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Improve dSYM loading for Mach-O binaries #425

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3 changes: 3 additions & 0 deletions CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -373,6 +373,7 @@ file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/src)
if(PROTOC_FOUND)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.cc
${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.h
DEPENDS protoc ${CMAKE_CURRENT_SOURCE_DIR}/src/bloaty.proto
COMMAND protoc ${CMAKE_CURRENT_SOURCE_DIR}/src/bloaty.proto
--cpp_out=${CMAKE_CURRENT_BINARY_DIR}/src
Expand All @@ -381,6 +382,7 @@ add_custom_command(
else()
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.cc
${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.h
COMMAND protoc ${CMAKE_CURRENT_SOURCE_DIR}/src/bloaty.proto
--cpp_out=${CMAKE_CURRENT_BINARY_DIR}/src
-I${CMAKE_CURRENT_SOURCE_DIR}/src
Expand All @@ -395,6 +397,7 @@ add_library(libbloaty STATIC
src/bloaty.h
src/disassemble.cc
${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.cc
${CMAKE_CURRENT_BINARY_DIR}/src/bloaty.pb.h
src/dwarf/attr.h
src/dwarf/attr.cc
src/dwarf/dwarf_util.cc
Expand Down
17 changes: 17 additions & 0 deletions doc/using.md
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Expand Up @@ -327,6 +327,23 @@ can create with `dsymutil`:
```
$ dsymutil bloaty
$ strip bloaty (optional)
$ ./bloaty -d symbols bloaty # Auto-discovers bloaty.dSYM
```

Bloaty will automatically discover and load debug symbols from
`bloaty.dSYM/Contents/Resources/DWARF/bloaty` when analyzing `bloaty`.

If you used `dsymutil -o` to write the dSYM to a different directory,
you can explicitly specify its location:

```
$ dsymutil bloaty -o /path/to/symbols/bloaty.dSYM
$ ./bloaty -d symbols --dsym=/path/to/symbols/bloaty.dSYM bloaty
```

For cross-platform compatibility, `--debug-file` still works:

```
$ ./bloaty -d symbols --debug-file=bloaty.dSYM/Contents/Resources/DWARF/bloaty bloaty
```

Expand Down
158 changes: 156 additions & 2 deletions src/bloaty.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@ typedef size_t z_size_t;

#include <atomic>
#include <cmath>
#include <filesystem>
#include <fstream>
#include <iostream>
#include <limits>
Expand Down Expand Up @@ -1526,6 +1527,9 @@ class Bloaty {
void AddFilename(const std::string& filename, bool base_file);
void AddDebugFilename(const std::string& filename);
void AddSourceMapFilename(const std::string& filename);
void AddDsymFilename(const Options& options, const std::string& filename);
void TryAutoLoadDsym(const std::string& binary_path,
const std::string& build_id);

size_t GetSourceCount() const { return sources_.size(); }

Expand Down Expand Up @@ -1573,6 +1577,15 @@ class Bloaty {

std::unique_ptr<ObjectFile> GetObjectFile(const std::string& filename) const;

bool TryGetBuildId(const std::string& filename, std::string* build_id);
bool TryValidateAndAddDsym(const std::string& dsym_path,
const std::string& main_binary_path,
bool strict_validation);

std::filesystem::path ConstructDsymPath(const std::filesystem::path& binary_path);
std::filesystem::path GetDsymInternalPath(const std::filesystem::path& dsym_bundle,
const std::string& binary_name);

const InputFileFactory& file_factory_;
const Options options_;

Expand Down Expand Up @@ -1634,6 +1647,9 @@ void Bloaty::AddFilename(const std::string& filename, bool is_base) {
auto object_file = GetObjectFile(filename);
std::string build_id = object_file->GetBuildId();

// Automatically try to load dSYM file for Mach-O binaries
TryAutoLoadDsym(filename, build_id);

if (is_base) {
base_files_.push_back({filename, build_id});
} else {
Expand Down Expand Up @@ -1663,6 +1679,137 @@ void Bloaty::AddSourceMapFilename(const std::string& filename) {
sourcemap_files_[sourcemap_build_id] = sourcemap_filename;
}

bool Bloaty::TryGetBuildId(const std::string& filename, std::string* build_id) {
try {
auto object_file = GetObjectFile(filename);
*build_id = object_file->GetBuildId();
return true;
} catch (const std::exception&) {
return false;
}
}

bool Bloaty::TryValidateAndAddDsym(const std::string& dsym_path,
const std::string& main_binary_path,
bool strict_validation) {
std::string main_build_id, dsym_build_id;

if (!TryGetBuildId(main_binary_path, &main_build_id) ||
!TryGetBuildId(dsym_path, &dsym_build_id)) {
if (strict_validation) {
THROWF("Cannot read build ID from files: main=$0, dsym=$1",
main_binary_path, dsym_path);
}
return false;
}

if (main_build_id != dsym_build_id) {
if (strict_validation) {
THROWF("dSYM file $0 has mismatched build ID (expected $1, got $2)",
dsym_path,
absl::BytesToHexString(main_build_id),
absl::BytesToHexString(dsym_build_id));
} else {
WARN("dSYM file $0 has mismatched build ID (expected $1, got $2) - skipping",
dsym_path,
absl::BytesToHexString(main_build_id),
absl::BytesToHexString(dsym_build_id));
}
return false;
}

AddDebugFilename(dsym_path);
return true;
}

std::filesystem::path Bloaty::ConstructDsymPath(const std::filesystem::path& binary_path) {
std::filesystem::path binary_file(binary_path);
std::string binary_name = binary_file.filename().string();
std::filesystem::path binary_dir = binary_file.parent_path();

std::filesystem::path dsym_bundle = binary_dir / (binary_name + ".dSYM");
return dsym_bundle / "Contents" / "Resources" / "DWARF" / binary_name;
}

std::filesystem::path Bloaty::GetDsymInternalPath(const std::filesystem::path& dsym_bundle,
const std::string& binary_name) {
return dsym_bundle / "Contents" / "Resources" / "DWARF" / binary_name;
}

// Loads dSYM files explicitly specified by the user using the --dsym flag.
//
// This function processes dSYM bundle paths with strict validation:
// - Warns if no dSYM files match the input binaries
// - Throws if the dSYM bundle path doesn't exist or isn't a directory
// - Throws if dSYM files are invalid or unreadable
// - Throws if buildids don't match between binary and dSYM
//
// The strict behaviour is intentional: when users explicitly specify --dsym,
// they expect it to work and want to know immediately if there are problems.
void Bloaty::AddDsymFilename(const Options& options,
const std::string& filename) {
if (!std::filesystem::exists(filename)) {
THROWF("couldn't open '$0'", filename.c_str());
}

std::filesystem::path dsym_bundle(filename);
// Only support dSYM bundles
if (std::filesystem::is_directory(dsym_bundle)) {
bool found_any = false;
for (const auto& binary_filename : options.filename()) {
std::filesystem::path binary_name = std::filesystem::path(binary_filename).filename();
std::filesystem::path dsym_file = GetDsymInternalPath(dsym_bundle, binary_name.string());

if (std::filesystem::exists(dsym_file)) {
if (TryValidateAndAddDsym(dsym_file.string(), binary_filename,
/*strict_validation=*/true)) {
found_any = true;
}
}
}
if (!found_any) {
WARN("dSYM bundle '$0' contains no debug files matching input binaries",
filename);
}
} else {
THROWF("'$0' is not a dSYM bundle directory", filename);
}
}

// Automatically discovers and loads dSYM files using macOS conventions.
//
// This function attempts to find dSYM bundles using the standard macOS layout:
// <binary_dir>/<binary_name>.dSYM/Contents/Resources/DWARF/<binary_name>
//
// Key behavioural differences from AddDsymFilename():
// - Never throws exceptions or breaks analysis if discovery fails
// - Ignores invalid/corrupted dSYM files
// - Warns on buildid mismatches but continues without the dSYM
//
// The permissive behaviour is intentional because auto-loading is speculative and
// shouldn't break analysis of the main binary. Users didn't explicitly
// request the dSYM so failures should be silent.
void Bloaty::TryAutoLoadDsym(const std::string& binary_path,
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@haberman haberman Dec 6, 2025

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From the user docs, I would expect that auto-loading of a dSYM performs exactly the same logic as explicitly specifying --dsym, but that doesn't seem to be the case.

Is there a reason to have two distinct functions for this? If so, can we add more explanation in the user docs?

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@rjmansfield rjmansfield Dec 6, 2025

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They have slightly different semantics and have different error handling.

For auto loading, I think lookup should be silent best effort but not throw/error if it fails. Where if a user specifies explicitly a --dsym and it's invalid or missing, that it should be reported to the user e.g. AddDsymFilename will throw if if file doesn't exist or if not a dSYM bundle directory.

I can explain in the documentation, but I think there's probably a little bit of refactoring that I can do here.

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@rjmansfield rjmansfield Dec 8, 2025

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I've updated the diff which I hope explains the differences between the two functions in terms of behaviour. i.e. autoloading is silent best effort, whereas the explicit --dsym option loading is validated more strictly. Please let me know you have any feedback on it.

This is a bit of a sale pitch to add the option but I think this patch, specifically the autoloading part, will definitely improve the out of the box usability for users with mach-o binaries. I hit both of these scenarios when first using bloaty.

With patch:

$ clang hw.c -g
$ bloaty -d compileunits ~/a.out
    FILE SIZE        VM SIZE    
 --------------  -------------- 
  48.7%  16.0Ki  33.3%  16.0Ki    [__DATA_CONST]
   2.6%     866  33.3%  16.0Ki    [__LINKEDIT]
  45.0%  14.8Ki  31.0%  14.9Ki    [__TEXT]
   3.2%  1.06Ki   2.0%     984    [Mach-O Headers]
   0.3%      88   0.2%      88    [__TEXT,__unwind_info]
   0.2%      82   0.2%      82    hw.c
   0.0%       8   0.0%       8    [__DATA_CONST,__got]
 100.0%  32.9Ki 100.0%  48.0Ki    TOTAL

vs

$ bloaty -d compileunits ~/a.out
bloaty: missing debug info

vs manually specifying the full path into the dSYM companion file

$ bloaty -d compileunits ~/a.out  --debug-file ~/a.out.dSYM/Contents/Resources/DWARF/a.out
    FILE SIZE        VM SIZE    
 --------------  -------------- 
  48.7%  16.0Ki  33.3%  16.0Ki    [__DATA_CONST]
   2.6%     866  33.3%  16.0Ki    [__LINKEDIT]
  45.0%  14.8Ki  31.0%  14.9Ki    [__TEXT]
   3.2%  1.06Ki   2.0%     984    [Mach-O Headers]
   0.3%      88   0.2%      88    [__TEXT,__unwind_info]
   0.2%      82   0.2%      82    hw.c
   0.0%       8   0.0%       8    [__DATA_CONST,__got]
 100.0%  32.9Ki 100.0%  48.0Ki    TOTAL

const std::string& build_id) {
if (build_id.empty()) {
return;
}

std::filesystem::path dsym_file = ConstructDsymPath(binary_path);

if (std::filesystem::exists(dsym_file)) {
// Use non-strict validation because auto-loading is speculative
// GetObjectFile() will throw if the file isn't a valid Mach-O and
// we want auto-loading to silently fail rather than break analysis.
if (TryValidateAndAddDsym(dsym_file.string(), binary_path,
/*strict_validation=*/false)) {
if (verbose_level > 1) {
printf("Found matching dSYM file: %s\n", dsym_file.string().c_str());
}
}
}
}

void Bloaty::DefineCustomDataSource(const CustomDataSource& source) {
if (source.base_data_source() == "symbols") {
THROW(
Expand Down Expand Up @@ -2030,6 +2177,7 @@ USAGE: bloaty [OPTION]... FILE... [-- BASE_FILE...]
-c FILE Load configuration from <file>.
-d SOURCE,SOURCE Comma-separated list of sources to scan.
--debug-file=FILE Use this file for debug symbols and/or symbol table.
--dsym=FILE Use this dSYM file or bundle (Mach-O only).
--source-map=ID=FILE
Use this source map file for the binary. The ID can be
the build ID (or Wasm sourceMappingURL) or the file path
Expand Down Expand Up @@ -2224,6 +2372,8 @@ bool DoParseOptions(bool skip_unknown, int* argc, char** argv[],
}
} else if (args.TryParseOption("--debug-file", &option)) {
options->add_debug_filename(std::string(option));
} else if (args.TryParseOption("--dsym", &option)) {
options->add_dsym_path(std::string(option));
} else if (args.TryParseUint64Option("--debug-fileoff", &uint64_option)) {
if (options->has_debug_fileoff()) {
THROW("currently we only support a single debug fileoff");
Expand Down Expand Up @@ -2349,6 +2499,8 @@ void BloatyDoMain(const Options& options, const InputFileFactory& file_factory,
THROW("max_rows_per_level must be at least 1");
}

verbose_level = options.verbose_level();

for (auto& filename : options.filename()) {
bloaty.AddFilename(filename, false);
}
Expand All @@ -2361,6 +2513,10 @@ void BloatyDoMain(const Options& options, const InputFileFactory& file_factory,
bloaty.AddDebugFilename(debug_filename);
}

for (const auto& dsym_path : options.dsym_path()) {
bloaty.AddDsymFilename(options, dsym_path);
}

for (auto& sourcemap : options.source_map()) {
bloaty.AddSourceMapFilename(sourcemap);
}
Expand All @@ -2380,8 +2536,6 @@ void BloatyDoMain(const Options& options, const InputFileFactory& file_factory,
}
}

verbose_level = options.verbose_level();

if (options.data_source_size() > 0) {
bloaty.ScanAndRollup(options, output);
} else if (options.has_disassemble_function()) {
Expand Down
3 changes: 3 additions & 0 deletions src/bloaty.proto
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Expand Up @@ -32,6 +32,9 @@ message Options {
// Build id to source map file names, delimited by '='.
repeated string source_map = 15;

// Mach-O specific option to specify path to a dSYM file.
repeated string dsym_path = 16;

// The data sources to scan in each file. At least one data source must be
// specified. If more than one source is specified, the output is
// hierarchical.
Expand Down
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