Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 function tracer guts 2 ==================== 3 By Mike Frysinger 4 5 Introduction 6 ------------ 7 8 Here we will cover the architecture pieces that the common function tracing 9 code relies on for proper functioning. Things are broken down into increasing 10 complexity so that you can start simple and at least get basic functionality. 11 12 Note that this focuses on architecture implementation details only. If you 13 want more explanation of a feature in terms of common code, review the common 14 ftrace.txt file. 15 16 Ideally, everyone who wishes to retain performance while supporting tracing in 17 their kernel should make it all the way to dynamic ftrace support. 18 19 20 Prerequisites 21 ------------- 22 23 Ftrace relies on these features being implemented: 24 STACKTRACE_SUPPORT - implement save_stack_trace() 25 TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h 26 27 28 HAVE_FUNCTION_TRACER 29 -------------------- 30 31 You will need to implement the mcount and the ftrace_stub functions. 32 33 The exact mcount symbol name will depend on your toolchain. Some call it 34 "mcount", "_mcount", or even "__mcount". You can probably figure it out by 35 running something like: 36 $ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount 37 call mcount 38 We'll make the assumption below that the symbol is "mcount" just to keep things 39 nice and simple in the examples. 40 41 Keep in mind that the ABI that is in effect inside of the mcount function is 42 *highly* architecture/toolchain specific. We cannot help you in this regard, 43 sorry. Dig up some old documentation and/or find someone more familiar than 44 you to bang ideas off of. Typically, register usage (argument/scratch/etc...) 45 is a major issue at this point, especially in relation to the location of the 46 mcount call (before/after function prologue). You might also want to look at 47 how glibc has implemented the mcount function for your architecture. It might 48 be (semi-)relevant. 49 50 The mcount function should check the function pointer ftrace_trace_function 51 to see if it is set to ftrace_stub. If it is, there is nothing for you to do, 52 so return immediately. If it isn't, then call that function in the same way 53 the mcount function normally calls __mcount_internal -- the first argument is 54 the "frompc" while the second argument is the "selfpc" (adjusted to remove the 55 size of the mcount call that is embedded in the function). 56 57 For example, if the function foo() calls bar(), when the bar() function calls 58 mcount(), the arguments mcount() will pass to the tracer are: 59 "frompc" - the address bar() will use to return to foo() 60 "selfpc" - the address bar() (with mcount() size adjustment) 61 62 Also keep in mind that this mcount function will be called *a lot*, so 63 optimizing for the default case of no tracer will help the smooth running of 64 your system when tracing is disabled. So the start of the mcount function is 65 typically the bare minimum with checking things before returning. That also 66 means the code flow should usually be kept linear (i.e. no branching in the nop 67 case). This is of course an optimization and not a hard requirement. 68 69 Here is some pseudo code that should help (these functions should actually be 70 implemented in assembly): 71 72 void ftrace_stub(void) 73 { 74 return; 75 } 76 77 void mcount(void) 78 { 79 /* save any bare state needed in order to do initial checking */ 80 81 extern void (*ftrace_trace_function)(unsigned long, unsigned long); 82 if (ftrace_trace_function != ftrace_stub) 83 goto do_trace; 84 85 /* restore any bare state */ 86 87 return; 88 89 do_trace: 90 91 /* save all state needed by the ABI (see paragraph above) */ 92 93 unsigned long frompc = ...; 94 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE; 95 ftrace_trace_function(frompc, selfpc); 96 97 /* restore all state needed by the ABI */ 98 } 99 100 Don't forget to export mcount for modules ! 101 extern void mcount(void); 102 EXPORT_SYMBOL(mcount); 103 104 105 HAVE_FUNCTION_GRAPH_TRACER 106 -------------------------- 107 108 Deep breath ... time to do some real work. Here you will need to update the 109 mcount function to check ftrace graph function pointers, as well as implement 110 some functions to save (hijack) and restore the return address. 111 112 The mcount function should check the function pointers ftrace_graph_return 113 (compare to ftrace_stub) and ftrace_graph_entry (compare to 114 ftrace_graph_entry_stub). If either of those is not set to the relevant stub 115 function, call the arch-specific function ftrace_graph_caller which in turn 116 calls the arch-specific function prepare_ftrace_return. Neither of these 117 function names is strictly required, but you should use them anyway to stay 118 consistent across the architecture ports -- easier to compare & contrast 119 things. 120 121 The arguments to prepare_ftrace_return are slightly different than what are 122 passed to ftrace_trace_function. The second argument "selfpc" is the same, 123 but the first argument should be a pointer to the "frompc". Typically this is 124 located on the stack. This allows the function to hijack the return address 125 temporarily to have it point to the arch-specific function return_to_handler. 126 That function will simply call the common ftrace_return_to_handler function and 127 that will return the original return address with which you can return to the 128 original call site. 129 130 Here is the updated mcount pseudo code: 131 void mcount(void) 132 { 133 ... 134 if (ftrace_trace_function != ftrace_stub) 135 goto do_trace; 136 137 +#ifdef CONFIG_FUNCTION_GRAPH_TRACER 138 + extern void (*ftrace_graph_return)(...); 139 + extern void (*ftrace_graph_entry)(...); 140 + if (ftrace_graph_return != ftrace_stub || 141 + ftrace_graph_entry != ftrace_graph_entry_stub) 142 + ftrace_graph_caller(); 143 +#endif 144 145 /* restore any bare state */ 146 ... 147 148 Here is the pseudo code for the new ftrace_graph_caller assembly function: 149 #ifdef CONFIG_FUNCTION_GRAPH_TRACER 150 void ftrace_graph_caller(void) 151 { 152 /* save all state needed by the ABI */ 153 154 unsigned long *frompc = &...; 155 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE; 156 /* passing frame pointer up is optional -- see below */ 157 prepare_ftrace_return(frompc, selfpc, frame_pointer); 158 159 /* restore all state needed by the ABI */ 160 } 161 #endif 162 163 For information on how to implement prepare_ftrace_return(), simply look at the 164 x86 version (the frame pointer passing is optional; see the next section for 165 more information). The only architecture-specific piece in it is the setup of 166 the fault recovery table (the asm(...) code). The rest should be the same 167 across architectures. 168 169 Here is the pseudo code for the new return_to_handler assembly function. Note 170 that the ABI that applies here is different from what applies to the mcount 171 code. Since you are returning from a function (after the epilogue), you might 172 be able to skimp on things saved/restored (usually just registers used to pass 173 return values). 174 175 #ifdef CONFIG_FUNCTION_GRAPH_TRACER 176 void return_to_handler(void) 177 { 178 /* save all state needed by the ABI (see paragraph above) */ 179 180 void (*original_return_point)(void) = ftrace_return_to_handler(); 181 182 /* restore all state needed by the ABI */ 183 184 /* this is usually either a return or a jump */ 185 original_return_point(); 186 } 187 #endif 188 189 190 HAVE_FUNCTION_GRAPH_FP_TEST 191 --------------------------- 192 193 An arch may pass in a unique value (frame pointer) to both the entering and 194 exiting of a function. On exit, the value is compared and if it does not 195 match, then it will panic the kernel. This is largely a sanity check for bad 196 code generation with gcc. If gcc for your port sanely updates the frame 197 pointer under different optimization levels, then ignore this option. 198 199 However, adding support for it isn't terribly difficult. In your assembly code 200 that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument. 201 Then in the C version of that function, do what the x86 port does and pass it 202 along to ftrace_push_return_trace() instead of a stub value of 0. 203 204 Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer. 205 206 HAVE_FUNCTION_GRAPH_RET_ADDR_PTR 207 -------------------------------- 208 209 An arch may pass in a pointer to the return address on the stack. This 210 prevents potential stack unwinding issues where the unwinder gets out of 211 sync with ret_stack and the wrong addresses are reported by 212 ftrace_graph_ret_addr(). 213 214 Adding support for it is easy: just define the macro in asm/ftrace.h and 215 pass the return address pointer as the 'retp' argument to 216 ftrace_push_return_trace(). 217 218 HAVE_FTRACE_NMI_ENTER 219 --------------------- 220 221 If you can't trace NMI functions, then skip this option. 222 223 <details to be filled> 224 225 226 HAVE_SYSCALL_TRACEPOINTS 227 ------------------------ 228 229 You need very few things to get the syscalls tracing in an arch. 230 231 - Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig). 232 - Have a NR_syscalls variable in <asm/unistd.h> that provides the number 233 of syscalls supported by the arch. 234 - Support the TIF_SYSCALL_TRACEPOINT thread flags. 235 - Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace 236 in the ptrace syscalls tracing path. 237 - If the system call table on this arch is more complicated than a simple array 238 of addresses of the system calls, implement an arch_syscall_addr to return 239 the address of a given system call. 240 - If the symbol names of the system calls do not match the function names on 241 this arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h and 242 implement arch_syscall_match_sym_name with the appropriate logic to return 243 true if the function name corresponds with the symbol name. 244 - Tag this arch as HAVE_SYSCALL_TRACEPOINTS. 245 246 247 HAVE_FTRACE_MCOUNT_RECORD 248 ------------------------- 249 250 See scripts/recordmcount.pl for more info. Just fill in the arch-specific 251 details for how to locate the addresses of mcount call sites via objdump. 252 This option doesn't make much sense without also implementing dynamic ftrace. 253 254 255 HAVE_DYNAMIC_FTRACE 256 ------------------- 257 258 You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so 259 scroll your reader back up if you got over eager. 260 261 Once those are out of the way, you will need to implement: 262 - asm/ftrace.h: 263 - MCOUNT_ADDR 264 - ftrace_call_adjust() 265 - struct dyn_arch_ftrace{} 266 - asm code: 267 - mcount() (new stub) 268 - ftrace_caller() 269 - ftrace_call() 270 - ftrace_stub() 271 - C code: 272 - ftrace_dyn_arch_init() 273 - ftrace_make_nop() 274 - ftrace_make_call() 275 - ftrace_update_ftrace_func() 276 277 First you will need to fill out some arch details in your asm/ftrace.h. 278 279 Define MCOUNT_ADDR as the address of your mcount symbol similar to: 280 #define MCOUNT_ADDR ((unsigned long)mcount) 281 Since no one else will have a decl for that function, you will need to: 282 extern void mcount(void); 283 284 You will also need the helper function ftrace_call_adjust(). Most people 285 will be able to stub it out like so: 286 static inline unsigned long ftrace_call_adjust(unsigned long addr) 287 { 288 return addr; 289 } 290 <details to be filled> 291 292 Lastly you will need the custom dyn_arch_ftrace structure. If you need 293 some extra state when runtime patching arbitrary call sites, this is the 294 place. For now though, create an empty struct: 295 struct dyn_arch_ftrace { 296 /* No extra data needed */ 297 }; 298 299 With the header out of the way, we can fill out the assembly code. While we 300 did already create a mcount() function earlier, dynamic ftrace only wants a 301 stub function. This is because the mcount() will only be used during boot 302 and then all references to it will be patched out never to return. Instead, 303 the guts of the old mcount() will be used to create a new ftrace_caller() 304 function. Because the two are hard to merge, it will most likely be a lot 305 easier to have two separate definitions split up by #ifdefs. Same goes for 306 the ftrace_stub() as that will now be inlined in ftrace_caller(). 307 308 Before we get confused anymore, let's check out some pseudo code so you can 309 implement your own stuff in assembly: 310 311 void mcount(void) 312 { 313 return; 314 } 315 316 void ftrace_caller(void) 317 { 318 /* save all state needed by the ABI (see paragraph above) */ 319 320 unsigned long frompc = ...; 321 unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE; 322 323 ftrace_call: 324 ftrace_stub(frompc, selfpc); 325 326 /* restore all state needed by the ABI */ 327 328 ftrace_stub: 329 return; 330 } 331 332 This might look a little odd at first, but keep in mind that we will be runtime 333 patching multiple things. First, only functions that we actually want to trace 334 will be patched to call ftrace_caller(). Second, since we only have one tracer 335 active at a time, we will patch the ftrace_caller() function itself to call the 336 specific tracer in question. That is the point of the ftrace_call label. 337 338 With that in mind, let's move on to the C code that will actually be doing the 339 runtime patching. You'll need a little knowledge of your arch's opcodes in 340 order to make it through the next section. 341 342 Every arch has an init callback function. If you need to do something early on 343 to initialize some state, this is the time to do that. Otherwise, this simple 344 function below should be sufficient for most people: 345 346 int __init ftrace_dyn_arch_init(void) 347 { 348 return 0; 349 } 350 351 There are two functions that are used to do runtime patching of arbitrary 352 functions. The first is used to turn the mcount call site into a nop (which 353 is what helps us retain runtime performance when not tracing). The second is 354 used to turn the mcount call site into a call to an arbitrary location (but 355 typically that is ftracer_caller()). See the general function definition in 356 linux/ftrace.h for the functions: 357 ftrace_make_nop() 358 ftrace_make_call() 359 The rec->ip value is the address of the mcount call site that was collected 360 by the scripts/recordmcount.pl during build time. 361 362 The last function is used to do runtime patching of the active tracer. This 363 will be modifying the assembly code at the location of the ftrace_call symbol 364 inside of the ftrace_caller() function. So you should have sufficient padding 365 at that location to support the new function calls you'll be inserting. Some 366 people will be using a "call" type instruction while others will be using a 367 "branch" type instruction. Specifically, the function is: 368 ftrace_update_ftrace_func() 369 370 371 HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER 372 ------------------------------------------------ 373 374 The function grapher needs a few tweaks in order to work with dynamic ftrace. 375 Basically, you will need to: 376 - update: 377 - ftrace_caller() 378 - ftrace_graph_call() 379 - ftrace_graph_caller() 380 - implement: 381 - ftrace_enable_ftrace_graph_caller() 382 - ftrace_disable_ftrace_graph_caller() 383 384 <details to be filled> 385 Quick notes: 386 - add a nop stub after the ftrace_call location named ftrace_graph_call; 387 stub needs to be large enough to support a call to ftrace_graph_caller() 388 - update ftrace_graph_caller() to work with being called by the new 389 ftrace_caller() since some semantics may have changed 390 - ftrace_enable_ftrace_graph_caller() will runtime patch the 391 ftrace_graph_call location with a call to ftrace_graph_caller() 392 - ftrace_disable_ftrace_graph_caller() will runtime patch the 393 ftrace_graph_call location with nops