xref: /external/valgrind/main/coregrind/m_gdbserver/README_DEVELOPERS

This file contains various notes/ideas/history/... related
to gdbserver in valgrind.

How to use Valgrind gdbserver ?
-------------------------------
This is described in the Valgrind user manual.
Before reading the below, you better read the user manual first.

What is gdbserver ?
-------------------
gdb debugger typically is used to debug a process running
on the same machine : gdb uses system calls (such as ptrace) 
to fetch data from the process being debugged
or to change data in the process 
or interrupt the process 
or ...

gdb can also debug processes running in a different computer
(e.g. it can debug a process running on a small real time
board).

gdb does this by sending some commands (e.g. using tcp/ip) to a piece
of code running on the remote computer. This piece of code (called a
gdb stub in small boards, or gdbserver when the remote computer runs
an OS such as GNU/linux) will provide a set of commands allowing gdb
to remotely debug the process.  Examples of commands are: "get the
registers", "get the list of running threads", "read xxx bytes at
address yyyyyyyy", etc.  The definition of all these commands and the
associated replies is the gdb remote serial protocol, which is
documented in Appendix D of gdb user manual.

The standard gdb distribution has a standalone gdbserver (a small
executable) which implements this protocol and the needed system calls
to allow gdb to remotely debug process running on a linux or MacOS or
...

Activation of gdbserver code inside valgrind
--------------------------------------------
The gdbserver code (from gdb 6.6, GPL2+) has been modified so as to
link it with valgrind and allow the valgrind guest process to be
debugged by a gdb speaking to this gdbserver embedded in valgrind.
The ptrace system calls inside gdbserver have been replaced by reading
the state of the guest.

The gdbserver functionality is activated with valgrind command line
options. If gdbserver is not enabled, then the impact on valgrind
runtime is minimal: basically it just checks at startup the command
line option to see that there is nothing to do for what concerns gdb
server: there is a "if gdbserver is active" check in the translate
function of translate.c and an "if" in the valgrind scheduler.
If the valgrind gdbserver is activated (--vgdb=yes), the impact
is minimal (from time to time, the valgrind scheduler checks a counter
in memory). Option --vgdb-poll=yyyyy controls how often the scheduler
will do a (somewhat) more heavy check to see if gdbserver needs to
stop execution of the guest to allow debugging.
If valgrind gdbserver is activated with --vgdb=full, then
each instruction is instrumented with an additional call to a dirty
helper. 

How does gdbserver code interacts with valgrind ?
-------------------------------------------------
When an error is reported, the gdbserver code is called.  It reads
commands from gdb using read system call on a FIFO (e.g. a command
such as "get the registers").  It executes the command (e.g. fetches
the registers from the guest state) and writes the reply (e.g. a
packet containing the register data).  When gdb instructs gdbserver to
"continue", the control is returned to valgrind, which then continues
to execute guest code.  The FIFOs used to communication between
valgrind and gdb are created at startup if gdbserver is activated
according to the --vgdb=no/yes/full command line option.

How are signals "handled" ?
---------------------------
When a signal is to be given to the guest, valgrind core first calls
gdbserver (if a gdb is currently connected to valgrind, otherwise the
signal is delivered immediately). If gdb instructs to give the signal
to the process, the signal is delivered to the guest.  Otherwise, the
signal is ignored (not given to the guest). The user can
with gdb further decide to pass (or not pass) the signal.
Note that some (fatal) signals cannot be ignored.

How are "break/step/stepi/next/..." implemented ?
-------------------------------------------------
When a break is put by gdb on an instruction, a command is sent to the
gdbserver in valgrind. This causes the basic block of this instruction
to be discarded and then re-instrumented so as to insert calls to a
dirty helper which calls the gdb server code.  When a block is
instrumented for gdbserver, all the "jump targets" of this block are
invalidated, so as to allow step/stepi/next to properly work: these
blocks will themselves automatically be re-instrumented for gdbserver
if they are jumped to.
The valgrind gdbserver remembers which blocks have been instrumented
due to this "lazy 'jump targets' debugging instrumentation" so as to
discard these "debugging translation" when gdb instructs to continue
the execution normally.
The blocks in which an explicit break has been put by the user
are kept instrumented for gdbserver.
(but note that by default, gdb removes all breaks when the
process is stopped, and re-inserts all breaks when the process
is continued). This behaviour can be changed using the gdb
command 'set breakpoint always-inserted'.

How are watchpoints implemented ?
---------------------------------
Watchpoints implies support from the tool to detect that
a location is read and/or written. Currently, only memcheck
supports this : when a watchpoint is placed, memcheck changes
the addressability bits of the watched memory zone to be unacessible.
Before an access, memcheck then detects an error, but sees this error
is due to a watchpoint and gives the control back to gdb.
Stopping on the exact instruction for a write watchpoint implies
to use --vgdb=full. This is because the error is detected by memcheck
before modifying the value. gdb checks that the value has not changed
and so "does not believe" the information that the write watchpoint
was triggered, and continues the execution. At the next watchpoint
occurence, gdb sees the value has changed. But the watchpoints are all
reported "off by one". To avoid this, Valgrind gdbserver must
terminate the current instruction before reporting the write watchpoint.
Terminating precisely the current instruction implies to have
instrumented all the instructions of the block for gdbserver even
if there is no break in this block. This is ensured by --vgdb=full.
See m_gdbserver.c Bool VG_(is_watched) where watchpoint handling
is implemented.

How is the Valgrind gdbserver receiving commands/packets from gdb ?
-------------------------------------------------------------------
The embedded gdbserver reads gdb commands on a named pipe having
(by default) the name   /tmp/vgdb-pipe-from-vgdb-to-PID-by-USER-on-HOST
where PID, USER, and HOST will be replaced by the actual pid, the user id,
and the host name, respectively.
The embedded gdbserver will reply to gdb commands on a named pipe
/tmp/vgdb-pipe-to-vgdb-from-PID-by-USER-on-HOST

gdb does not speak directly with gdbserver in valgrind: a relay application
called vgdb is needed between gdb and the valgrind-ified process.
gdb writes commands on the stdin of vgdb. vgdb reads these
commands and writes them on FIFO /tmp/vgdb-pipe-from-vgdb-to-PID-by-USER-on-HOST.
vgdb reads replies on FIFO /tmp/vgdb-pipe-to-vgdb-from-PID-by-USER-on-HOST
and writes them on its stdout. 

Note: The solution of named pipes was preferred to tcp ip connections as
it allows a discovery of which valgrind-ified processes are ready to accept
command by looking at files starting with the /tmp/vgdb-pipe- prefix
(changeable by a command line option).
Also, the usual unix protections are protecting 
the valgrind process against other users sending commands.
The relay process also takes into account the wake up of the valgrind
process in case all threads are blocked in a system call.
The relay process can also be used in a shell to send commands
without a gdb (this allows to have a standard mechanism to control
valgrind tools from the command line, rather than specialized mechanism
e.g. in callgrind).

How is gdbserver activated if all Valgrind threads are blocked in a syscall ?
-----------------------------------------------------------------------------
vgdb relays characters from gdb to valgrind. The scheduler will from
time to time check if gdbserver has to handle incoming characters.
(the check is efficient i.e. most of the time consists in checking
a counter in (shared) memory).

However, it might be that all the threads in the valgrind process are
blocked in a system call. In such a case, no polling will be done by
the valgrind scheduler (as no activity takes place).  By default, vgdb
will check after 100ms if the characters it has written have been read
by valgrind. If not, vgdb will force the invocation of the gdbserver
code inside the valgrind process.

This forced invocation is implemented using the ptrace system call:
using ptrace, vgdb will cause the valgrind process to call the
gdbserver code.

This wake up is *not* done using signals as this would imply to
implement a syscall restart logic in valgrind for all system
calls. When using ptrace as above, the linux kernel is responsible to
restart the system call.

This wakeup is also *not* implemented by having a "system thread"
started by valgrind as this would transform all non-threaded programs
in threaded programs when running under valgrind. Also, such a 'system
thread' for gdbserver was tried by Greg Parker in the early MacOS
port, and was unreliable.  

So, the ptrace based solution was chosen instead.

There used to be some bugs in the kernel when using ptrace on 
a process blocked in a system call : the symptom is that the system
call fails with an unknown errno 512. This typically happens
with a vgdb in 64bits ptrace-ing a 32 bits process.
A bypass for old kernels has been integrated in vgdb.c (sign extend
register rax).

At least on a fedora core 12 (kernel 2.6.32), syscall restart of read
and select are working ok and red-hat 5.3 (an old kernel), everything
works properly.

Need to investigate if darwin can similarly do syscall
restart with ptrace.

The vgdb argument --max-invoke-ms=xxx allows to control the nr of
milli-seconds after which vgdb will force the invocation of gdbserver
code.  If xxx is 0, this disables the forced invocation.
Also, disabling this ptrace mechanism is necessary in case you are
debugging the valgrind code at the same time as debugging the guest
process using gdbserver.

Do not kill -9 vgdb while it has interrupted the valgrind process,
otherwise the valgrind process will very probably stay stopped or die.


Implementation is based on the gdbserver code from gdb 6.6
----------------------------------------------------------
The gdbserver implementation is derived from the gdbserver included
in the gdb distribution.
The files originating from gdb are : inferiors.c, regcache.[ch],
regdef.h, remote-utils.c, server.[ch], signals.c, target.[ch], utils.c,
version.c.
valgrind-low-* are inspired from gdb files.

This code had to be changed to integrate properly within valgrind
(e.g. no libc usage).  Some of these changes have been ensured by
using the preprocessor to replace calls by valgrind equivalent,
e.g. #define strcmp(...) VG_(strcmp) (...).

Some "control flow" changes are due to the fact that gdbserver inside
valgrind must return the control to valgrind when the 'debugged'
process has to run, while in a classical gdbserver usage, the
gdbserver process waits for a debugged process to stop on a break or
similar.  This has implied to have some variables to remember the
state of gdbserver before returning to valgrind (search for
resume_packet_needed in server.c) and "goto" the place where gdbserver
expects a stopped process to return control to gdbserver.

How does a tool need to be changed to be "debuggable" ?
-------------------------------------------------------
There is no need to modify a tool to have it "debuggable" via
gdbserver : e.g. reports of errors, break etc will work "out of the
box".  If an interactive usage of tool client requests or similar is
desired for a tool, then simple code can be written for that via a
specific client request VG_USERREQ__GDB_MONITOR_COMMAND code. The tool
function "handle_client_request" must then parse the string received
in argument and call the expected valgrind or tool code.  See
e.g. massif ms_handle_client_request as an example.


Automatic regression tests:
---------------------------
Automatic Valgrind gdbserver tests are in the directory
$(top_srcdir)/gdbserver_tests.
Read $(top_srcdir)/gdbserver_tests/README_DEVELOPERS for more
info about testing.

How to integrate support for a new architecture xxx?
----------------------------------------------------
Let's imagine a new architecture hal9000 has to be supported.

Mandatory:
The main thing to do is to make a file valgrind-low-hal9000.c.
Start from an existing file (e.g. valgrind-low-x86.c).
The data structures 'struct reg regs'
and 'const char *expedite_regs' are build from files
in the gdb sources, e.g. for an new arch hal9000
   cd gdb/regformats
   ./regdat.sh reg-hal9000.dat hal9000

From the generated file hal9000, you copy/paste in
valgrind-low-hal9000.c the two needed data structures and change their
name to 'regs' and 'expedite_regs'

Then adapt the set of functions needed to initialize the structure
'static struct valgrind_target_ops low_target'.

Optional but heavily recommended:
To have a proper wake up of a Valgrind process with all threads
blocked in a system call, some architecture specific code
has to be done in vgdb.c : search for PTRACEINVOKER processor symbol
to see what has to be completed.

For Linux based platforms, all the ptrace calls should be ok.
The only thing needed is the code needed to "push a dummy call" on the stack,
i.e. assign the relevant registers in the struct user_regs_struct, and push
values on the stack according to the ABI.

For other platforms (i.e. Macos), more work is needed as the ptrace calls
on Macos are either different and/or incomplete (and so, 'Mach' specific
things are needed e.g. to attach to threads etc).
A courageous Mac aficionado is welcome on this aspect.

Optional:
To let gdb see the Valgrind shadow registers, xml description
files have to be provided + valgrind-low-hal9000.c has
to give the top xml file.
Start from the xml files found in the gdb distribution directory
gdb/features. You need to duplicate and modify these files to provide
shadow1 and shadow2 register sets description.

Modify coregrind/Makefile.am:
    add valgrind-low-hal9000.c
    If you have target xml description, also add them to GDBSERVER_XML_FILES


TODO and/or additional nice things to have
------------------------------------------
* many options can be changed on-line without problems.
  => would be nice to have a v.option command that would evaluate
  its arguments like the  startup options of m_main.c and tool clo processing.

* have a memcheck monitor command
  show_dangling_pointers [last_n_recently_released_blocks]
  showing which of the n last recently released blocks are still
  referenced. These references are (potential) dangling pointers.

* some GDBTD in the code 

(GDBTD = GDB To Do = something still to look at and/or a question)

* All architectures and platforms are done.
  But there are still some "GDBTD" to convert between gdb registers
  and VEX registers :
  e.g. some registers in x86 or amd64 that I could not
  translate to VEX registers. Someone with a good knowledge
  of these architectures might complete this 
  (see the GDBTD in valgrind-low-*.c)

* Currently, at least on recent linux kernel, vgdb can properly wake
  up a valgrind process which is blocked in system calls. Maybe we
  need to see till which kernel version the ptrace + syscall restart
  is broken, and put the default value of --max-invoke-ms to 0 in this
  case.

* more client requests can be programmed in various tools.  Currently,
  there are only a few standard valgrind or memcheck client requests
  implemented.
  v.suppression [generate|add|delete] might be an interesting command: 
     generate would output a suppression, add/delete would add a suppression
     in memory for the last (or selected?) error.
  v.break on fn calls/entry/exit + commands associated to it 
    (such as search leaks)?



* currently jump(s) and inferior call(s) are somewhat dangerous
  when called from a block not yet instrumented : instead
  of continuing till the next Imark, where there will be a
  debugger call that can properly jump at an instruction boundary,
  the jump/call will quit the "middle" of an instruction.
  We could detect if the current block is instrumented by a trick
  like this:
     /* Each time helperc_CallDebugger is called, we will store
        the address from which is it called and the nr of bbs_done
        when called. This allows to detect that gdbserver is called
        from a block which is instrumented. */
     static HWord CallDebugger_addr;
     static ULong CallDebugger_bbs_done;

     Bool VG_(gdbserver_current_IP_instrumented) (ThreadId tid)
     {
        if (VG_(get_IP) (tid) != CallDebugger_addr
            || CallDebugger_bbs_done != VG_(bbs_done)())
           return False;
        return True;
     }

  Alternatively, we ensure we can re-instrument the current
  block for gdbserver while executing it.
  Something like:
  keep current block till the end of the current instruction, then
  go back to scheduler.
  Unsure if and how this is do-able.


* ensure that all non static symbols of gdbserver files are #define
  xxxxx VG_(xxxxx) ???? Is this really needed ? I have tried to put in
  a test program variables and functions with the same name as valgrind
  stuff, and everything seems to be ok.
  I see that all exported symbols in valgrind have a unique prefix
  created with VG_ or MC_ or ...
  This is not done for the "gdb gdbserver code", where I have kept
  the original names. Is this a problem ? I could not create
  a "symbol" collision between the user symbol and the valgrind
  core gdbserver symbol.

* currently, gdbserver can only stop/continue the whole process. It
  might be interesting to have a fine-grained thread control (vCont
  packet) maybe for tools such as helgrind, drd.  This would allow the
  user to stop/resume specific threads.  Also, maybe this would solve
  the following problem: wait for a breakpoint to be encountered,
  switch thread, next. This sometimes causes an internal error in gdb,
  probably because gdb believes the current thread will be continued ?

* would be nice to have some more tests.

* better valgrind target support in gdb (see comments of Tom Tromey).


-------- description of how gdb invokes a function in the inferior
to call a function in the inferior (below is for x86):
gdb writes ESP and EBP to have some more stack space
push a return address equal to  0x8048390 <_start>
puts a break                at  0x8048390
put address of the function to call (e.g. hello_world in EIP (0x8048444))
continue
break encountered at 0x8048391 (90 after decrement)
  => report stop to gdb
  => gdb restores esp/ebp/eip to what it was (eg. 0x804848C)
  => gdb "s" => causes the EIP to go to the new EIP (i.e. 0x804848C)
     gdbserver tells "resuming from 0x804848c"
                     "stop pc is 0x8048491" => informed gdb of this