GEM - GNU C Compiler Extensions Framework

GCC Hacks Alexey Smirnov GRC’06 http://gcchacks.info

Introduction GNU Compiler Collection includes C, C++, Java, etc. compilers and libraries for them Standard compiler for Linux Latest release: GCC 4.1 http://gcc.gnu.org

Introduction GEM – compiler extensibility framework Examples: syntactic sugar, BCC, Propolice, etc. Dynamically loaded modules simplify development and deployment

Overview GCC 3.4 Tutorial GEM Overview Hacks, hacks, hacks.

GCC Architecture Driver program gcc . Finds appropriate compiler. Calls compiler, assembler, linker C language: cc1, as, collect2 This presentation: cc1

GCC Architecture Front end, middle end, back end.

Representations AST – abstract syntax tree RTL – register transfer language Object – assembly code of target platform Other representations used for optimizations

GCC Initialization cc1 is preprocessor and compiler toplev.c: toplev_main() command-line option processing, front end/back end initialization, global scope creation Front end is initialized with standard types: char_type_node , integer_type_node , unsigned_type_node . built-in functions: builtin_memcpy , builtin_strlen These objects are instances of tree .

Tree data type Code, operands. MODIFY_EXPR – an assignment expression. TREE_OPERAND(t,0), TREE_OPERAND(t,1) ARRAY_TYPE – declaration of type. TREE_TYPE(t) – type of array element, TYPE_DOMAIN(t) – type of index. CALL_EXPR – function call. TREE_OPERAND(t,0) – function definition, TERE_OPERAND(t,1) – function arguments. debug_tree() prints out AST

Parser Identifier after identifier get_identifier() char* -> tree with IDENTIFIER_NODE code. A declaration is a tree node with _DECL code. lookup_name() returns declaration corresponding to the symbol Symbol table not constructed. C_DECL_INVISIBLE attribute used instead.

AST to RTL to assembly start_decl() / finish_decl() start_function() / finish_function() tree build_function_call (tree function, tree params) When a function is parsed it is converted to RTL immediately or after the file is parsed. Option –funit-at-a-time finish_function() Assembly code is generated from RTL. output_asm_insn() is executed for each instruction

GEM Framework The idea is similar to that of LSM Module loaded using an option: -fextension-module=test.gem Hooks throughout GCC code AST Assembly output New hooks added when needed

GEM Framework Hooks gem_handle_option gem_c_common_nodes_and_builtins gem_macro_name, gem_macro_def gem_start_decl, gem_start_func gem_finish_function gem_output_asm_insn

Traversing an AST walk_tree static tree callback(tree *tp, …) { switch (TREE_CODE(*tp)) { case CALL_EXPR: … case VAR_DECL: … } return NULL_TREE; } walk_tree(&t, callback, NULL, NULL);

Creating trees t =build_int_2(val, 0); build1(ADDR_EXPR, build_pointer_type(T_T(t)), t); build(MODIFY_EXPR, TREE_TYPE(left), left, val);

Hacks Syntactic sugar Operating systems Security

Syntactic Sugar When a compiler error occurs, fix compiler rather than program. Examples: Function overloading as in C++ toString() in each structure as in Java Invoke block of code from a function Ruby Use functions to initialize a variable Default argument values

Security DIRA: detection, identification, and repair of control hijacking attacks PASAN: signature and patch generation Propolice -fstack-protector

Operating Systems Dusk: develop in userland, install at kernel level.

Function Overloading Two functions: void add(int, int); void add(int, char*); The idea is to replace function name so that it includes argument types: add_i_i add_i_pch gem_start_decl() gem_start_function() gem_build_function_call()

Alias Each Declaraiton cfo_find_symtab(&t_func, func_name); if (t_func==NULL_TREE || DECL_BUILT_IN(t_func)) { return; } If found then alias and create new declaration.

Alias Each Declaration strcpy(new_name, func_name); strcat(new_name, cfo_build_name(TREE_PURPOSE(T_O(declarator, 1)))); cfo_find_symtab(&t_func_alias, name); If not found: t_alias_attr=tree_cons(get_identifier("alias"), tree_cons(NULL_TREE, get_identifier(name), NULL_TREE), NULL_TREE); TYPE_ATTRIBUTES(T_T(t_func)) = t_alias_attr; DECL_ATTRIBUTES(t_func)=t_alias_attr; T_O(declarator,0) = get_identifier(new_name);

Replace function calls name = cfo_build_decl_name(t_func, t_parm); t_new_func = get_identifier(name); if (t_new_func) { t_new_func = lookup_name(t_new_func); } *func = t_new_func;

Conclusion GCC is a big program so we thought it’s a good idea to document it: http://en.wikibooks.org/GNU_C_Compiler_Internals GEM allows to implement GCC extensions. http://www.ecsl.cs.sunysb.edu/gem Examples: programming languages, security, OS.

Thank you http://gcchacks.info

GEM - GNU C Compiler Extensions Framework

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