| 1 | ============================================
|
| 2 | The object-lifetime debugging infrastructure
|
| 3 | ============================================
|
| 4 |
|
| 5 | :Author: Thomas Gleixner
|
| 6 |
|
| 7 | Introduction
|
| 8 | ============
|
| 9 |
|
| 10 | debugobjects is a generic infrastructure to track the life time of
|
| 11 | kernel objects and validate the operations on those.
|
| 12 |
|
| 13 | debugobjects is useful to check for the following error patterns:
|
| 14 |
|
| 15 | - Activation of uninitialized objects
|
| 16 |
|
| 17 | - Initialization of active objects
|
| 18 |
|
| 19 | - Usage of freed/destroyed objects
|
| 20 |
|
| 21 | debugobjects is not changing the data structure of the real object so it
|
| 22 | can be compiled in with a minimal runtime impact and enabled on demand
|
| 23 | with a kernel command line option.
|
| 24 |
|
| 25 | Howto use debugobjects
|
| 26 | ======================
|
| 27 |
|
| 28 | A kernel subsystem needs to provide a data structure which describes the
|
| 29 | object type and add calls into the debug code at appropriate places. The
|
| 30 | data structure to describe the object type needs at minimum the name of
|
| 31 | the object type. Optional functions can and should be provided to fixup
|
| 32 | detected problems so the kernel can continue to work and the debug
|
| 33 | information can be retrieved from a live system instead of hard core
|
| 34 | debugging with serial consoles and stack trace transcripts from the
|
| 35 | monitor.
|
| 36 |
|
| 37 | The debug calls provided by debugobjects are:
|
| 38 |
|
| 39 | - debug_object_init
|
| 40 |
|
| 41 | - debug_object_init_on_stack
|
| 42 |
|
| 43 | - debug_object_activate
|
| 44 |
|
| 45 | - debug_object_deactivate
|
| 46 |
|
| 47 | - debug_object_destroy
|
| 48 |
|
| 49 | - debug_object_free
|
| 50 |
|
| 51 | - debug_object_assert_init
|
| 52 |
|
| 53 | Each of these functions takes the address of the real object and a
|
| 54 | pointer to the object type specific debug description structure.
|
| 55 |
|
| 56 | Each detected error is reported in the statistics and a limited number
|
| 57 | of errors are printk'ed including a full stack trace.
|
| 58 |
|
| 59 | The statistics are available via /sys/kernel/debug/debug_objects/stats.
|
| 60 | They provide information about the number of warnings and the number of
|
| 61 | successful fixups along with information about the usage of the internal
|
| 62 | tracking objects and the state of the internal tracking objects pool.
|
| 63 |
|
| 64 | Debug functions
|
| 65 | ===============
|
| 66 |
|
| 67 | .. kernel-doc:: lib/debugobjects.c
|
| 68 | :functions: debug_object_init
|
| 69 |
|
| 70 | This function is called whenever the initialization function of a real
|
| 71 | object is called.
|
| 72 |
|
| 73 | When the real object is already tracked by debugobjects it is checked,
|
| 74 | whether the object can be initialized. Initializing is not allowed for
|
| 75 | active and destroyed objects. When debugobjects detects an error, then
|
| 76 | it calls the fixup_init function of the object type description
|
| 77 | structure if provided by the caller. The fixup function can correct the
|
| 78 | problem before the real initialization of the object happens. E.g. it
|
| 79 | can deactivate an active object in order to prevent damage to the
|
| 80 | subsystem.
|
| 81 |
|
| 82 | When the real object is not yet tracked by debugobjects, debugobjects
|
| 83 | allocates a tracker object for the real object and sets the tracker
|
| 84 | object state to ODEBUG_STATE_INIT. It verifies that the object is not
|
| 85 | on the callers stack. If it is on the callers stack then a limited
|
| 86 | number of warnings including a full stack trace is printk'ed. The
|
| 87 | calling code must use debug_object_init_on_stack() and remove the
|
| 88 | object before leaving the function which allocated it. See next section.
|
| 89 |
|
| 90 | .. kernel-doc:: lib/debugobjects.c
|
| 91 | :functions: debug_object_init_on_stack
|
| 92 |
|
| 93 | This function is called whenever the initialization function of a real
|
| 94 | object which resides on the stack is called.
|
| 95 |
|
| 96 | When the real object is already tracked by debugobjects it is checked,
|
| 97 | whether the object can be initialized. Initializing is not allowed for
|
| 98 | active and destroyed objects. When debugobjects detects an error, then
|
| 99 | it calls the fixup_init function of the object type description
|
| 100 | structure if provided by the caller. The fixup function can correct the
|
| 101 | problem before the real initialization of the object happens. E.g. it
|
| 102 | can deactivate an active object in order to prevent damage to the
|
| 103 | subsystem.
|
| 104 |
|
| 105 | When the real object is not yet tracked by debugobjects debugobjects
|
| 106 | allocates a tracker object for the real object and sets the tracker
|
| 107 | object state to ODEBUG_STATE_INIT. It verifies that the object is on
|
| 108 | the callers stack.
|
| 109 |
|
| 110 | An object which is on the stack must be removed from the tracker by
|
| 111 | calling debug_object_free() before the function which allocates the
|
| 112 | object returns. Otherwise we keep track of stale objects.
|
| 113 |
|
| 114 | .. kernel-doc:: lib/debugobjects.c
|
| 115 | :functions: debug_object_activate
|
| 116 |
|
| 117 | This function is called whenever the activation function of a real
|
| 118 | object is called.
|
| 119 |
|
| 120 | When the real object is already tracked by debugobjects it is checked,
|
| 121 | whether the object can be activated. Activating is not allowed for
|
| 122 | active and destroyed objects. When debugobjects detects an error, then
|
| 123 | it calls the fixup_activate function of the object type description
|
| 124 | structure if provided by the caller. The fixup function can correct the
|
| 125 | problem before the real activation of the object happens. E.g. it can
|
| 126 | deactivate an active object in order to prevent damage to the subsystem.
|
| 127 |
|
| 128 | When the real object is not yet tracked by debugobjects then the
|
| 129 | fixup_activate function is called if available. This is necessary to
|
| 130 | allow the legitimate activation of statically allocated and initialized
|
| 131 | objects. The fixup function checks whether the object is valid and calls
|
| 132 | the debug_objects_init() function to initialize the tracking of this
|
| 133 | object.
|
| 134 |
|
| 135 | When the activation is legitimate, then the state of the associated
|
| 136 | tracker object is set to ODEBUG_STATE_ACTIVE.
|
| 137 |
|
| 138 |
|
| 139 | .. kernel-doc:: lib/debugobjects.c
|
| 140 | :functions: debug_object_deactivate
|
| 141 |
|
| 142 | This function is called whenever the deactivation function of a real
|
| 143 | object is called.
|
| 144 |
|
| 145 | When the real object is tracked by debugobjects it is checked, whether
|
| 146 | the object can be deactivated. Deactivating is not allowed for untracked
|
| 147 | or destroyed objects.
|
| 148 |
|
| 149 | When the deactivation is legitimate, then the state of the associated
|
| 150 | tracker object is set to ODEBUG_STATE_INACTIVE.
|
| 151 |
|
| 152 | .. kernel-doc:: lib/debugobjects.c
|
| 153 | :functions: debug_object_destroy
|
| 154 |
|
| 155 | This function is called to mark an object destroyed. This is useful to
|
| 156 | prevent the usage of invalid objects, which are still available in
|
| 157 | memory: either statically allocated objects or objects which are freed
|
| 158 | later.
|
| 159 |
|
| 160 | When the real object is tracked by debugobjects it is checked, whether
|
| 161 | the object can be destroyed. Destruction is not allowed for active and
|
| 162 | destroyed objects. When debugobjects detects an error, then it calls the
|
| 163 | fixup_destroy function of the object type description structure if
|
| 164 | provided by the caller. The fixup function can correct the problem
|
| 165 | before the real destruction of the object happens. E.g. it can
|
| 166 | deactivate an active object in order to prevent damage to the subsystem.
|
| 167 |
|
| 168 | When the destruction is legitimate, then the state of the associated
|
| 169 | tracker object is set to ODEBUG_STATE_DESTROYED.
|
| 170 |
|
| 171 | .. kernel-doc:: lib/debugobjects.c
|
| 172 | :functions: debug_object_free
|
| 173 |
|
| 174 | This function is called before an object is freed.
|
| 175 |
|
| 176 | When the real object is tracked by debugobjects it is checked, whether
|
| 177 | the object can be freed. Free is not allowed for active objects. When
|
| 178 | debugobjects detects an error, then it calls the fixup_free function of
|
| 179 | the object type description structure if provided by the caller. The
|
| 180 | fixup function can correct the problem before the real free of the
|
| 181 | object happens. E.g. it can deactivate an active object in order to
|
| 182 | prevent damage to the subsystem.
|
| 183 |
|
| 184 | Note that debug_object_free removes the object from the tracker. Later
|
| 185 | usage of the object is detected by the other debug checks.
|
| 186 |
|
| 187 |
|
| 188 | .. kernel-doc:: lib/debugobjects.c
|
| 189 | :functions: debug_object_assert_init
|
| 190 |
|
| 191 | This function is called to assert that an object has been initialized.
|
| 192 |
|
| 193 | When the real object is not tracked by debugobjects, it calls
|
| 194 | fixup_assert_init of the object type description structure provided by
|
| 195 | the caller, with the hardcoded object state ODEBUG_NOT_AVAILABLE. The
|
| 196 | fixup function can correct the problem by calling debug_object_init
|
| 197 | and other specific initializing functions.
|
| 198 |
|
| 199 | When the real object is already tracked by debugobjects it is ignored.
|
| 200 |
|
| 201 | Fixup functions
|
| 202 | ===============
|
| 203 |
|
| 204 | Debug object type description structure
|
| 205 | ---------------------------------------
|
| 206 |
|
| 207 | .. kernel-doc:: include/linux/debugobjects.h
|
| 208 | :internal:
|
| 209 |
|
| 210 | fixup_init
|
| 211 | -----------
|
| 212 |
|
| 213 | This function is called from the debug code whenever a problem in
|
| 214 | debug_object_init is detected. The function takes the address of the
|
| 215 | object and the state which is currently recorded in the tracker.
|
| 216 |
|
| 217 | Called from debug_object_init when the object state is:
|
| 218 |
|
| 219 | - ODEBUG_STATE_ACTIVE
|
| 220 |
|
| 221 | The function returns true when the fixup was successful, otherwise
|
| 222 | false. The return value is used to update the statistics.
|
| 223 |
|
| 224 | Note, that the function needs to call the debug_object_init() function
|
| 225 | again, after the damage has been repaired in order to keep the state
|
| 226 | consistent.
|
| 227 |
|
| 228 | fixup_activate
|
| 229 | ---------------
|
| 230 |
|
| 231 | This function is called from the debug code whenever a problem in
|
| 232 | debug_object_activate is detected.
|
| 233 |
|
| 234 | Called from debug_object_activate when the object state is:
|
| 235 |
|
| 236 | - ODEBUG_STATE_NOTAVAILABLE
|
| 237 |
|
| 238 | - ODEBUG_STATE_ACTIVE
|
| 239 |
|
| 240 | The function returns true when the fixup was successful, otherwise
|
| 241 | false. The return value is used to update the statistics.
|
| 242 |
|
| 243 | Note that the function needs to call the debug_object_activate()
|
| 244 | function again after the damage has been repaired in order to keep the
|
| 245 | state consistent.
|
| 246 |
|
| 247 | The activation of statically initialized objects is a special case. When
|
| 248 | debug_object_activate() has no tracked object for this object address
|
| 249 | then fixup_activate() is called with object state
|
| 250 | ODEBUG_STATE_NOTAVAILABLE. The fixup function needs to check whether
|
| 251 | this is a legitimate case of a statically initialized object or not. In
|
| 252 | case it is it calls debug_object_init() and debug_object_activate()
|
| 253 | to make the object known to the tracker and marked active. In this case
|
| 254 | the function should return false because this is not a real fixup.
|
| 255 |
|
| 256 | fixup_destroy
|
| 257 | --------------
|
| 258 |
|
| 259 | This function is called from the debug code whenever a problem in
|
| 260 | debug_object_destroy is detected.
|
| 261 |
|
| 262 | Called from debug_object_destroy when the object state is:
|
| 263 |
|
| 264 | - ODEBUG_STATE_ACTIVE
|
| 265 |
|
| 266 | The function returns true when the fixup was successful, otherwise
|
| 267 | false. The return value is used to update the statistics.
|
| 268 |
|
| 269 | fixup_free
|
| 270 | -----------
|
| 271 |
|
| 272 | This function is called from the debug code whenever a problem in
|
| 273 | debug_object_free is detected. Further it can be called from the debug
|
| 274 | checks in kfree/vfree, when an active object is detected from the
|
| 275 | debug_check_no_obj_freed() sanity checks.
|
| 276 |
|
| 277 | Called from debug_object_free() or debug_check_no_obj_freed() when
|
| 278 | the object state is:
|
| 279 |
|
| 280 | - ODEBUG_STATE_ACTIVE
|
| 281 |
|
| 282 | The function returns true when the fixup was successful, otherwise
|
| 283 | false. The return value is used to update the statistics.
|
| 284 |
|
| 285 | fixup_assert_init
|
| 286 | -------------------
|
| 287 |
|
| 288 | This function is called from the debug code whenever a problem in
|
| 289 | debug_object_assert_init is detected.
|
| 290 |
|
| 291 | Called from debug_object_assert_init() with a hardcoded state
|
| 292 | ODEBUG_STATE_NOTAVAILABLE when the object is not found in the debug
|
| 293 | bucket.
|
| 294 |
|
| 295 | The function returns true when the fixup was successful, otherwise
|
| 296 | false. The return value is used to update the statistics.
|
| 297 |
|
| 298 | Note, this function should make sure debug_object_init() is called
|
| 299 | before returning.
|
| 300 |
|
| 301 | The handling of statically initialized objects is a special case. The
|
| 302 | fixup function should check if this is a legitimate case of a statically
|
| 303 | initialized object or not. In this case only debug_object_init()
|
| 304 | should be called to make the object known to the tracker. Then the
|
| 305 | function should return false because this is not a real fixup.
|
| 306 |
|
| 307 | Known Bugs And Assumptions
|
| 308 | ==========================
|
| 309 |
|
| 310 | None (knock on wood).
|