align end of elf archives

The end of the archive needs to also be aligned to a two-byte boundary,
not just the start of records. This was causing lld to reject archives.

Notably, this was happening with compiler_rt when rebuilding in fuzz
mode, which is why this commit is included in this patchset.

Author: gooncreeper

lib/std/Build/Step/CheckObject.zig

@@ -1701,6 +1701,10 @@ const ElfDumper = struct {
             return error.InvalidArchiveMagicNumber;
         }
 
+        if (!mem.isAligned(bytes.len, 2)) {
+            return error.InvalidArchivePadding;
+        }
+
         var ctx = ArchiveContext{
             .gpa = gpa,
             .data = bytes,
@@ -1714,8 +1718,8 @@ const ElfDumper = struct {
         }
 
         while (true) {
-            if (reader.seek >= ctx.data.len) break;
             if (!mem.isAligned(reader.seek, 2)) reader.seek += 1;
+            if (reader.seek >= ctx.data.len) break;
 
             const hdr = try reader.takeStruct(elf.ar_hdr, .little);
 

src/link/Elf/relocatable.zig

@@ -74,10 +74,11 @@ pub fn flushStaticLib(elf_file: *Elf, comp: *Compilation) !void {
     const total_size: usize = blk: {
         var pos: usize = elf.ARMAG.len;
         pos += @sizeOf(elf.ar_hdr) + ar_symtab.size(.p64);
+        pos = mem.alignForward(usize, pos, 2);
 
         if (ar_strtab.size() > 0) {
-            pos = mem.alignForward(usize, pos, 2);
             pos += @sizeOf(elf.ar_hdr) + ar_strtab.size();
+            pos = mem.alignForward(usize, pos, 2);
         }
 
         for (files.items) |index| {
@@ -87,9 +88,9 @@ pub fn flushStaticLib(elf_file: *Elf, comp: *Compilation) !void {
                 .object => |x| &x.output_ar_state,
                 else => unreachable,
             };
-            pos = mem.alignForward(usize, pos, 2);
             state.file_off = pos;
             pos += @sizeOf(elf.ar_hdr) + (math.cast(usize, state.size) orelse return error.Overflow);
+            pos = mem.alignForward(usize, pos, 2);
         }
 
         break :blk pos;
@@ -110,17 +111,18 @@ pub fn flushStaticLib(elf_file: *Elf, comp: *Compilation) !void {
 
     // Write symtab
     try ar_symtab.write(.p64, elf_file, &writer);
+    if (!mem.isAligned(writer.end, 2)) try writer.writeByte(0);
 
     // Write strtab
     if (ar_strtab.size() > 0) {
-        if (!mem.isAligned(writer.end, 2)) try writer.writeByte(0);
         try ar_strtab.write(&writer);
+        if (!mem.isAligned(writer.end, 2)) try writer.writeByte(0);
     }
 
     // Write object files
     for (files.items) |index| {
-        if (!mem.isAligned(writer.end, 2)) try writer.writeByte(0);
         try elf_file.file(index).?.writeAr(elf_file, &writer);
+        if (!mem.isAligned(writer.end, 2)) try writer.writeByte(0);
     }
 
     assert(writer.buffered().len == total_size);