, 2);
+        try ar_strtab.writer().writeByteNTimes(0, aligned - end);
+    }
+
     // Encode ar symtab in 64bit format.
     var ar_symtab = std.ArrayList(u8).init(gpa);
     defer ar_symtab.deinit();
@@ -1599,7 +1606,7 @@ pub fn flushStaticLib(self: *Elf, comp: *Compilation) link.File.FlushError!void
         try ar_symtab.writer().print("{s}\x00", .{name});
     }
 
-    // Align to 8bytes if required
+    // Align to 8 bytes if required
     {
         const end = ar_symtab.items.len;
         const aligned = mem.alignForward(usize, end, 8);
@@ -1617,8 +1624,6 @@ pub fn flushStaticLib(self: *Elf, comp: *Compilation) link.File.FlushError!void
         file_off += @sizeOf(Archive.ar_hdr) + @as(u64, @intCast(ar_symtab.items.len));
         // Strtab
         file_off += @sizeOf(Archive.ar_hdr) + @as(u64, @intCast(ar_strtab.items.len));
-        // And because we are nice, we will align to 8 bytes.
-        file_off = mem.alignForward(u64, file_off, 8);
 
         const files_ptr = files.getPtr(zig_object.index).?;
         files_ptr[1] = file_off;
@@ -1661,7 +1666,7 @@ pub fn flushStaticLib(self: *Elf, comp: *Compilation) link.File.FlushError!void
         const hdr = setArHdr(.{
             .kind = .strtab,
             .name_off = 0,
-            .size = @intCast(mem.alignForward(usize, ar_strtab.items.len, 8)),
+            .size = @intCast(ar_strtab.items.len),
         });
         try self.base.file.?.pwriteAll(mem.asBytes(&hdr), pos);
         pos += @sizeOf(Archive.ar_hdr);
@@ -1674,8 +1679,16 @@ pub fn flushStaticLib(self: *Elf, comp: *Compilation) link.File.FlushError!void
         const entry = files.get(zig_object.index).?;
         const hdr = setArHdr(.{ .kind = .object, .name_off = entry[0], .size = @intCast(entry[2]) });
         try self.base.file.?.pwriteAll(mem.asBytes(&hdr), entry[1]);
+        pos += @sizeOf(Archive.ar_hdr) + entry[2];
     }
 
+    if (pos % 2 != 0) {
+        pos += 1;
+        try self.base.file.?.pwriteAll(&[1]u8{0}, pos);
+    }
+
+    assert(mem.isAligned(pos, 2));
+
     // TODO parsed positionals
 
     // Magic bytes.
diff --git a/src/link/Elf/Archive.zig b/src/link/Elf/Archive.zig
index 49496ec7b5..bdefbfad04 100644
--- a/src/link/Elf/Archive.zig
+++ b/src/link/Elf/Archive.zig
@@ -88,7 +88,7 @@ pub fn parse(self: *Archive, elf_file: *Elf) !void {
         if (stream.pos % 2 != 0) {
             stream.pos += 1;
         }
-
+        // TODO flag an error if stream.pos > self.data.len after alignment
         const hdr = reader.readStruct(ar_hdr) catch break;
 
         if (!mem.eql(u8, &hdr.ar_fmag, ARFMAG)) {