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[SR-3712] Mach-O files use non-aligned pointers #46297

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swift-ci opened this issue Jan 24, 2017 · 4 comments
Closed

[SR-3712] Mach-O files use non-aligned pointers #46297

swift-ci opened this issue Jan 24, 2017 · 4 comments
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself IRGen LLVM IR generation

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@swift-ci
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Previous ID SR-3712
Radar None
Original Reporter mpetrov (JIRA User)
Type Bug
Status Resolved
Resolution Invalid
Environment

Apple Swift version 3.0.1 (swiftlang-800.0.58.6 clang-800.0.42.1)
ld64-274.1

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, IRGen
Assignee mpetrov (JIRA)
Priority Medium

md5: 8a872b674041ac969b54d108104bff60

Issue Description:

Generated Mach-O binaries use non word-aligned references that can break some Mach-O parsing libraries.

For example, the pointer for a Swift class' class_ro_t* is at 0x1000042d9, when in practice it starts at 0x1000042d8. This may just be a linker bug, but I've only seen this happen with Swift classes (and not with ObjC).

Repro case is below:

~> cat hello.swift
class Foo {}
class Bar {}
class Baz {}

~> swiftc hello.swift -v -Xlinker -v
Apple Swift version 3.0.1 (swiftlang-800.0.58.6 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file hello.swift -target x86_64-apple-macosx10.9 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -color-diagnostics -module-name hello -o /var/folders/r7/q5y959494nl8rg1yy6th6gd40045kr/T/hello-522055.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld /var/folders/r7/q5y959494nl8rg1yy6th6gd40045kr/T/hello-522055.o -force_load /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/arc/libarclite_macosx.a -framework CoreFoundation -v -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -lobjc -lSystem -arch x86_64 -L /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx -rpath /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx -macosx_version_min 10.9.0 -no_objc_category_merging -o hello
@(#)PROGRAM:ld PROJECT:ld64-274.1
configured to support archs: armv6 armv7 armv7s arm64 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em (tvOS)
Library search paths:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/lib
Framework search paths:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/

~> otool hello -o | grep "Swift class"
data 0x1000042d9 (struct class_ro_t *) Swift class
data 0x100004369 (struct class_ro_t *) Swift class
data 0x1000043f9 (struct class_ro_t *) Swift class
@belkadan
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This is correct and necessary behavior for Swift class metadata records, which have information located at negative offsets. That said I'm a little concerned about not respecting alignment within the class_ro_t.

@belkadan
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@rjmccall, @jckarter, thoughts?

@jckarter
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The low bit of class_ro_t is the "is Swift, using an app-packaged runtime" bit. The rodata pointer itself is still aligned, but the alignment bits (and on 64-bit platforms, unused address space high bits) are reserved to the ObjC runtime.

@rjmccall
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Right. The pointer is not "unaligned". It's a pointer to a correctly-aligned ro_t, and a low bit is being used to encode extra information in that pointer. Those Mach-O parsing libraries should just be fixed to ignore sub-word offsets.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. compiler The Swift compiler in itself IRGen LLVM IR generation
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@jckarter @belkadan @rjmccall @swift-ci