Re: [PATCH] [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 10/28/22 10:53, Luis Machado wrote:
> Hi,
> 
> On 10/28/22 03:42, Simon Marchi wrote:
>>
>>
>> On 10/26/22 04:41, Luis Machado via Gdb-patches wrote:
>>> Investigating PR29727, it was mentioned a particular test used to work on
>>
>> Typo, 727 <-> 272
> 
> Fixed.
> 
>>
>>> 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.
>>>
>>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29272
>>> ---
>>>   gdb/displaced-stepping.c | 15 ++++++++++++---
>>>   1 file changed, 12 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c
>>> index eac2c5dab94..19e4df33085 100644
>>> --- a/gdb/displaced-stepping.c
>>> +++ b/gdb/displaced-stepping.c
>>> @@ -139,15 +139,20 @@ 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.  */
>>> +  regcache_write_pc (regcache, buffer->addr);
>>
>> If the implementation under regcache_write_pc needs to look at the
>> displaced step buffers (like ARM does), it indeed seems like a good idea
>> for current_thread and copy_insn_closure to be set, so the buffer
>> information is complete at that point.  However, my worry would be that
>> if regcache_write_pc throws, for some reason, current_thread stays set
>> and the buffer will forever stay marked as busy.  Now, when writing the
>> PC fails, things don't look so good for the debug session, but still it
>> would be nice to leave the displaced stepping buffer information in good
>> shape to not make things worse.  If all the displaced step buffers (and
>> there's just one on ARM) are busy, threads waiting for a displaced step
>> will be stuck forever.
>>
>> So, perhaps use a try / catch to clear those fields if an exception is
>> thrown (or maybe you can find a cleaner solution).
> 
> Would a scoped restore be cleaner here?
> 

Nevermind, that doesn't help us here.

>>
>> Simon
>