1 /**
2 * syslinux/module.h
3 *
4 * Dynamic ELF modules definitions and services.
5 */
6
7 #ifndef MODULE_H_
8 #define MODULE_H_
9
10 #include <stdio.h>
11 #include <elf.h>
12 #include <stdint.h>
13 #include <setjmp.h>
14 #include <stdbool.h>
15 #include <linux/list.h>
16
17 #if __SIZEOF_POINTER__ == 4
18 #include <i386/module.h>
19 #elif __SIZEOF_POINTER__ == 8
20 #include <x86_64/module.h>
21 #else
22 #error "unsupported architecture"
23 #endif
24
25 /*
26 * The maximum length of the module file name (including path), stored
27 * in the struct module descriptor.
28 */
29 #define MODULE_NAME_SIZE 256
30
31 /*
32 * Some common information about what kind of modules we're dealing with
33 */
34 #define EXEC_MODULE 0
35 #define LIB_MODULE 1
36
37 #define MAX_NR_DEPS 64
38
39 /*
40 * Initialization and finalization function signatures
41 */
42
43 /**
44 * module_main_t - pointer to an entry routine
45 *
46 * The entry routine is present only in executable modules, and represents
47 * the entry point for the program.
48 */
49 typedef int (*module_main_t)(int, char**);
50
51 /**
52 * module_ctor_t - pointer to a constructor or destructor routine
53 *
54 * A module may have multiple routines that need to be executed before
55 * or after the main routine. These are the constructors and
56 * destructors, respectively.
57 */
58 typedef void (*module_ctor_t) (void);
59
60 /**
61 * struct elf_module - structure encapsulating a module loaded in memory.
62 *
63 * Each SYSLINUX ELF module must have an associated struct elf_module descriptor
64 * that keeps track of memory allocations, symbol information, and various other
65 * resources needed by the module itself or by other modules that depend on it.
66 *
67 * There are two types of modules:
68 * - regular modules, which are actual memory images of a loaded & linked shared
69 * object (ELF file). Memory is reserved for the struct elf_module structure itself
70 * and for the object loadable sections read from the file.
71 * - shallow modules, which are not associated with an ELF shared object, but contain
72 * metainformation about a memory region already present and containing the
73 * actual code and data. One particular usage of shallow modules is to access
74 * symbol information from the root COM32 module loaded by the SYSLINUX core.
75 * As their name suggests, memory is reserved only for the elf_module structure
76 * itself and optionally for a usually small memory region containing metainformation
77 * (symbol information).
78 *
79 * Module descriptors are related to each other through dependency information. A module
80 * can depend on symbols from other modules, and in turn it can provide symbols used
81 * by other dependant modules. This relationship can be described as a directed
82 * acyclic graph (DAG). The graph is stored using double linked lists of
83 * predecessors and successors. There is also a global linked list containing all
84 * the modules currently loaded.
85 */
86 struct atexit;
87 struct elf_module {
88 char name[MODULE_NAME_SIZE]; // The module name
89
90 bool shallow; // Whether the module contains any code
91
92 struct list_head required; // Head of the required modules list
93 struct list_head dependants; // Head of module dependants list
94 struct list_head list; // The list entry in the module list
95
96 module_ctor_t *ctors; // module constructors
97 module_ctor_t *dtors; // module destructors
98 module_main_t main_func; // The main function (for executable modules)
99
100 void *module_addr; // The module location in the memory
101 Elf_Addr base_addr; // The base address of the module
102 Elf_Word module_size; // The module size in memory
103
104 Elf_Word *hash_table; // The symbol hash table
105 Elf_Word *ghash_table; // The GNU style hash table
106 char *str_table; // The string table
107 void *sym_table; // The symbol table
108 void *got; // The Global Offset Table
109 Elf_Dyn *dyn_table; // Dynamic loading information table
110
111 Elf_Word strtable_size; // The size of the string table
112 Elf_Word syment_size; // The size of a symbol entry
113 Elf_Word symtable_size; // The size of the symbol table
114
115
116 union {
117 // Transient - Data available while the module is loading
118 struct {
119 FILE *_file; // The file object of the open file
120 Elf_Off _cr_offset; // The current offset in the open file
121 } l;
122
123 // Process execution data
124 struct {
125 jmp_buf process_exit; // Exit state
126 struct atexit *atexit_list; // atexit() chain
127 } x;
128 } u;
129
130 // ELF DT_NEEDED entries for this module
131 int nr_needed;
132 Elf_Word needed[MAX_NR_DEPS];
133 };
134
135 /**
136 * struct module_dep - structure encapsulating a module dependency need
137 *
138 * This structure represents an item in a double linked list of predecessors or
139 * successors. The item contents is a pointer to the corresponding module descriptor.
140 */
141 struct module_dep {
142 struct list_head list; // The list entry in the dependency list
143
144 struct elf_module *module; // The target module descriptor
145 };
146
147
148 /**
149 * Unload all modules that have been loaded since @name.
150 *
151 * Returns the struct elf_module * for @name or %NULL if no modules
152 * have been loaded since @name.
153 */
154 extern struct elf_module *unload_modules_since(const char *name);
155
156 extern FILE *findpath(char *name);
157
158
159 /**
160 * Names of symbols with special meaning (treated as special cases at linking)
161 */
162 #define MODULE_ELF_INIT_PTR "__module_init_ptr" // Initialization pointer symbol name
163 #define MODULE_ELF_EXIT_PTR "__module_exit_ptr" // Finalization pointer symbol name
164 #define MODULE_ELF_MAIN_PTR "__module_main_ptr" // Entry pointer symbol name
165
166 /**
167 * modules_head - A global linked list containing all the loaded modules.
168 */
169 extern struct list_head modules_head;
170
171
172 /**
173 * for_each_module - iterator loop through the list of loaded modules.
174 */
175 #define for_each_module(m) list_for_each_entry(m, &modules_head, list)
176
177 /**
178 * for_each_module_safe - iterator loop through the list of loaded modules safe against removal.
179 */
180 #define for_each_module_safe(m, n) \
181 list_for_each_entry_safe(m, n, &modules_head, list)
182
183 /**
184 * module_current - return the module at the head of the module list.
185 */
module_current(void)186 static inline struct elf_module *module_current(void)
187 {
188 struct elf_module *head;
189
190 head = list_entry((&modules_head)->next, typeof(*head), list);
191 return head;
192 }
193
194 /**
195 * modules_init - initialize the module subsystem.
196 *
197 * This function must be called before any module operation is to be performed.
198 */
199 extern int modules_init(void);
200
201
202 /**
203 * modules_term - releases all resources pertaining to the module subsystem.
204 *
205 * This function should be called after all module operations.
206 */
207 extern void modules_term(void);
208
209
210 /**
211 * module_alloc - reserves space for a new module descriptor.
212 * @name: the file name of the module to be loaded.
213 *
214 * The function simply allocates a new module descriptor and initializes its fields
215 * in order to be used by subsequent loading operations.
216 */
217 extern struct elf_module *module_alloc(const char *name);
218
219
220 /**
221 * module_load - loads a regular ELF module into memory.
222 * @module: the module descriptor returned by module_alloc.
223 *
224 * The function reads the module file, checks whether the file has a
225 * valid structure, then loads into memory the code and the data and performs
226 * any symbol relocations. A module dependency is created automatically when the
227 * relocated symbol is defined in a different module.
228 *
229 * The function returns 0 if the operation is completed successfully, and
230 * a non-zero value if an error occurs. Possible errors include invalid module
231 * structure, missing symbol definitions (unsatisfied dependencies) and memory
232 * allocation issues.
233 */
234 extern int module_load(struct elf_module *module);
235
236
237 /**
238 * module_unload - unloads the module from the system.
239 * @module: the module descriptor structure.
240 *
241 * The function checks to see whether the module can be safely
242 * removed, then it executes any destructors and releases all the
243 * associated memory. This function can be applied both for standard
244 * modules and for shallow modules.
245 *
246 * A module can be safely removed from the system when no other modules reference
247 * symbols from it.
248 */
249 extern int module_unload(struct elf_module *module);
250
251 /**
252 * _module_unload - unloads the module without running destructors
253 * @module: the module descriptor structure.
254 *
255 * This function is the same as module_unload(), except that the
256 * module's destructors are not executed.
257 */
258 extern int _module_unload(struct elf_module *module);
259
260 /**
261 * get_module_type - get type of the module
262 * @module: the module descriptor structure.
263 *
264 * This function returns the type of module we're dealing with
265 * either a library module ( LIB_MODULE ), executable module ( EXEC_MODULE ),
266 * or an error ( UNKNOWN_MODULE ). The way it checks teh type is by checking to see
267 * if the module has its main_func set ( in which case it's an executable ). In case
268 * it doesn't it then checks to see if init_func is set ( in which case it's a
269 * library module. If this isn't the case either we don't know what it is so bail out
270 */
271 extern int get_module_type(struct elf_module *module);
272
273 /**
274 * module_unloadable - checks whether the given module can be unloaded.
275 * @module: the module descriptor structure
276 *
277 * A module can be unloaded from the system when no other modules depend on it,
278 * that is, no symbols are referenced from it.
279 */
280 extern int module_unloadable(struct elf_module *module);
281
282 /**
283 * module_find - searches for a module by its name.
284 * @name: the name of the module, as it was specified in module_alloc.
285 *
286 * The function returns a pointer to the module descriptor, if found, or
287 * NULL otherwise.
288 */
289 extern struct elf_module *module_find(const char *name);
290
291 /**
292 * module_find_symbol - searches for a symbol definition in a given module.
293 * @name: the name of the symbol to be found.
294 * @module: the module descriptor structure.
295 *
296 * The function searches the module symbol table for a symbol matching exactly
297 * the name provided. The operation uses the following search algorithms, in this
298 * order:
299 * - If a GNU hash table is present in the module, it is used to find the symbol.
300 * - If the symbol cannot be found with the first method (either the hash table
301 * is not present or the symbol is not found) and if a regular (SysV) hash table
302 * is present, a search is performed on the SysV hash table. If the symbol is not
303 * found, NULL is returned.
304 * - If the second method cannot be applied, a linear search is performed by
305 * inspecting every symbol in the symbol table.
306 *
307 * If the symbol is found, a pointer to its descriptor structure is returned, and
308 * NULL otherwise.
309 */
310 extern Elf_Sym *module_find_symbol(const char *name, struct elf_module *module);
311
312 /**
313 * global_find_symbol - searches for a symbol definition in the entire module namespace.
314 * @name: the name of the symbol to be found.
315 * @module: an optional (may be NULL) pointer to a module descriptor variable that
316 * will hold the module where the symbol was found.
317 *
318 * The function search for the given symbol name in all the modules currently
319 * loaded in the system, in the reverse module loading order. That is, the most
320 * recently loaded module is searched first, followed by the previous one, until
321 * the first loaded module is reached.
322 *
323 * If no module contains the symbol, NULL is returned, otherwise the return value is
324 * a pointer to the symbol descriptor structure. If the module parameter is not NULL,
325 * it is filled with the address of the module descriptor where the symbol is defined.
326 */
327 extern Elf_Sym *global_find_symbol(const char *name, struct elf_module **module);
328
329 /**
330 * module_get_absolute - converts an memory address relative to a module base address
331 * to its absolute value in RAM.
332 * @addr: the relative address to convert.
333 * @module: the module whose base address is used for the conversion.
334 *
335 * The function returns a pointer to the absolute memory address.
336 */
module_get_absolute(Elf_Addr addr,struct elf_module * module)337 static inline void *module_get_absolute(Elf_Addr addr, struct elf_module *module) {
338 return (void*)(module->base_addr + addr);
339 }
340
341 /**
342 * syslinux_current - get the current module process
343 */
344 extern struct elf_module *__syslinux_current;
syslinux_current(void)345 static inline const struct elf_module *syslinux_current(void)
346 {
347 return __syslinux_current;
348 }
349
350
351 #endif // MODULE_H_
352