README_DEVELOPERS
1 This file contains various notes/ideas/history/... related 2 to gdbserver in valgrind. 3 4 How to use Valgrind gdbserver ? 5 ------------------------------- 6 This is described in the Valgrind user manual. 7 Before reading the below, you better read the user manual first. 8 9 What is gdbserver ? 10 ------------------- 11 gdb debugger typically is used to debug a process running 12 on the same machine : gdb uses system calls (such as ptrace) 13 to fetch data from the process being debugged 14 or to change data in the process 15 or interrupt the process 16 or ... 17 18 gdb can also debug processes running in a different computer 19 (e.g. it can debug a process running on a small real time 20 board). 21 22 gdb does this by sending some commands (e.g. using tcp/ip) to a piece 23 of code running on the remote computer. This piece of code (called a 24 gdb stub in small boards, or gdbserver when the remote computer runs 25 an OS such as GNU/linux) will provide a set of commands allowing gdb 26 to remotely debug the process. Examples of commands are: "get the 27 registers", "get the list of running threads", "read xxx bytes at 28 address yyyyyyyy", etc. The definition of all these commands and the 29 associated replies is the gdb remote serial protocol, which is 30 documented in Appendix D of gdb user manual. 31 32 The standard gdb distribution has a standalone gdbserver (a small 33 executable) which implements this protocol and the needed system calls 34 to allow gdb to remotely debug process running on a linux or MacOS or 35 ... 36 37 Activation of gdbserver code inside valgrind 38 -------------------------------------------- 39 The gdbserver code (from gdb 6.6, GPL2+) has been modified so as to 40 link it with valgrind and allow the valgrind guest process to be 41 debugged by a gdb speaking to this gdbserver embedded in valgrind. 42 The ptrace system calls inside gdbserver have been replaced by reading 43 the state of the guest. 44 45 The gdbserver functionality is activated with valgrind command line 46 options. If gdbserver is not enabled, then the impact on valgrind 47 runtime is minimal: basically it just checks at startup the command 48 line option to see that there is nothing to do for what concerns gdb 49 server: there is a "if gdbserver is active" check in the translate 50 function of translate.c and an "if" in the valgrind scheduler. 51 If the valgrind gdbserver is activated (--vgdb=yes), the impact 52 is minimal (from time to time, the valgrind scheduler checks a counter 53 in memory). Option --vgdb-poll=yyyyy controls how often the scheduler 54 will do a (somewhat) more heavy check to see if gdbserver needs to 55 stop execution of the guest to allow debugging. 56 If valgrind gdbserver is activated with --vgdb=full, then 57 each instruction is instrumented with an additional call to a dirty 58 helper. 59 60 How does gdbserver code interacts with valgrind ? 61 ------------------------------------------------- 62 When an error is reported, the gdbserver code is called. It reads 63 commands from gdb using read system call on a FIFO (e.g. a command 64 such as "get the registers"). It executes the command (e.g. fetches 65 the registers from the guest state) and writes the reply (e.g. a 66 packet containing the register data). When gdb instructs gdbserver to 67 "continue", the control is returned to valgrind, which then continues 68 to execute guest code. The FIFOs used to communication between 69 valgrind and gdb are created at startup if gdbserver is activated 70 according to the --vgdb=no/yes/full command line option. 71 72 How are signals "handled" ? 73 --------------------------- 74 When a signal is to be given to the guest, valgrind core first calls 75 gdbserver (if a gdb is currently connected to valgrind, otherwise the 76 signal is delivered immediately). If gdb instructs to give the signal 77 to the process, the signal is delivered to the guest. Otherwise, the 78 signal is ignored (not given to the guest). The user can 79 with gdb further decide to pass (or not pass) the signal. 80 Note that some (fatal) signals cannot be ignored. 81 82 How are "break/step/stepi/next/..." implemented ? 83 ------------------------------------------------- 84 When a break is put by gdb on an instruction, a command is sent to the 85 gdbserver in valgrind. This causes the basic block of this instruction 86 to be discarded and then re-instrumented so as to insert calls to a 87 dirty helper which calls the gdb server code. When a block is 88 instrumented for gdbserver, all the "jump targets" of this block are 89 invalidated, so as to allow step/stepi/next to properly work: these 90 blocks will themselves automatically be re-instrumented for gdbserver 91 if they are jumped to. 92 The valgrind gdbserver remembers which blocks have been instrumented 93 due to this "lazy 'jump targets' debugging instrumentation" so as to 94 discard these "debugging translation" when gdb instructs to continue 95 the execution normally. 96 The blocks in which an explicit break has been put by the user 97 are kept instrumented for gdbserver. 98 (but note that by default, gdb removes all breaks when the 99 process is stopped, and re-inserts all breaks when the process 100 is continued). This behaviour can be changed using the gdb 101 command 'set breakpoint always-inserted'. 102 103 How are watchpoints implemented ? 104 --------------------------------- 105 Watchpoints implies support from the tool to detect that 106 a location is read and/or written. Currently, only memcheck 107 supports this : when a watchpoint is placed, memcheck changes 108 the addressability bits of the watched memory zone to be unacessible. 109 Before an access, memcheck then detects an error, but sees this error 110 is due to a watchpoint and gives the control back to gdb. 111 Stopping on the exact instruction for a write watchpoint implies 112 to use --vgdb=full. This is because the error is detected by memcheck 113 before modifying the value. gdb checks that the value has not changed 114 and so "does not believe" the information that the write watchpoint 115 was triggered, and continues the execution. At the next watchpoint 116 occurence, gdb sees the value has changed. But the watchpoints are all 117 reported "off by one". To avoid this, Valgrind gdbserver must 118 terminate the current instruction before reporting the write watchpoint. 119 Terminating precisely the current instruction implies to have 120 instrumented all the instructions of the block for gdbserver even 121 if there is no break in this block. This is ensured by --vgdb=full. 122 See m_gdbserver.c Bool VG_(is_watched) where watchpoint handling 123 is implemented. 124 125 How is the Valgrind gdbserver receiving commands/packets from gdb ? 126 ------------------------------------------------------------------- 127 The embedded gdbserver reads gdb commands on a named pipe having 128 (by default) the name /tmp/vgdb-pipe-from-vgdb-to-PID-by-USER-on-HOST 129 where PID, USER, and HOST will be replaced by the actual pid, the user id, 130 and the host name, respectively. 131 The embedded gdbserver will reply to gdb commands on a named pipe 132 /tmp/vgdb-pipe-to-vgdb-from-PID-by-USER-on-HOST 133 134 gdb does not speak directly with gdbserver in valgrind: a relay application 135 called vgdb is needed between gdb and the valgrind-ified process. 136 gdb writes commands on the stdin of vgdb. vgdb reads these 137 commands and writes them on FIFO /tmp/vgdb-pipe-from-vgdb-to-PID-by-USER-on-HOST. 138 vgdb reads replies on FIFO /tmp/vgdb-pipe-to-vgdb-from-PID-by-USER-on-HOST 139 and writes them on its stdout. 140 141 Note: The solution of named pipes was preferred to tcp ip connections as 142 it allows a discovery of which valgrind-ified processes are ready to accept 143 command by looking at files starting with the /tmp/vgdb-pipe- prefix 144 (changeable by a command line option). 145 Also, the usual unix protections are protecting 146 the valgrind process against other users sending commands. 147 The relay process also takes into account the wake up of the valgrind 148 process in case all threads are blocked in a system call. 149 The relay process can also be used in a shell to send commands 150 without a gdb (this allows to have a standard mechanism to control 151 valgrind tools from the command line, rather than specialized mechanism 152 e.g. in callgrind). 153 154 How is gdbserver activated if all Valgrind threads are blocked in a syscall ? 155 ----------------------------------------------------------------------------- 156 vgdb relays characters from gdb to valgrind. The scheduler will from 157 time to time check if gdbserver has to handle incoming characters. 158 (the check is efficient i.e. most of the time consists in checking 159 a counter in (shared) memory). 160 161 However, it might be that all the threads in the valgrind process are 162 blocked in a system call. In such a case, no polling will be done by 163 the valgrind scheduler (as no activity takes place). By default, vgdb 164 will check after 100ms if the characters it has written have been read 165 by valgrind. If not, vgdb will force the invocation of the gdbserver 166 code inside the valgrind process. 167 168 This forced invocation is implemented using the ptrace system call: 169 using ptrace, vgdb will cause the valgrind process to call the 170 gdbserver code. 171 172 This wake up is *not* done using signals as this would imply to 173 implement a syscall restart logic in valgrind for all system 174 calls. When using ptrace as above, the linux kernel is responsible to 175 restart the system call. 176 177 This wakeup is also *not* implemented by having a "system thread" 178 started by valgrind as this would transform all non-threaded programs 179 in threaded programs when running under valgrind. Also, such a 'system 180 thread' for gdbserver was tried by Greg Parker in the early MacOS 181 port, and was unreliable. 182 183 So, the ptrace based solution was chosen instead. 184 185 There used to be some bugs in the kernel when using ptrace on 186 a process blocked in a system call : the symptom is that the system 187 call fails with an unknown errno 512. This typically happens 188 with a vgdb in 64bits ptrace-ing a 32 bits process. 189 A bypass for old kernels has been integrated in vgdb.c (sign extend 190 register rax). 191 192 At least on a fedora core 12 (kernel 2.6.32), syscall restart of read 193 and select are working ok and red-hat 5.3 (an old kernel), everything 194 works properly. 195 196 Need to investigate if darwin can similarly do syscall 197 restart with ptrace. 198 199 The vgdb argument --max-invoke-ms=xxx allows to control the nr of 200 milli-seconds after which vgdb will force the invocation of gdbserver 201 code. If xxx is 0, this disables the forced invocation. 202 Also, disabling this ptrace mechanism is necessary in case you are 203 debugging the valgrind code at the same time as debugging the guest 204 process using gdbserver. 205 206 Do not kill -9 vgdb while it has interrupted the valgrind process, 207 otherwise the valgrind process will very probably stay stopped or die. 208 209 210 Implementation is based on the gdbserver code from gdb 6.6 211 ---------------------------------------------------------- 212 The gdbserver implementation is derived from the gdbserver included 213 in the gdb distribution. 214 The files originating from gdb are : inferiors.c, regcache.[ch], 215 regdef.h, remote-utils.c, server.[ch], signals.c, target.[ch], utils.c, 216 version.c. 217 valgrind-low-* are inspired from gdb files. 218 219 This code had to be changed to integrate properly within valgrind 220 (e.g. no libc usage). Some of these changes have been ensured by 221 using the preprocessor to replace calls by valgrind equivalent, 222 e.g. #define strcmp(...) VG_(strcmp) (...). 223 224 Some "control flow" changes are due to the fact that gdbserver inside 225 valgrind must return the control to valgrind when the 'debugged' 226 process has to run, while in a classical gdbserver usage, the 227 gdbserver process waits for a debugged process to stop on a break or 228 similar. This has implied to have some variables to remember the 229 state of gdbserver before returning to valgrind (search for 230 resume_packet_needed in server.c) and "goto" the place where gdbserver 231 expects a stopped process to return control to gdbserver. 232 233 How does a tool need to be changed to be "debuggable" ? 234 ------------------------------------------------------- 235 There is no need to modify a tool to have it "debuggable" via 236 gdbserver : e.g. reports of errors, break etc will work "out of the 237 box". If an interactive usage of tool client requests or similar is 238 desired for a tool, then simple code can be written for that via a 239 specific client request VG_USERREQ__GDB_MONITOR_COMMAND code. The tool 240 function "handle_client_request" must then parse the string received 241 in argument and call the expected valgrind or tool code. See 242 e.g. massif ms_handle_client_request as an example. 243 244 245 Automatic regression tests: 246 --------------------------- 247 Automatic Valgrind gdbserver tests are in the directory 248 $(top_srcdir)/gdbserver_tests. 249 Read $(top_srcdir)/gdbserver_tests/README_DEVELOPERS for more 250 info about testing. 251 252 How to integrate support for a new architecture xxx? 253 ---------------------------------------------------- 254 Let's imagine a new architecture hal9000 has to be supported. 255 256 Mandatory: 257 The main thing to do is to make a file valgrind-low-hal9000.c. 258 Start from an existing file (e.g. valgrind-low-x86.c). 259 The data structures 'struct reg regs' 260 and 'const char *expedite_regs' are build from files 261 in the gdb sources, e.g. for an new arch hal9000 262 cd gdb/regformats 263 ./regdat.sh reg-hal9000.dat hal9000 264 265 From the generated file hal9000, you copy/paste in 266 valgrind-low-hal9000.c the two needed data structures and change their 267 name to 'regs' and 'expedite_regs' 268 269 Then adapt the set of functions needed to initialize the structure 270 'static struct valgrind_target_ops low_target'. 271 272 Optional but heavily recommended: 273 To have a proper wake up of a Valgrind process with all threads 274 blocked in a system call, some architecture specific code 275 has to be done in vgdb.c : search for PTRACEINVOKER processor symbol 276 to see what has to be completed. 277 278 For Linux based platforms, all the ptrace calls should be ok. 279 The only thing needed is the code needed to "push a dummy call" on the stack, 280 i.e. assign the relevant registers in the struct user_regs_struct, and push 281 values on the stack according to the ABI. 282 283 For other platforms (i.e. Macos), more work is needed as the ptrace calls 284 on Macos are either different and/or incomplete (and so, 'Mach' specific 285 things are needed e.g. to attach to threads etc). 286 A courageous Mac aficionado is welcome on this aspect. 287 288 Optional: 289 To let gdb see the Valgrind shadow registers, xml description 290 files have to be provided + valgrind-low-hal9000.c has 291 to give the top xml file. 292 Start from the xml files found in the gdb distribution directory 293 gdb/features. You need to duplicate and modify these files to provide 294 shadow1 and shadow2 register sets description. 295 296 Modify coregrind/Makefile.am: 297 add valgrind-low-hal9000.c 298 If you have target xml description, also add them to GDBSERVER_XML_FILES 299 300 301 TODO and/or additional nice things to have 302 ------------------------------------------ 303 * many options can be changed on-line without problems. 304 => would be nice to have a v.option command that would evaluate 305 its arguments like the startup options of m_main.c and tool clo processing. 306 307 * have a memcheck monitor command 308 show_dangling_pointers [last_n_recently_released_blocks] 309 showing which of the n last recently released blocks are still 310 referenced. These references are (potential) dangling pointers. 311 312 * some GDBTD in the code 313 314 (GDBTD = GDB To Do = something still to look at and/or a question) 315 316 * All architectures and platforms are done. 317 But there are still some "GDBTD" to convert between gdb registers 318 and VEX registers : 319 e.g. some registers in x86 or amd64 that I could not 320 translate to VEX registers. Someone with a good knowledge 321 of these architectures might complete this 322 (see the GDBTD in valgrind-low-*.c) 323 324 * Currently, at least on recent linux kernel, vgdb can properly wake 325 up a valgrind process which is blocked in system calls. Maybe we 326 need to see till which kernel version the ptrace + syscall restart 327 is broken, and put the default value of --max-invoke-ms to 0 in this 328 case. 329 330 * more client requests can be programmed in various tools. Currently, 331 there are only a few standard valgrind or memcheck client requests 332 implemented. 333 v.suppression [generate|add|delete] might be an interesting command: 334 generate would output a suppression, add/delete would add a suppression 335 in memory for the last (or selected?) error. 336 v.break on fn calls/entry/exit + commands associated to it 337 (such as search leaks)? 338 339 340 341 * currently jump(s) and inferior call(s) are somewhat dangerous 342 when called from a block not yet instrumented : instead 343 of continuing till the next Imark, where there will be a 344 debugger call that can properly jump at an instruction boundary, 345 the jump/call will quit the "middle" of an instruction. 346 We could detect if the current block is instrumented by a trick 347 like this: 348 /* Each time helperc_CallDebugger is called, we will store 349 the address from which is it called and the nr of bbs_done 350 when called. This allows to detect that gdbserver is called 351 from a block which is instrumented. */ 352 static HWord CallDebugger_addr; 353 static ULong CallDebugger_bbs_done; 354 355 Bool VG_(gdbserver_current_IP_instrumented) (ThreadId tid) 356 { 357 if (VG_(get_IP) (tid) != CallDebugger_addr 358 || CallDebugger_bbs_done != VG_(bbs_done)()) 359 return False; 360 return True; 361 } 362 363 Alternatively, we ensure we can re-instrument the current 364 block for gdbserver while executing it. 365 Something like: 366 keep current block till the end of the current instruction, then 367 go back to scheduler. 368 Unsure if and how this is do-able. 369 370 371 * ensure that all non static symbols of gdbserver files are #define 372 xxxxx VG_(xxxxx) ???? Is this really needed ? I have tried to put in 373 a test program variables and functions with the same name as valgrind 374 stuff, and everything seems to be ok. 375 I see that all exported symbols in valgrind have a unique prefix 376 created with VG_ or MC_ or ... 377 This is not done for the "gdb gdbserver code", where I have kept 378 the original names. Is this a problem ? I could not create 379 a "symbol" collision between the user symbol and the valgrind 380 core gdbserver symbol. 381 382 * currently, gdbserver can only stop/continue the whole process. It 383 might be interesting to have a fine-grained thread control (vCont 384 packet) maybe for tools such as helgrind, drd. This would allow the 385 user to stop/resume specific threads. Also, maybe this would solve 386 the following problem: wait for a breakpoint to be encountered, 387 switch thread, next. This sometimes causes an internal error in gdb, 388 probably because gdb believes the current thread will be continued ? 389 390 * would be nice to have some more tests. 391 392 * better valgrind target support in gdb (see comments of Tom Tromey). 393 394 395 -------- description of how gdb invokes a function in the inferior 396 to call a function in the inferior (below is for x86): 397 gdb writes ESP and EBP to have some more stack space 398 push a return address equal to 0x8048390 <_start> 399 puts a break at 0x8048390 400 put address of the function to call (e.g. hello_world in EIP (0x8048444)) 401 continue 402 break encountered at 0x8048391 (90 after decrement) 403 => report stop to gdb 404 => gdb restores esp/ebp/eip to what it was (eg. 0x804848C) 405 => gdb "s" => causes the EIP to go to the new EIP (i.e. 0x804848C) 406 gdbserver tells "resuming from 0x804848c" 407 "stop pc is 0x8048491" => informed gdb of this 408 409
