Re: [PATCH, v2] [PR gdb/29272] Make sure a copy_insn_closure is available when we have a match in copy_insn_closure_by_addr

[Luis Machado <luis.machado@arm.com>]

On 11/2/22 18:22, Simon Marchi wrote:
> On 11/2/22 14:06, Luis Machado wrote:
>> On 11/2/22 17:44, Simon Marchi wrote:
>>> On 11/2/22 10:33, Luis Machado via Gdb-patches wrote:
>>>> v2: Add try/catch block
>>>>
>>>> Investigating PR29272, it was mentioned a particular test used to work on
>>>> GDB 10, but it started failing with GDB 11 onwards. I tracked it down to
>>>> some displaced stepping improvements on commit
>>>> 187b041e2514827b9d86190ed2471c4c7a352874.
>>>>
>>>> In particular, one of the corner cases using copy_insn_closure_by_addr got
>>>> silently broken. It is hard to spot because it doesn't have any good tests
>>>> for it, and the situation is quite specific to the Arm target.
>>>>
>>>> Essentially, the change from the displaced stepping improvements made it so
>>>> we could still invoke copy_insn_closure_by_addr correctly to return the
>>>> pointer to a copy_insn_closure, but it always returned nullptr due to
>>>> the order of the statements in displaced_step_buffer::prepare.
>>>>
>>>> The way it is now, we first write the address of the displaced step buffer
>>>> to PC and then save the copy_insn_closure pointer.
>>>>
>>>> The problem is that writing to PC for the Arm target requires figuring
>>>> out if the new PC is thumb mode or not.
>>>>
>>>> With no copy_insn_closure data, the logic to determine the thumb mode
>>>> during displaced stepping doesn't work, and gives random results that
>>>> are difficult to track (SIGILL, SIGSEGV etc).
>>>>
>>>> Fix this by reordering the PC write in displaced_step_buffer::prepare
>>>> and, for safety, add an assertion to
>>>> displaced_step_buffer::copy_insn_closure_by_addr so GDB stops right
>>>> when it sees this invalid situation. If this gets broken again in the
>>>> future, it will be easier to spot.
>>>>
>>>> Guard the code in a try/catch block to handle the case where we can't
>>>> write the PC, so as to not leave partial state in the displaced step
>>>> machinery.
>>>>
>>>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29272
>>>> ---
>>>>    gdb/displaced-stepping.c | 26 +++++++++++++++++++++++---
>>>>    1 file changed, 23 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c
>>>> index eac2c5dab94..3b5376cf31b 100644
>>>> --- a/gdb/displaced-stepping.c
>>>> +++ b/gdb/displaced-stepping.c
>>>> @@ -139,15 +139,31 @@ displaced_step_buffers::prepare (thread_info *thread, CORE_ADDR &displaced_pc)
>>>>          return DISPLACED_STEP_PREPARE_STATUS_CANT;
>>>>        }
>>>>    -  /* Resume execution at the copy.  */
>>>> -  regcache_write_pc (regcache, buffer->addr);
>>>> -
>>>>      /* This marks the buffer as being in use.  */
>>>>      buffer->current_thread = thread;
>>>>        /* Save this, now that we know everything went fine.  */
>>>>      buffer->copy_insn_closure = std::move (copy_insn_closure);
>>>>    +  /* Adjust the PC so it points to the displaced step buffer address that will
>>>> +     be used.  This needs to be done after we save the copy_insn_closure, as
>>>> +     some architectures (Arm, for one) need that information so they can adjust
>>>> +     other data as needed.  In particular, Arm needs to know if the instruction
>>>> +     being executed in the displaced step buffer is thumb or not.  Without that
>>>> +     information, things will be very wrong in a random way.  */
>>>> +  try
>>>> +    {
>>>> +      regcache_write_pc (regcache, buffer->addr);
>>>> +    }
>>>> +  catch (const gdb_exception_error &except)
>>>> +    {
>>>> +      /* Reset the displaced step buffer state if we failed to write PC.
>>>> +     Otherwise we will prevent this buffer from being used, as it will
>>>> +     always have a thread in buffer->current_thread.  */
>>>> +      buffer->current_thread = nullptr;
>>>> +      copy_insn_closure = std::move (buffer->copy_insn_closure);
>>>
>>> The intention would be clearer by just doing:
>>>
>>>     buffer->copy_insn_closure.reset ()
>>>
>>>> +      return DISPLACED_STEP_PREPARE_STATUS_CANT;
>>>
>>> I think we should just let the exception escape,
>>> DISPLACED_STEP_PREPARE_STATUS_CANT isn't meant to convey an error.
>>
>> Wouldn't letting it escape completely abort the single-stepping operation? I was expecting a return of
>> DISPLACED_STEP_PREPARE_STATUS_CANT to have a fallback of stepping in-place. Isn't that the case?
> 
> Yeah, but I think that's what we want.  Failing to write the PC is an
> "abort mission" kind of failure, IMO.  Something is very broken.
> 
> DISPLACED_STEP_PREPARE_STATUS_CANT is not equivalent to an errorp, it's "we have
> successfully analyzed the instruction and concluded it can't be
> displaced-step".  If we wanted to return a status code, I would suggest
> to introduce a new one (e.g. DISPLACED_STEP_PREPARE_STATUS_ERROR).  But
> I think the exception is fine, this is how other kinds of failure that
> happen when resuming are reported, like when we fail to insert
> breakpoints.  We arguably are not very good at handling those
> gracefully, but that's the problem of this code here.

Yeah. That's a reasonable point. I was hoping to salvage something from this bad situation and at least
let the user complete a single-stepping.

Let me get a v3 going.

> 
> Simon