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

by Alexey Smirnov on Nov 13, 2009

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