| Debugging with GDB: Process Record and Replay |
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On some platforms, GDB provides a special process record and replay target that can record a log of the process execution, and replay it later with both forward and reverse execution commands.
When this target is in use, if the execution log includes the record for the next instruction, GDB will debug in replay mode. In the replay mode, the inferior does not really execute code instructions. Instead, all the events that normally happen during code execution are taken from the execution log. While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would. Their contents are taken from the execution log.
If the record for the next instruction is not in the execution log, GDB will debug in record mode. In this mode, the inferior executes normally, and GDB records the execution log for future replay.
The process record and replay target supports reverse execution (see Reverse Execution), even if the platform on which the inferior runs does not. However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log. In other words, reverse execution on platforms that don’t support it directly can only be done in the replay mode.
When debugging in the reverse direction, GDB will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not.
For architecture environments that support process record and replay, GDB provides the following commands:
record method
This command starts the process record and replay target. The
recording method can be specified as parameter. Without a parameter
the command uses the full recording method. The following
recording methods are available:
full
Full record/replay recording using GDB’s software record and replay implementation. This method allows replaying and reverse execution.
btrace format
Hardware-supported instruction recording. This method does not record
data. Further, the data is collected in a ring buffer so old data will
be overwritten when the buffer is full. It allows limited reverse
execution. Variables and registers are not available during reverse
execution. In remote debugging, recording continues on disconnect.
Recorded data can be inspected after reconnecting. The recording may
be stopped using record stop.
The recording format can be specified as parameter. Without a parameter the command chooses the recording format. The following recording formats are available:
bts
Use the Branch Trace Store (BTS) recording format. In this format, the processor stores a from/to record for each executed branch in the btrace ring buffer.
pt
Use the Intel Processor Trace recording format. In this format, the processor stores the execution trace in a compressed form that is afterwards decoded by GDB.
The trace can be recorded with very low overhead. The compressed trace format also allows small trace buffers to already contain a big number of instructions compared to BTS.
Decoding the recorded execution trace, on the other hand, is more expensive than decoding BTS trace. This is mostly due to the increased number of instructions to process. You should increase the buffer-size with care.
Not all recording formats may be available on all processors.
The process record and replay target can only debug a process that is already running. Therefore, you need first to start the process with the run or start commands, and then start the recording with the record method command.
Displaced stepping (see displaced stepping) will be automatically disabled when process record and replay target is started. That’s because the process record and replay target doesn’t support displaced stepping.
If the inferior is in the non-stop mode (see Non-Stop Mode) or in
the asynchronous execution mode (see Background Execution), not
all recording methods are available. The full recording method
does not support these two modes.
record stop
Stop the process record and replay target. When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state.
When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded. In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened.
On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become “live” at that earlier state, and it will then be possible to continue the usual “live” debugging of the process from that state.
When the inferior process exits, or GDB detaches from it, process record and replay target will automatically stop itself.
record goto
Go to a specific location in the execution log. There are several ways to specify the location to go to:
record goto begin
record goto start
Go to the beginning of the execution log.
record goto end
Go to the end of the execution log.
record goto n
Go to instruction number n in the execution log.
record save filename
Save the execution log to a file filename . Default filename is gdb_record.process_id , where process_id is the process ID of the inferior.
This command may not be available for all recording methods.
record restore filename
Restore the execution log from a file
filename
.
File must have been created with record save.
set record full insn-number-max limit
set record full insn-number-max unlimited
Set the limit of instructions to be recorded for the full
recording method. Default value is 200000.
If limit is a positive number, then GDB will start
deleting instructions from the log once the number of the record
instructions becomes greater than limit. For every new recorded
instruction, GDB will delete the earliest recorded
instruction to keep the number of recorded instructions at the limit.
(Since deleting recorded instructions loses information, GDB
lets you control what happens when the limit is reached, by means of
the stop-at-limit option, described below.)
If limit is unlimited or zero, GDB will never
delete recorded instructions from the execution log. The number of
recorded instructions is limited only by the available memory.
show record full insn-number-max
Show the limit of instructions to be recorded with the full
recording method.
set record full stop-at-limit
Control the behavior of the full recording method when the
number of recorded instructions reaches the limit. If ON (the
default), GDB will stop when the limit is reached for the
first time and ask you whether you want to stop the inferior or
continue running it and recording the execution log. If you decide
to continue recording, each new recorded instruction will cause the
oldest one to be deleted.
If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking.
show record full stop-at-limit
Show the current setting of stop-at-limit.
set record full memory-query
Control the behavior when GDB is unable to record memory
changes caused by an instruction for the full recording method.
If ON, GDB will query whether to stop the inferior in that
case.
If this option is OFF (the default), GDB will automatically ignore the effect of such instructions on memory. Later, when GDB replays this execution log, it will mark the log of this instruction as not accessible, and it will not affect the replay results.
show record full memory-query
Show the current setting of memory-query.
The btrace record target does not trace data. As a
convenience, when replaying, GDB reads read-only memory off
the live program directly, assuming that the addresses of the
read-only areas don’t change. This for example makes it possible to
disassemble code while replaying, but not to print variables.
In some cases, being able to inspect variables might be useful.
You can use the following command for that:
set record btrace replay-memory-access
Control the behavior of the btrace recording method when
accessing memory during replay. If read-only (the default),
GDB will only allow accesses to read-only memory.
If read-write, GDB will allow accesses to read-only
and to read-write memory. Beware that the accessed memory corresponds
to the live target and not necessarily to the current replay
position.
show record btrace replay-memory-access
Show the current setting of replay-memory-access.
set record btrace bts buffer-size size
set record btrace bts buffer-size unlimited
Set the requested ring buffer size for branch tracing in BTS format. Default is 64KB.
If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the BTS format.
The actually obtained buffer size may differ from the requested
size. Use the info record command to see the actual
buffer size for each thread that uses the btrace recording method and
the BTS format.
If limit is unlimited or zero, GDB will try to
allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will also need longer to process the branch trace data before it can be used.
show record btrace bts buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in BTS format.
set record btrace pt buffer-size size
set record btrace pt buffer-size unlimited
Set the requested ring buffer size for branch tracing in Intel Processor Trace format. Default is 16KB.
If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the Intel Processor Trace
format. The actually obtained buffer size may differ from the
requested size. Use the info record command to see the
actual buffer size for each thread.
If limit is unlimited or zero, GDB will try to
allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will also need longer to process the branch trace data before it can be used.
show record btrace pt buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in Intel Processor Trace format.
info record
Show various statistics about the recording depending on the recording method:
full
For the full recording method, it shows the state of process
record and its in-memory execution log buffer, including:
btrace
For the btrace recording method, it shows:
For the bts recording format, it also shows:
For the pt recording format, it also shows:
record delete
When record target runs in replay mode (“in the past”), delete the subsequent execution log and begin to record a new execution log starting from the current address. This means you will abandon the previously recorded “future” and begin recording a new “future”.
record instruction-history
Disassembles instructions from the recorded execution log. By
default, ten instructions are disassembled. This can be changed using
the set record instruction-history-size command. Instructions
are printed in execution order.
It can also print mixed source+disassembly if you specify the the
/m or /s modifier, and print the raw instructions in hex
as well as in symbolic form by specifying the /r modifier.
The current position marker is printed for the instruction at the
current program counter value. This instruction can appear multiple
times in the trace and the current position marker will be printed
every time. To omit the current position marker, specify the
/p modifier.
To better align the printed instructions when the trace contains
instructions from more than one function, the function name may be
omitted by specifying the /f modifier.
Speculatively executed instructions are prefixed with ‘?’. This feature is not available for all recording formats.
There are several ways to specify what part of the execution log to disassemble:
record instruction-history insn
Disassembles ten instructions starting from instruction number insn.
record instruction-history insn, +/-n
Disassembles n instructions around instruction number
insn. If n is preceded with +, disassembles
n instructions after instruction number insn. If
n is preceded with -, disassembles n
instructions before instruction number insn.
record instruction-history
Disassembles ten more instructions after the last disassembly.
record instruction-history -
Disassembles ten more instructions before the last disassembly.
record instruction-history begin, end
Disassembles instructions beginning with instruction number begin until instruction number end. The instruction number end is included.
This command may not be available for all recording methods.
set record instruction-history-size size
set record instruction-history-size unlimited
Define how many instructions to disassemble in the record
instruction-history command. The default value is 10.
A size of unlimited means unlimited instructions.
show record instruction-history-size
Show how many instructions to disassemble in the record
instruction-history command.
record function-call-history
Prints the execution history at function granularity. It prints one
line for each sequence of instructions that belong to the same
function giving the name of that function, the source lines
for this instruction sequence (if the /l modifier is
specified), and the instructions numbers that form the sequence (if
the /i modifier is specified). The function names are indented
to reflect the call stack depth if the /c modifier is
specified. The /l, /i, and /c modifiers can be
given together.
(gdb) list 1, 10
1 void foo (void)
2 {
3 }
4
5 void bar (void)
6 {
7 ...
8 foo ();
9 ...
10 }
(gdb) record function-call-history /ilc
1 bar inst 1,4 at foo.c:6,8
2 foo inst 5,10 at foo.c:2,3
3 bar inst 11,13 at foo.c:9,10
By default, ten lines are printed. This can be changed using the
set record function-call-history-size command. Functions are
printed in execution order. There are several ways to specify what
to print:
record function-call-history func
Prints ten functions starting from function number func.
record function-call-history func, +/-n
Prints n functions around function number func. If
n is preceded with +, prints n functions after
function number func. If n is preceded with -,
prints n functions before function number func.
record function-call-history
Prints ten more functions after the last ten-line print.
record function-call-history -
Prints ten more functions before the last ten-line print.
record function-call-history begin, end
Prints functions beginning with function number begin until function number end. The function number end is included.
This command may not be available for all recording methods.
set record function-call-history-size size
set record function-call-history-size unlimited
Define how many lines to print in the
record function-call-history command. The default value is 10.
A size of unlimited means unlimited lines.
show record function-call-history-size
Show how many lines to print in the
record function-call-history command.
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