Reduce the size of `__LINKEDIT Export Info` section in stripped binaries #18408

ivanpovazan · 2023-06-05T19:52:33Z

Description

This PR reduces the application's SOD (size on disk) by making __LINKEDIT Export Info section smaller in the stripped Mach-O binaries.

The feature is controlled by _ExportSymbolsExplicitly MSBuild property and can be disabled by specifying: -p:_ExportSymbolsExplicitly=true

Fixes #18332

Initial problem

It has been noticed that during stripping, the strip tool does not resize the export info section after it removes the symbols. Instead it only zeroes out the entries (achieved by calling prune_trie function):

Thanks @lambdageek for helping to track this down.

Proposed approach

As Xamarin build process already collects all the required symbols needed for the application to run and preserves them during the strip phase, we can use the same file to instruct clang toolchain to export only those symbols via the command line options: -exported_symbols_list <file> (source). This will make the export info section only include what is necessary for the runtime - and at the same time eliminate the problem of the strip tool which does not resize stripped symbols.

Investigation setup

The issue is observable by building and inspecting the test application: https://github.com/xamarin/xamarin-macios/blob/main/tests/dotnet/MySingleView/MySingleView.csproj and targeting iOS platform in Release mode.

Results:

Measure	MySingleView - main	MySingleView - this PR	Diff (%)
SOD (bytes)	13668940	13458476	-1.5%
.ipa (bytes)	4829368	4827928	-0.03%

Even though zeroes are compressed well, the SOD is still affected and unused section takes around 1.5% of the most simplistic app size.
Much bigger impact has been noted when trying out a MAUI iOS template app with NativeAOT where the __LINKEDIT Export Info zeroes take up to 20MB of the SOD, but also with the regular macOS applications: dotnet/runtime#86707

Repros (outdated - see note)

NOTE I will leave the detailed analysis as part of this PR description for reference. However it should be noted that the used make targets listed bellow are outdated and are used during investigation.

Bellow I listed 3 scenarios for reproducing the current app sizes (with and without stripping) and the app size that we can get by using changes of this PR:

Repro current state of MySingleView.app with stripped binary
Repro current state of MySingleView.app with unstripped binary
Repro the new approach (-exported_symbols_list <file> + strip)

The repro steps assume that this PR has been checkout and that tools are invoked from the test sample directory: cd tests/dotnet/MySingleView

Repro current state of MySingleView.app with stripped binary

Build the app (you can ignore the need to run the sample, I just did it to make sure the changes do not break anything)

make run-device

Print the load commands - load_cmds_strip.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_strip.list

We are interested in the export info section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942960
export_size 207712

Create a hex dump of the export info section - hex_dump_strip.list

xxd -s 5942960 -l 207712 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_strip.list

NOTE: Notice around ~200kb of zeroes from ~ 0x005ab490 to ~ 0x005dda00

Verify exported symbols are correct - dyld_info_strip.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_strip.list

Repro current state of MySingleView.app with unstripped binary

Build the app (the make target preserves the symbols)

make run-device-no-strip

Print the load commands - load_cmds_nostrip.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_nostrip.list

We are interested in the export info section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942960
export_size 207712

Create a hex dump of the export info section - hex_dump_nostrip.list

xxd -s 5942960 -l 207712 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_nostrip.list

Notice that the range: ~ 0x005ab490 to ~ 0x005dda00 now includes exported symbol entries

Verify exported symbols are correct - dyld_info_nostrip.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_nostrip.list

Repro the new approach

Build the app (the make target uses the new approach)

make run-device-export-syms

Print the load commands - load_cmds_export.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_export.list

We are interested in the export info section notice the reduced size of the section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942432
export_size 1048

Create a hex dump of the export info section - hex_dump_export.list

xxd -s 5942432 -l 1048 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_export.list

Verify exported symbols are correct - dyld_info_export.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_export.list

Additional benefits

With this approach we could also switch the way strip tool is invoked - to always strip all debug and local symbols via strip -S -x instead of passing the file with symbols to preserve. This would remove the warning that we are currently getting (which is being ignored):

/Applications/Xcode_14.3.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip: warning: removing global symbols from a final linked no longer supported.  Use -exported_symbols_list at link time when building...

Other references:

https://github.com/qyang-nj/llios/blob/main/exported_symbol/README.md

…round the strip tool not resizing the stripped export section

ivanpovazan · 2023-06-05T19:53:13Z

/cc: @rolfbjarne

ivanpovazan · 2023-06-05T19:54:29Z

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -1173,6 +1173,10 @@
 	</PropertyGroup>

 	<Target Name="_ComputeLinkNativeExecutableInputs" Condition="'$(IsMacEnabled)' == 'true'">
+		<PropertyGroup>
+			<_ExportedSymbolsFile Condition="'$(_ExportedSymbolsFile)' == '' and '$(_MtouchSymbolsList)' == ''">/dev/null</_ExportedSymbolsFile> <!-- nothing to export -->


In case there are no symbols to export, this workaround works just fine making the export info section empty.

github-actions · 2023-06-05T19:56:29Z

⚠️ Your code has been reformatted. ⚠️

If this is not desired, add the actions-disable-autoformat label, and revert the reformatting commit.

If files unrelated to your change were modified, try reverting the reformatting commit + merging with the target branch (and push those changes).

dotnet/targets/Xamarin.Shared.Sdk.targets

msbuild/Xamarin.MacDev.Tasks/Tasks/LinkNativeCodeTaskBase.cs

dotnet/targets/Xamarin.Shared.Sdk.targets

rolfbjarne · 2023-06-06T10:32:35Z

/azp run

azure-pipelines · 2023-06-06T10:32:56Z

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rolfbjarne · 2023-06-06T12:27:34Z

/azp run

azure-pipelines · 2023-06-06T12:27:55Z

Azure Pipelines successfully started running 1 pipeline(s).

rolfbjarne · 2023-06-06T19:34:57Z

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

ivanpovazan · 2023-06-07T08:03:18Z

The CI looks 🆗 should I also try adding this:

Additional benefits
With this approach we could also switch the way strip tool is invoked - to always strip all debug and local symbols via strip -S -x instead of passing the file with symbols to preserve. This would remove the warning that we are currently getting (which is being ignored):

rolfbjarne · 2023-06-07T14:49:00Z

The CI looks 🆗 should I also try adding this:

Additional benefits
With this approach we could also switch the way strip tool is invoked - to always strip all debug and local symbols via strip -S -x instead of passing the file with symbols to preserve. This would remove the warning that we are currently getting (which is being ignored):

That should be in a different PR.

rolfbjarne · 2023-06-07T14:51:20Z

Actually I think we can just enable this by default, since there's an escape hatch if something goes wrong at some point.

tests/dotnet/MySingleView/Makefile

dotnet/targets/Xamarin.Shared.Sdk.props

ivanpovazan · 2023-06-08T10:12:32Z

/azp run

azure-pipelines · 2023-06-08T10:12:38Z

Commenter does not have sufficient privileges for PR 18408 in repo xamarin/xamarin-macios

rolfbjarne · 2023-06-08T10:17:28Z

/azp run

azure-pipelines · 2023-06-08T10:17:50Z

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vs-mobiletools-engineering-service2 · 2023-06-08T11:39:49Z

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vs-mobiletools-engineering-service2 · 2023-06-08T11:44:39Z

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vs-mobiletools-engineering-service2 · 2023-06-08T12:05:40Z

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

~~iOS~~ (no change detected)
~~tvOS~~ (no change detected)
~~watchOS~~ (no change detected)
~~macOS~~ (no change detected)

NET (empty diffs)

~~iOS~~: (empty diff detected)
~~tvOS~~: (empty diff detected)
~~MacCatalyst~~: (empty diff detected)
~~macOS~~: (empty diff detected)

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iOS: vsdrops gist (No breaking changes)
tvOS: vsdrops gist (No breaking changes)
watchOS: vsdrops gist (No breaking changes)
macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

iOS: vsdrops gist (No breaking changes)
tvOS: vsdrops gist (No breaking changes)
MacCatalyst: vsdrops gist (No breaking changes)
macOS: vsdrops gist (No breaking changes)
Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

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iOS: vsdrops gist
tvOS: vsdrops gist
macOS: vsdrops gist

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vs-mobiletools-engineering-service2 · 2023-06-08T15:39:13Z

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Add an option to explicitly export global symbols with clang to worka…

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…round the strip tool not resizing the stripped export section

ivanpovazan commented Jun 5, 2023

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FIXME: Enabling the feature by default to run CI testing

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Merge branch 'main' into linkedit-fix

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ivanpovazan commented Jun 7, 2023

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ivanpovazan added 2 commits June 8, 2023 11:35

Merge remote-tracking branch 'upstream/main' into linkedit-fix

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ivanpovazan changed the title ~~[DRAFT] Reduce the size of __LINKEDIT Export Info section in stripped binaries~~ Reduce the size of __LINKEDIT Export Info section in stripped binaries Jun 8, 2023

ivanpovazan marked this pull request as ready for review June 8, 2023 10:04

ivanpovazan requested review from filipnavara and rolfbjarne June 8, 2023 10:05

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MSR: Remove use of reflection in UCOs of generic types #18421

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ICU causes crash on iOS 17.4 simulator dotnet/runtime#98941

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Compile error: strip exited with code 139 getsentry/sentry-dotnet#3620

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angularsen mentioned this pull request Oct 11, 2024

[META] Xcode 16.0 Support for .NET 8/9 and MAUI dotnet/maui#24819

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Reduce the size of __LINKEDIT Export Info section in stripped binaries #18408

Reduce the size of __LINKEDIT Export Info section in stripped binaries #18408

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ivanpovazan commented Jun 5, 2023 • edited Loading

Description

Initial problem

Proposed approach

Investigation setup

Results:

Repros (outdated - see note)

Repro current state of MySingleView.app with stripped binary

Repro current state of MySingleView.app with unstripped binary

Repro the new approach

Additional benefits

Other references:

ivanpovazan commented Jun 5, 2023

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💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) passed 💻

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💻 [PR Build] Tests on macOS M1 - Mac Ventura (13.0) passed 💻

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Reduce the size of `__LINKEDIT Export Info` section in stripped binaries #18408

Reduce the size of `__LINKEDIT Export Info` section in stripped binaries #18408

ivanpovazan commented Jun 5, 2023 •

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