Commit 6ff267dc26

@@ -453,7 +453,7 @@ fn emitReloc(elf_file: *Elf, r_offset: u64, sym: *const Symbol, rel: elf.Elf64_R
     };
 }
 
-pub fn writeEhFrameRelocs(elf_file: *Elf, writer: anytype) !void {
+pub fn writeEhFrameRelocs(elf_file: *Elf, relocs: *std.ArrayList(elf.Elf64_Rela)) !void {
     relocs_log.debug("{x}: .eh_frame", .{
         elf_file.sections.items(.shdr)[elf_file.section_indexes.eh_frame.?].sh_addr,
     });
@@ -466,8 +466,7 @@ pub fn writeEhFrameRelocs(elf_file: *Elf, writer: anytype) !void {
         for (atom_ptr.relocs(elf_file)) |rel| {
             const ref = zo.resolveSymbol(rel.r_sym(), elf_file);
             const target = elf_file.symbol(ref).?;
-            const out_rel = emitReloc(elf_file, rel.r_offset, target, rel);
-            try writer.writeStruct(out_rel);
+            try relocs.append(emitReloc(elf_file, rel.r_offset, target, rel));
         }
     }
 
@@ -480,8 +479,7 @@ pub fn writeEhFrameRelocs(elf_file: *Elf, writer: anytype) !void {
                 const ref = object.resolveSymbol(rel.r_sym(), elf_file);
                 const sym = elf_file.symbol(ref).?;
                 const r_offset = cie.address(elf_file) + rel.r_offset - cie.offset;
-                const out_rel = emitReloc(elf_file, r_offset, sym, rel);
-                try writer.writeStruct(out_rel);
+                try relocs.append(emitReloc(elf_file, r_offset, sym, rel));
             }
         }
 
@@ -491,8 +489,7 @@ pub fn writeEhFrameRelocs(elf_file: *Elf, writer: anytype) !void {
                 const ref = object.resolveSymbol(rel.r_sym(), elf_file);
                 const sym = elf_file.symbol(ref).?;
                 const r_offset = fde.address(elf_file) + rel.r_offset - fde.offset;
-                const out_rel = emitReloc(elf_file, r_offset, sym, rel);
-                try writer.writeStruct(out_rel);
+                try relocs.append(emitReloc(elf_file, r_offset, sym, rel));
             }
         }
     }

@@ -398,6 +398,10 @@ fn parseEhFrame(
     defer gpa.free(relocs);
     const rel_start: u32 = @intCast(self.relocs.items.len);
     try self.relocs.appendUnalignedSlice(gpa, relocs);
+
+    // We expect relocations to be sorted by r_offset as per this comment in mold linker:
+    // https://github.com/rui314/mold/blob/8e4f7b53832d8af4f48a633a8385cbc932d1944e/src/input-files.cc#L653
+    // Except for RISCV and Loongarch which do not seem to be uphold this convention.
     if (target.cpu.arch == .riscv64) {
         sortRelocs(self.relocs.items[rel_start..][0..relocs.len]);
     }

@@ -362,6 +362,14 @@ fn writeSyntheticSections(elf_file: *Elf) !void {
     const gpa = elf_file.base.comp.gpa;
     const slice = elf_file.sections.slice();
 
+    const SortRelocs = struct {
+        pub fn lessThan(ctx: void, lhs: elf.Elf64_Rela, rhs: elf.Elf64_Rela) bool {
+            _ = ctx;
+            assert(lhs.r_offset != rhs.r_offset);
+            return lhs.r_offset < rhs.r_offset;
+        }
+    };
+
     for (slice.items(.shdr), slice.items(.atom_list), 0..) |shdr, atom_list, shndx| {
         if (shdr.sh_type != elf.SHT_RELA) continue;
         if (atom_list.items.len == 0) continue;
@@ -378,15 +386,8 @@ fn writeSyntheticSections(elf_file: *Elf) !void {
             try atom_ptr.writeRelocs(elf_file, &relocs);
         }
         assert(relocs.items.len == num_relocs);
-
-        const SortRelocs = struct {
-            pub fn lessThan(ctx: void, lhs: elf.Elf64_Rela, rhs: elf.Elf64_Rela) bool {
-                _ = ctx;
-                assert(lhs.r_offset != rhs.r_offset);
-                return lhs.r_offset < rhs.r_offset;
-            }
-        };
-
+        // Sort output relocations by r_offset which is usually an expected (and desired) condition
+        // by the linkers.
         mem.sortUnstable(elf.Elf64_Rela, relocs.items, {}, SortRelocs.lessThan);
 
         log.debug("writing {s} from 0x{x} to 0x{x}", .{
@@ -418,16 +419,21 @@ fn writeSyntheticSections(elf_file: *Elf) !void {
     }
     if (elf_file.section_indexes.eh_frame_rela) |shndx| {
         const shdr = slice.items(.shdr)[shndx];
-        const sh_size = math.cast(usize, shdr.sh_size) orelse return error.Overflow;
-        var buffer = try std.ArrayList(u8).initCapacity(gpa, sh_size);
-        defer buffer.deinit();
-        try eh_frame.writeEhFrameRelocs(elf_file, buffer.writer());
-        assert(buffer.items.len == sh_size);
+        const num_relocs = math.cast(usize, @divExact(shdr.sh_size, shdr.sh_entsize)) orelse
+            return error.Overflow;
+        var relocs = try std.ArrayList(elf.Elf64_Rela).initCapacity(gpa, num_relocs);
+        defer relocs.deinit();
+        try eh_frame.writeEhFrameRelocs(elf_file, &relocs);
+        assert(relocs.items.len == num_relocs);
+        // Sort output relocations by r_offset which is usually an expected (and desired) condition
+        // by the linkers.
+        mem.sortUnstable(elf.Elf64_Rela, relocs.items, {}, SortRelocs.lessThan);
+
         log.debug("writing .rela.eh_frame from 0x{x} to 0x{x}", .{
             shdr.sh_offset,
             shdr.sh_offset + shdr.sh_size,
         });
-        try elf_file.base.file.?.pwriteAll(buffer.items, shdr.sh_offset);
+        try elf_file.base.file.?.pwriteAll(mem.sliceAsBytes(relocs.items), shdr.sh_offset);
     }
 
     try writeComdatGroups(elf_file);

@@ -2744,32 +2744,52 @@ fn testRelocatableEhFrame(b: *Build, opts: Options) *Step {
         ,
     });
     obj2.linkLibCpp();
+    const obj3 = addObject(b, opts, .{ .name = "obj3", .cpp_source_bytes = 
+    \\#include <iostream>
+    \\#include <stdexcept>
+    \\extern int try_again();
+    \\int main() {
+    \\  try {
+    \\    try_again();
+    \\  } catch (const std::exception &e) {
+    \\    std::cout << "exception=" << e.what();
+    \\  }
+    \\  return 0;
+    \\}
+    });
+    obj3.linkLibCpp();
 
-    const obj = addObject(b, opts, .{ .name = "obj" });
-    obj.addObject(obj1);
-    obj.addObject(obj2);
-    obj.linkLibCpp();
+    {
+        const obj = addObject(b, opts, .{ .name = "obj" });
+        obj.addObject(obj1);
+        obj.addObject(obj2);
+        obj.linkLibCpp();
 
-    const exe = addExecutable(b, opts, .{ .name = "test1" });
-    addCppSourceBytes(exe,
-        \\#include <iostream>
-        \\#include <stdexcept>
-        \\extern int try_again();
-        \\int main() {
-        \\  try {
-        \\    try_again();
-        \\  } catch (const std::exception &e) {
-        \\    std::cout << "exception=" << e.what();
-        \\  }
-        \\  return 0;
-        \\}
-    , &.{});
-    exe.addObject(obj);
-    exe.linkLibCpp();
+        const exe = addExecutable(b, opts, .{ .name = "test1" });
+        exe.addObject(obj3);
+        exe.addObject(obj);
+        exe.linkLibCpp();
 
-    const run = addRunArtifact(exe);
-    run.expectStdOutEqual("exception=Oh no!");
-    test_step.dependOn(&run.step);
+        const run = addRunArtifact(exe);
+        run.expectStdOutEqual("exception=Oh no!");
+        test_step.dependOn(&run.step);
+    }
+    {
+        // Flipping the order should not influence the end result.
+        const obj = addObject(b, opts, .{ .name = "obj" });
+        obj.addObject(obj2);
+        obj.addObject(obj1);
+        obj.linkLibCpp();
+
+        const exe = addExecutable(b, opts, .{ .name = "test2" });
+        exe.addObject(obj3);
+        exe.addObject(obj);
+        exe.linkLibCpp();
+
+        const run = addRunArtifact(exe);
+        run.expectStdOutEqual("exception=Oh no!");
+        test_step.dependOn(&run.step);
+    }
 
     return test_step;
 }