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Clang Interface Stubs Syntax Directed Stub Library Generation Puyan Lotfi Facebook Interface Stubs echo "" | clang -shared -fPIC -x c - -o - | llvm-objdump -section-headers a.out: file format ELF64-x86-64 Sections: Idx Name

  1. Clang Interface Stubs Syntax Directed Stub Library Generation Puyan Lotfi Facebook

  2. Interface Stubs echo "" | clang -shared -fPIC -x c - -o - | llvm-objdump -section-headers a.out: file format ELF64-x86-64 Sections: Idx Name Size Address Type 0 00000000 0000000000000000 1 .dynsym 00000108 00000000000001d0 2 .dynstr 0000008f 00000000000002d8 3 .symtab 00000498 0000000000000000 4 .strtab 00000178 0000000000000000 5 .shstrtab 000000cc 0000000000000000 6 .gnu.hash 0000003c 0000000000000190 7 .gnu.version 00000016 0000000000000368 8 .gnu.version_r 00000020 0000000000000380 9 .rela.dyn 000000a8 00000000000003a0 10 .init 00000017 0000000000000448 TEXT 11 .plt 00000010 0000000000000460 TEXT 12 .plt.got 00000008 0000000000000470 TEXT 13 .text 000000c6 0000000000000480 TEXT 14 .fini 00000009 0000000000000548 TEXT 15 .eh_frame_hdr 00000024 0000000000000554 DATA 16 .eh_frame 0000007c 0000000000000578 DATA 17 .init_array 00000008 0000000000200e40 18 .fini_array 00000008 0000000000200e48 19 .dynamic 00000190 0000000000200e50 20 .got 00000020 0000000000200fe0 DATA 21 .got.plt 00000018 0000000000201000 DATA 22 .data 00000008 0000000000201018 DATA 23 .bss 00000008 0000000000201020 BSS 24 .comment 0000008f 0000000000000000

  3. Interface Stubs echo "" | clang -shared -fPIC -x c - -o - | llvm-objdump -section-headers a.out: file format ELF64-x86-64 Sections: Idx Name Size Address Type 0 00000000 0000000000000000 1 .dynsym 00000108 00000000000001d0 2 .dynstr 0000008f 00000000000002d8 3 .symtab 00000498 0000000000000000 4 .strtab 00000178 0000000000000000 5 .shstrtab 000000cc 0000000000000000 6 .gnu.hash 0000003c 0000000000000190 7 .gnu.version 00000016 0000000000000368 8 .gnu.version_r 00000020 0000000000000380 9 .rela.dyn 000000a8 00000000000003a0 10 .init 00000017 0000000000000448 TEXT 11 .plt 00000010 0000000000000460 TEXT 12 .plt.got 00000008 0000000000000470 TEXT 13 .text 000000c6 0000000000000480 TEXT 14 .fini 00000009 0000000000000548 TEXT 15 .eh_frame_hdr 00000024 0000000000000554 DATA 16 .eh_frame 0000007c 0000000000000578 DATA 17 .init_array 00000008 0000000000200e40 18 .fini_array 00000008 0000000000200e48 19 .dynamic 00000190 0000000000200e50 20 .got 00000020 0000000000200fe0 DATA 21 .got.plt 00000018 0000000000201000 DATA 22 .data 00000008 0000000000201018 DATA 23 .bss 00000008 0000000000201020 BSS 24 .comment 0000008f 0000000000000000

  4. Motivation • Generation of lean SDKs • No Code • Explicit Symbol Exposure • Code as Source of Truth: Syntax Directed • Make use of visibility attributes (ie __attribute__((__visibility__(“hidden”))))

  5. Motivation • Generation of lean SDKs • No Code • Explicit Symbol Exposure • Code as Source of Truth: Syntax Directed • Make use of visibility attributes (ie __attribute__((__visibility__(“hidden”)))) clang -emit-interface-stubs -o libfoo.so a.cpp c.cpp sq.cpp #define hidden __attribute__(( \ __visibility__("hidden"))) .dynsym hidden int b; .dynstr int red() { return b; } .symtab hidden void green() { } .strtab hidden void blue() { } .shstrtab int a; hidden int c;

  6. Prior Art & Approaches • Microsoft’s Import Libraries • Generate stub code from compiler & linker • Syntax directed through __declspec(dllimport/dllexport) • Apple’s TAPI (Text API) • Header Scanning & stub generation • .so/.dylib/.dll scanning & stripping

  7. Clang Interface Stubs • Repurposes visibility attribute to direct symbol exposure using code syntax • Fine grain control (internal SPI vs external API) • Aggregates exposure across compilation units via text • Supports ELF • Yields smaller SDK and faster link times

  8. Clang Driver Pipeline • Traditional Pipeline: Preprocess, Compile, Backend, Assemble, and Link Phases PP COMPILE BE ASM LINK

  9. Clang Driver Pipeline • Traditional Pipeline: Preprocess, Compile, Backend, Assemble, and Link Phases clang -c PP COMPILE BE ASM

  10. Clang Driver Pipeline • Traditional Pipeline: Preprocess, Compile, Backend, Assemble, and Link Phases clang -S PP COMPILE BE

  11. Clang Driver Pipeline • Traditional Pipeline: Preprocess, Compile, Backend, Assemble, and Link Phases clang -fsyntax_only PP COMPILE

  12. Clang Driver Pipeline • Traditional Pipeline: Preprocess, Compile, Backend, Assemble, and Link Phases clang -E PP

  13. Driver Pipeline • Clang Interface Stubs Pipeline: Preprocess, Compile, and Merge phases • Compile Phase: Generates intermediate text (.ifs files) • Merge Phase: Invokes llvm-ifs to consume & merge .ifs files to produce ELF .so • Compile Phase invokes InterfaceStubFunctionsConsumer (clang -cc1) • Walks the AST scanning for visible decls PP COMPILE MERGE

  14. Driver Pipeline • Clang Interface Stubs Pipeline: Preprocess, Compile, and Merge phases • Compile Phase: Generates intermediate text (.ifs files) • Merge Phase: Invokes llvm-ifs to consume & merge .ifs files to produce ELF .so • Compile Phase invokes InterfaceStubFunctionsConsumer (clang -cc1) • Walks the AST scanning for visible decls #define weak \ __attribute__((__weak__)) --- !experimental-ifs-v1 #define hidden __attribute__(( \ IfsVersion: 1.0 __visibility__("hidden"))) Triple: x86_64-unknown-linux-gnu Symbols: COMPILE hidden int b; _Z3redv: { Type: Func } int red() { return b; } _Z5greenv: { Type: Func, weak void green() { } Weak: true } hidden void blue() { } a: { Type: Object, Size: 4 } int a; ... hidden int c;

  15. IFS Text Format --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: _Z5brownv: { Type: Func } _Z5greenv: { Type: Func } b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } _Z3redv: { Type: Func } _Z4bluev: { Type: Func } _Z3redi: { Type: Func, Weak: true } ...

  16. IFS Text Format --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } _Z3redv: { Type: Func } --- !experimental-ifs-v1 _Z4bluev: { Type: Func } IfsVersion: 1.0 ... Triple: x86_64-unknown-linux-gnu Symbols: --- !experimental-ifs-v1 _Z5brownv: { Type: Func } IfsVersion: 1.0 _Z5greenv: { Type: Func } llvm-ifs Triple: x86_64-unknown-linux-gnu b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } Symbols: _Z3redv: { Type: Func } _Z5brownv: { Type: Func } _Z4bluev: { Type: Func } _Z5greenv: { Type: Func } _Z3redi: { Type: Func, Weak: true } ... ... --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: _Z3redi: { Type: Func, Weak: true } ...

  17. llvm-ifs • A new llvm tool for consuming IFS files: produces a merged IFS file, or ELF shared object (.dynsym, .dynstr, .symtab only) --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } _Z3redv: { Type: Func } --- !experimental-ifs-v1 _Z4bluev: { Type: Func } IfsVersion: 1.0 ... Triple: x86_64-unknown-linux-gnu Symbols: --- !experimental-ifs-v1 _Z5brownv: { Type: Func } IfsVersion: 1.0 _Z5greenv: { Type: Func } llvm-ifs Triple: x86_64-unknown-linux-gnu b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } Symbols: _Z3redv: { Type: Func } _Z5brownv: { Type: Func } _Z4bluev: { Type: Func } _Z5greenv: { Type: Func } _Z3redi: { Type: Func, Weak: true } ... ... --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: _Z3redi: { Type: Func, Weak: true } ...

  18. llvm-ifs • A new llvm tool for consuming IFS files: produces a merged IFS file, or ELF shared object (.dynsym, .dynstr, .symtab only) --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: b: { Type: Object, Size: 4 } black: { Type: Object, Size: 1 } _Z3redv: { Type: Func } _Z4bluev: { Type: Func } ... .dynsym --- !experimental-ifs-v1 .dynstr IfsVersion: 1.0 llvm-ifs .symtab Triple: x86_64-unknown-linux-gnu .strtab Symbols: _Z5brownv: { Type: Func } .shstrtab _Z5greenv: { Type: Func } ... --- !experimental-ifs-v1 IfsVersion: 1.0 Triple: x86_64-unknown-linux-gnu Symbols: _Z3redi: { Type: Func, Weak: true } ...

  19. Example clang -emit-interface-stubs -o libfoo.so a.cpp c.cpp sq.cpp --- !experimental-ifs-v1 IfsVersion: 1.0 MERGE Triple: x86_64-unknown-linux-gnu COMPILE void brown() { } Symbols: (llvm-ifs) int green() { return 42; } _Z5brownv: { Type: Func } _Z5greenv: { Type: Func } ...

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