Re: [PATCH] [AArch64] Recognize more program breakpoint patterns

[Alan Hayward <Alan dot Hayward at arm dot com>]

> On 9 Jan 2020, at 15:45, Luis Machado <luis.machado@linaro.org> wrote:
> 
> On 1/9/20 11:52 AM, Alan Hayward wrote:
>>> On 23 Dec 2019, at 17:34, Luis Machado <luis.machado@linaro.org> wrote:
>>> 
>>> It was reported to me that program breakpoints (permanent ones inserted into
>>> the code itself) other than the one GDB uses for AArch64 (0xd4200000) do not
>>> generate visible stops when continuing, and GDB will continue spinning
>>> infinitely.
>>> 
>>> This happens because GDB, upon hitting one of those program breakpoints, thinks
>>> the SIGTRAP came from a delayed breakpoint hit...
>>> 
>>> (gdb) x/i $pc
>>> => 0x4005c0 <problem_function>: brk     #0x90f
>>> (gdb) c
>>> Continuing.
>>> infrun: clear_proceed_status_thread (process 14198)
>>> infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
>>> infrun: proceed: resuming process 14198
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14198] at 0x4005c0
>>> infrun: infrun_async(1)
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14198.14198.0 [process 14198],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: delayed software breakpoint trap, ignoring
>>> infrun: no stepping, continue
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14198] at 0x4005c0
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14198.14198.0 [process 14198],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: delayed software breakpoint trap, ignoring
>>> infrun: no stepping, continue
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14198] at 0x4005c0
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14198.14198.0 [process 14198],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: delayed software breakpoint trap, ignoring
>>> infrun: no stepping, continue
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14198] at 0x4005c0
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14198.14198.0 [process 14198],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: delayed software breakpoint trap, ignoring
>>> infrun: no stepping, continue
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14198] at 0x4005c0
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14198.14198.0 [process 14198],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> ...
>>> 
>>> ... which is not the case.
>>> 
>>> If the program breakpoint is one GDB recognizes, then it will stop when it
>>> hits it.
>>> 
>>> (gdb) x/i $pc
>>> => 0x4005c0 <problem_function>: brk     #0x0
>>> (gdb) c
>>> Continuing.
>>> infrun: clear_proceed_status_thread (process 14193)
>>> infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
>>> infrun: proceed: resuming process 14193
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 14193] at 0x4005c0
>>> infrun: infrun_async(1)
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   14193.14193.0 [process 14193],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: random signal (GDB_SIGNAL_TRAP)
>>> infrun: stop_waiting
>>> infrun: stop_all_threads
>>> infrun: stop_all_threads, pass=0, iterations=0
>>> infrun:   process 14193 not executing
>>> infrun: stop_all_threads, pass=1, iterations=1
>>> infrun:   process 14193 not executing
>>> infrun: stop_all_threads done
>>> 
>>> Program received signal SIGTRAP, Trace/breakpoint trap.
>>> problem_function () at brk_0.c:7
>>> 7        asm("brk %0\n\t" ::"n"(0x0));
>>> infrun: infrun_async(0)
>>> 
>>> Otherwise GDB will keep trying to resume the inferior and will keep
>>> seeing the SIGTRAP's, without stopping.
>>> 
>>> To the user it appears GDB has gone into an infinite loop, interruptible only
>>> by Ctrl-C.
>>> 
>>> Also, windbg seems to use a different variation of AArch64 breakpoint compared
>>> to GDB. This causes problems when debugging Windows on ARM binaries, when
>>> program breakpoints are being used.
>>> 
>>> The proposed patch creates a new gdbarch method (gdbarch_insn_is_breakpoint)
>>> that tells GDB whether the underlying instruction is a breakpoint instruction
>>> or not.
>>> 
>>> This is more general than only checking for the instruction GDB uses as
>>> breakpoint.
>>> 
>>> The existing logic is still preserved for targets that do not implement this
>>> new gdbarch method.
>>> 
>>> The end result is like so:
>>> 
>>> (gdb) x/i $pc
>>> => 0x4005c0 <problem_function>: brk     #0x90f
>>> (gdb) c
>>> Continuing.
>>> infrun: clear_proceed_status_thread (process 16417)
>>> infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
>>> infrun: proceed: resuming process 16417
>>> infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 16417] at 0x4005c0
>>> infrun: infrun_async(1)
>>> infrun: prepare_to_wait
>>> infrun: target_wait (-1.0.0, status) =
>>> infrun:   16417.16417.0 [process 16417],
>>> infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: handle_inferior_event status->kind = stopped, signal = GDB_SIGNAL_TRAP
>>> infrun: stop_pc = 0x4005c0
>>> infrun: random signal (GDB_SIGNAL_TRAP)
>>> infrun: stop_waiting
>>> infrun: stop_all_threads
>>> infrun: stop_all_threads, pass=0, iterations=0
>>> infrun:   process 16417 not executing
>>> infrun: stop_all_threads, pass=1, iterations=1
>>> infrun:   process 16417 not executing
>>> infrun: stop_all_threads done
>>> 
>>> Program received signal SIGTRAP, Trace/breakpoint trap.
>>> problem_function () at brk.c:7
>>> 7        asm("brk %0\n\t" ::"n"(0x900 + 0xf));
>>> infrun: infrun_async(0)
>>> 
>>> Does this change look ok?
>>> 
>>> gdb/ChangeLog:
>>> 
>>> 2019-12-23  Luis Machado  <luis.machado@linaro.org>
>>> 
>>> 	* aarch64-tdep.c (BRK_INSN_MASK): Define to 0xd4200000.
>>> 	(aarch64_insn_is_breakpoint): New function.
>>> 	(aarch64_gdbarch_init): Set gdbarch_insn_is_breakpoint hook.
>>> 	* arch-utils.c (default_insn_is_breakpoint): New function.
>>> 	* arch-utils.h (default_insn_is_breakpoint): New prototype.
>>> 	* breakpoint.c (program_breakpoint_here): Updated to use
>>> 	  gdbarch_insn_is_breakpoint.
>>> 	Update documentation to clarify behavior.
>>> 	* gdbarch.c: Regenerate.
>>> 	* gdbarch.h: Regenerate.
>>> 	* gdbarch.sh (gdbarch_insn_is_breakpoint): New method.
>>> 
>>> Change-Id: I96eb27151442f435560a58c87eac48b0f68432bc
>>> ---
>>> gdb/aarch64-tdep.c | 25 +++++++++++++++++++++++++
>>> gdb/arch-utils.c   |  7 +++++++
>>> gdb/arch-utils.h   |  3 +++
>>> gdb/breakpoint.c   | 19 ++++++++++++++-----
>>> gdb/gdbarch.c      | 23 +++++++++++++++++++++++
>>> gdb/gdbarch.h      |  7 +++++++
>>> gdb/gdbarch.sh     |  4 ++++
>>> 7 files changed, 83 insertions(+), 5 deletions(-)
>>> 
>> Do you have a test case for this? It could go in the gdb.arch/ directory.
>> It’d be fairly easy to check all the different brk patterns.
> 
> I'll work on this. I'm thinking we could auto-generate a number of brk patterns and verify we can continue and hit each of them.
> 
>>> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
>>> index 1d5fb2001d..c69361d4ea 100644
>>> --- a/gdb/aarch64-tdep.c
>>> +++ b/gdb/aarch64-tdep.c
>>> @@ -1201,6 +1201,28 @@ aarch64_execute_dwarf_cfa_vendor_op (struct gdbarch *gdbarch, gdb_byte op,
>>>   return false;
>>> }
>>> 
>>> +#define BRK_INSN_MASK 0xd4200000
>>> +
>>> +/* Implementation of gdbarch_insn_is_breakpoint for aarch64.  */
>>> +
>>> +static bool
>>> +aarch64_insn_is_breakpoint (gdbarch *gdbarch,
>>> +			    const gdb_byte *insn,
>>> +			    unsigned int insn_size)
>>> +{
>>> +  gdb_assert (insn != nullptr);
>>> +
>>> +  uint32_t i;
>>> +
>>> +  i = (uint32_t) extract_unsigned_integer (insn, insn_size,
>>> +					   gdbarch_byte_order (gdbarch));
>>> +
>>> +  /* Check if INSN is a BRK instruction pattern.  There are multiple choices
>>> +     of such instructions with different immediate values.  Different OS' may
>>> +     use a different variation, but they have the same outcome.  */
>>> +  return (i & BRK_INSN_MASK) == BRK_INSN_MASK;
>>> +}
>>> +
>>> /* When arguments must be pushed onto the stack, they go on in reverse
>>>    order.  The code below implements a FILO (stack) to do this.  */
>>> 
>>> @@ -3357,6 +3379,9 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
>>>   set_gdbarch_execute_dwarf_cfa_vendor_op (gdbarch,
>>> 					   aarch64_execute_dwarf_cfa_vendor_op);
>>> 
>>> +  /* Permanent/Program breakpoint handling.  */
>>> +  set_gdbarch_insn_is_breakpoint (gdbarch, aarch64_insn_is_breakpoint);
>>> +
>>>   /* Add some default predicates.  */
>>>   frame_unwind_append_unwinder (gdbarch, &aarch64_stub_unwind);
>>>   dwarf2_append_unwinders (gdbarch);
>>> diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
>>> index a1a003f91f..99c9f281be 100644
>>> --- a/gdb/arch-utils.c
>>> +++ b/gdb/arch-utils.c
>>> @@ -876,6 +876,13 @@ int default_insn_is_jump (struct gdbarch *gdbarch, CORE_ADDR addr)
>>>   return 0;
>>> }
>> Add "/* See arch-utils.h.  */" ...
> 
> Fixed.
> 
>>> 
>>> +bool default_insn_is_breakpoint (struct gdbarch *gdbarch,
>>> +				 const gdb_byte  *insn,
>>> +				 unsigned int insn_size)
>>> +{
>>> +  return false;
>> I don’t like that this is just returning false, as it’s not really doing what the function name says.
>> How about if the function did this:
>>   return (memcmp (target_mem, bpoint, len) == 0);
>> Then remove the memcmp from program_breakpoint_here_p.
> 
> I agree it would be cleaner, but ...
> 
>> You’ll probably have to move the call to gdbarch_breakpoint_from_pc into here too.
> 
> ... this depends on calling gdbarch_breakpoint_from_pc to fetch bpoint, and gdbarch_breakpoint_from_pc requires the address information so it can determine the breakpoint kind.
> 
> Passing in the address is a bit out of scope for what the function is supposed to do (verify if a particular instruction is a breakpoint).
> 
> I don't have a strong objection towards passing in the address (or NULL if no address) if others are OK with it.

How about replacing program_breakpoint_here_p with 
gdbarch_program_breakpoint_here_p(struct gdbarch *gdbarch, CORE_ADDR address) ?

default_program_breakpoint_here_p would be an exact copy of the existing program_breakpoint_here_p.

aarch64_program_breakpoint_here_p would do the copy to memory then the same as aarch64_insn_is_breakpoint.


> 
>>> +}
>>> +
>>> void
>>> default_skip_permanent_breakpoint (struct regcache *regcache)
>>> {
>>> diff --git a/gdb/arch-utils.h b/gdb/arch-utils.h
>>> index 48ff3bb9a1..77ffe8190c 100644
>>> --- a/gdb/arch-utils.h
>>> +++ b/gdb/arch-utils.h
>>> @@ -227,6 +227,9 @@ extern int default_return_in_first_hidden_param_p (struct gdbarch *,
>>> extern int default_insn_is_call (struct gdbarch *, CORE_ADDR);
>>> extern int default_insn_is_ret (struct gdbarch *, CORE_ADDR);
>>> extern int default_insn_is_jump (struct gdbarch *, CORE_ADDR);
>> ... plus a brief comment here.
>> >> +extern bool default_insn_is_breakpoint (struct gdbarch *gdbarch,
>>> +					const gdb_byte *insn,
>>> +					unsigned int insn_size);
>>> 
>>> /* Do-nothing version of vsyscall_range.  Returns false.  */
>>> 
> 
> Fixed as well.