Re: [PATCH,v3] [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 11/11/22 12:39, Simon Marchi wrote:
> On 11/11/22 04:32, Luis Machado wrote:
>> v2: Add try/catch block
>> v3: Let it throw after all, and make the rollback cleaner.
>>
>> PR gdb/29272
>>
>> 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.
>>
>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29272
> 
> LGTM, thanks:
> 
> Approved-By: Simon Marchi <simon.marchi@efficios.com>
> 
> Simon

Thanks. Pushed now.