objcopy: add support for `--compress-debug-sections`

1 files changed, 77 insertions, 2 deletions

diff --git a/src/objcopy.zig b/src/objcopy.zig
index 4555ee3f18..eb87c3ca23 100644
--- a/src/objcopy.zig
+++ b/src/objcopy.zig
@@ -27,6 +27,7 @@ pub fn cmdObjCopy(
     var strip_all: bool = false;
     var strip_debug: bool = false;
     var only_keep_debug: bool = false;
+    var compress_debug_sections: bool = false;
     var listen = false;
     while (i < args.len) : (i += 1) {
         const arg = args[i];
@@ -78,6 +79,8 @@ pub fn cmdObjCopy(
             strip_all = true;
         } else if (mem.eql(u8, arg, "--only-keep-debug")) {
             only_keep_debug = true;
+        } else if (mem.eql(u8, arg, "--compress-debug-sections")) {
+            compress_debug_sections = true;
         } else if (mem.startsWith(u8, arg, "--add-gnu-debuglink=")) {
             opt_add_debuglink = arg["--add-gnu-debuglink=".len..];
         } else if (mem.eql(u8, arg, "--add-gnu-debuglink")) {
@@ -152,6 +155,7 @@ pub fn cmdObjCopy(
                 .only_keep_debug = only_keep_debug,
                 .add_debuglink = opt_add_debuglink,
                 .extract_to = opt_extract,
+                .compress_debug = compress_debug_sections,
             });
             return std.process.cleanExit();
         },
@@ -210,6 +214,7 @@ const usage =
     \\  --only-keep-debug           Strip a file, removing contents of any sections that would not be stripped by --strip-debug and leaving the debugging sections intact.
     \\  --add-gnu-debuglink=<file>  Creates a .gnu_debuglink section which contains a reference to <file> and adds it to the output file.
     \\  --extract-to <file>         Extract the removed sections into <file>, and add a .gnu-debuglink section.
+    \\  --compress-debug-sections   Compress DWARF debug sections with zlib
     \\
 ;
 
@@ -660,6 +665,7 @@ const StripElfOptions = struct {
     strip_all: bool = false,
     strip_debug: bool = false,
     only_keep_debug: bool = false,
+    compress_debug: bool = false,
 };
 
 fn stripElf(
@@ -711,11 +717,11 @@ fn stripElf(
                 };
                 defer dbg_file.close();
 
-                try elf_file.emit(allocator, dbg_file, in_file, .{ .section_filter = flt });
+                try elf_file.emit(allocator, dbg_file, in_file, .{ .section_filter = flt, .compress_debug = options.compress_debug });
             }
 
             const debuglink: ?DebugLink = if (debuglink_path) |path| ElfFileHelper.createDebugLink(path) else null;
-            try elf_file.emit(allocator, out_file, in_file, .{ .section_filter = filter, .debuglink = debuglink });
+            try elf_file.emit(allocator, out_file, in_file, .{ .section_filter = filter, .debuglink = debuglink, .compress_debug = options.compress_debug });
         },
     }
 }
@@ -730,6 +736,7 @@ fn ElfFile(comptime is_64: bool) type {
     const Elf_Ehdr = if (is_64) elf.Elf64_Ehdr else elf.Elf32_Ehdr;
     const Elf_Phdr = if (is_64) elf.Elf64_Phdr else elf.Elf32_Phdr;
     const Elf_Shdr = if (is_64) elf.Elf64_Shdr else elf.Elf32_Shdr;
+    const Elf_Chdr = if (is_64) elf.Elf64_Chdr else elf.Elf32_Chdr;
     const Elf_Sym = if (is_64) elf.Elf64_Sym else elf.Elf32_Sym;
     const Elf_Verdef = if (is_64) elf.Elf64_Verdef else elf.Elf32_Verdef;
     const Elf_OffSize = if (is_64) elf.Elf64_Off else elf.Elf32_Off;
@@ -876,6 +883,7 @@ fn ElfFile(comptime is_64: bool) type {
         const EmitElfOptions = struct {
             section_filter: Filter = .all,
             debuglink: ?DebugLink = null,
+            compress_debug: bool = false,
         };
         fn emit(self: *const Self, gpa: Allocator, out_file: File, in_file: File, options: EmitElfOptions) !void {
             var arena = std.heap.ArenaAllocator.init(gpa);
@@ -939,6 +947,30 @@ fn ElfFile(comptime is_64: bool) type {
                 break :blk new_offset;
             };
 
+            // maybe compress .debug sections
+            if (options.compress_debug) {
+                for (self.sections[1..], sections_update[1..]) |section, *update| {
+                    if (update.action != .keep) continue;
+                    if (!std.mem.startsWith(u8, section.name, ".debug_")) continue;
+                    if ((section.section.sh_flags & elf.SHF_COMPRESSED) != 0) continue; // already compressed
+
+                    const chdr = Elf_Chdr{
+                        .ch_type = elf.COMPRESS.ZLIB,
+                        .ch_size = section.section.sh_size,
+                        .ch_addralign = section.section.sh_addralign,
+                    };
+
+                    const compressed_payload = try ElfFileHelper.tryCompressSection(allocator, in_file, section.section.sh_offset, section.section.sh_size, std.mem.asBytes(&chdr));
+                    if (compressed_payload) |payload| {
+                        update.payload = payload;
+                        update.section = section.section;
+                        update.section.?.sh_addralign = @alignOf(Elf_Chdr);
+                        update.section.?.sh_size = @intCast(Elf_OffSize, payload.len);
+                        update.section.?.sh_flags |= elf.SHF_COMPRESSED;
+                    }
+                }
+            }
+
             var cmdbuf = std.ArrayList(ElfFileHelper.WriteCmd).init(allocator);
             defer cmdbuf.deinit();
             try cmdbuf.ensureUnusedCapacity(3 + new_shnum);
@@ -1247,6 +1279,49 @@ const ElfFileHelper = struct {
         }
     }
 
+    fn tryCompressSection(allocator: Allocator, in_file: File, offset: u64, size: u64, prefix: []const u8) !?[]align(8) const u8 {
+        if (size < prefix.len) return null;
+
+        try in_file.seekTo(offset);
+        var section_reader = std.io.limitedReader(in_file.reader(), size);
+
+        // allocate as large as decompressed data. if the compression doesn't fit, keep the data uncompressed.
+        const compressed_data = try allocator.alignedAlloc(u8, 8, @intCast(usize, size));
+        var compressed_stream = std.io.fixedBufferStream(compressed_data);
+
+        try compressed_stream.writer().writeAll(prefix);
+
+        {
+            var compressor = try std.compress.zlib.compressStream(allocator, compressed_stream.writer(), .{});
+            defer compressor.deinit();
+
+            var buf: [8000]u8 = undefined;
+            while (true) {
+                const bytes_read = try section_reader.read(&buf);
+                if (bytes_read == 0) break;
+                const bytes_written = compressor.write(buf[0..bytes_read]) catch |err| switch (err) {
+                    error.NoSpaceLeft => {
+                        allocator.free(compressed_data);
+                        return null;
+                    },
+                    else => return err,
+                };
+                std.debug.assert(bytes_written == bytes_read);
+            }
+            compressor.finish() catch |err| switch (err) {
+                error.NoSpaceLeft => {
+                    allocator.free(compressed_data);
+                    return null;
+                },
+                else => return err,
+            };
+        }
+
+        const compressed_len = @intCast(usize, compressed_stream.getPos() catch unreachable);
+        const data = allocator.realloc(compressed_data, compressed_len) catch compressed_data;
+        return data[0..compressed_len];
+    }
+
     fn createDebugLink(path: []const u8) DebugLink {
         const file = std.fs.cwd().openFile(path, .{}) catch |err| {
             fatal("zig objcopy: could not open `{s}`: {s}\n", .{ path, @errorName(err) });