Re: GDB: etm traces decoding and breakpoints for arm targets

Zied Guermazi
2020-11-02T15:52:31Z

hi

Thanks Mike for your support, it was very helpful.

to put everything together, on arm, gdb inserts a sw breakpoint by
patching the code with an undefined instruction ( see comments in
arm-tdep.c line7687) when a breakpoint is hit, an exception number 9
"Undefined Instruction exception" is raised and a branch packet with
this info is generated in etm traces, the trap is get handled by the
kernel and it sends the appropriate signal to gdb process.

when the user continues the execution, gdb patches back the code and
executes the instruction. this leads to the instruction traced twice
with an exception in between, the same happens for next executed instruction

here is the log of decoded packets

[btrace] [ftrace] update insn: fun = main, file =
./function_call_history.c, level = 0, insn = [1; 2)
cs_etm_decoder_trace_element_callback: elem->elem_type
OCSD_GEN_TRC_ELEM_INSTR_RANGE */<= first execution attempt that raises
an undefined instruction exception/*
trace_chan_id: 18
isa: CS_ETM_ISA_T32
start addr = 0x400534
end addr   = 0x400536
instructions count = 1
last_i_type: OCSD_INSTR_OTHER
last_i_subtype: OCSD_S_INSTR_NONE
last instruction was executed
last instruction size: 2
[btrace] [ftrace] update insn: fun = main, file =
./function_call_history.c, level = 0, insn = [1; 3)
cs_etm_decoder_trace_element_callback: elem->elem_type
OCSD_GEN_TRC_ELEM_EXCEPTION */<= the exception is traced/*
trace_chan_id: 18
exception number: 9 */<= undefined instruction exception/*
cs_etm_decoder_trace_element_callback: elem->elem_type
OCSD_GEN_TRC_ELEM_TRACE_ON
cs_etm_decoder_trace_element_callback: elem->elem_type
OCSD_GEN_TRC_ELEM_PE_CONTEXT
cs_etm_decoder_trace_element_callback: elem->elem_type
OCSD_GEN_TRC_ELEM_INSTR_RANGE */<= execution of the original instruction/*
trace_chan_id: 18
isa: CS_ETM_ISA_T32
start addr = 0x400534
end addr   = 0x400536
instructions count = 1
last_i_type: OCSD_INSTR_OTHER
last_i_subtype: OCSD_S_INSTR_NONE
last instruction was executed
last instruction size: 2

as the code was changed during execution, it can not be reconstructed
during traces decoding.

in addition, and for tracing applications running on Linux, we are not
interested in capturing raised exceptions, we can consider rolling back
last instruction in ftraces. As this is not obvious, we can consider
ignoring the repeated instruction as a workaround.

for tracing bare metal software, we need to keep tracing exception, so
we can have a flag for ignoring exceptions, and activate or dis-activate
it according to the context.

what do you think about it, shall I go for implementing it as described
above?

Kind Regards

Zied Guermazi

On 02.11.20 12:59, Mike Leach wrote:
> Hi Zeid,
>
> On Sat, 31 Oct 2020 at 23:11, Zied Guermazi <zied.guermazi@trande.de> wrote:
>> hi,
>>
>> while testing the implementation in gdb of branch tracing on arm
>> processors using etm, I faced the the situation where a breakpoint was
>> set, was hit and then the execution of the program was continued. While
>> decoding generated traces, I got the address of the breakpoint
>> (0x400552) executed twice, and then the following address (0x400554)
>> also executed twice. the instruction at (0x400554) is a BL ( a function
>> call) and the second execution corrupts the function history.
>>
>> here is a dump of generated trace elements
>>
>>
>> ---------------------------------
>> trace_chan_id: 18
>> isa: CS_ETM_ISA_T32
>> start addr = 0x400552
>> end addr = 0x400554
>> instructions count = 1
>> last_i_type: OCSD_INSTR_OTHER
>> last_i_subtype: OCSD_S_INSTR_NONE
>> last instruction was executed
>> last instruction size: 2
>> ---------------------------------
>> trace_chan_id: 18
>> isa: CS_ETM_ISA_T32
>> start addr = 0x400552
>> end addr = 0x400554
>> instructions count = 1
>> last_i_type: OCSD_INSTR_OTHER
>> last_i_subtype: OCSD_S_INSTR_NONE
>> last instruction was executed
>> last instruction size: 2
>> ---------------------------------
>> trace_chan_id: 18
>> isa: CS_ETM_ISA_T32
>> start addr = 0x400554
>> end addr = 0x400558
>> instructions count = 1
>> last_i_type: OCSD_INSTR_BR
>> last_i_subtype: OCSD_S_INSTR_BR_LINK
>> last instruction was executed
>> last instruction size: 4
>> ---------------------------------
>> trace_chan_id: 18
>> isa: CS_ETM_ISA_T32
>> start addr = 0x400554
>> end addr = 0x400558
>> instructions count = 1
>> last_i_type: OCSD_INSTR_BR
>> last_i_subtype: OCSD_S_INSTR_BR_LINK
>> last instruction was executed
>> last instruction size: 4
>>
>> the explanation I have for this behavior is that :
>>
>> -when setting the software breakpoint, the memory content of the
>> instruction (at 0x400552) was altered to the instruction BKPT,
>>
>> -when the breakpoint was hit, the original opcode was set at (0x400552)
>> and a BKPT was set to the next instruction address (0x400554), then the
>> execution was continued
>>
>> -when the second breakpoint (0x400554) was hit, the a BKPT opcode was
>> set at (0x400552) and the original opcode was set at (0x400554) then the
>> execution was continued
>>
>> I am using the function "int target_read_code (CORE_ADDR memaddr,
>> gdb_byte *myaddr, ssize_t len)" to give program memory content to the
>> decoder. so the collected etm traces are correct, but, as memory was
>> altered in between, the decoder is "cheated".
>>
>> I need to identify the re-execution of code due to breakpoint handling,
>> and roll back its impact on etm decoding.
>>
>> is there a mean to get the actual content of program memory including
>> patched addresses?
>>
>> is there a means of getting the history of patched addresses during the
>> debugging of a program?
>>
>> what is the type and subtype of a BKPT instruction in a decoded trace
>> elements?
>>
> I can only really comment on this question. The type / subtype
> information in the output from the decoder is generated from the
> decoder walking the memory image of the executed trace - not from the
> trace packets themselves.
> The decoder classifies instructions according to how they will affect
> trace flow with the "other" category being set for the majority of
> instructions. The categories are: other, branch, indirect branch, ISB
> / DSB / DMB / WFI / WFE.
> These are important in program flow trace (PTM 1.x, ETM 4.x) as these
> determine which instruction we attach the E/N atoms to. BKPT will be
> classified as "other", if it is seen, as it has no effect on normal
> program flow. It will cause an exception which has a specific trace
> packet format.
>
> Regards
>
> Mike
>
>
>> do you have any other idea for handling this situation?
>>
>>
>> I am attaching the source code of the program as well as the
>> disassembled binary. the code was compiled as an application running on
>> linux on an ARMv7 A (STM32MP157 SoC). the breakpoint was set at line 43
>> in the source code (line 238 in the disassembled code)
>>
>>
>> Kind Regards
>>
>> Zied Guermazi
>>
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>