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>]

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?

> 
> Simon