From: Luis Machado <luis.machado@arm.com>
To: Simon Marchi <simark@simark.ca>,
gdb-patches@sourceware.org, simon.marchi@efficios.com
Subject: 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
Date: Wed, 2 Nov 2022 18:06:37 +0000 [thread overview]
Message-ID: <b3516f91-a3eb-0d5c-98a4-81aaf5634683@arm.com> (raw)
In-Reply-To: <376afc60-6910-56b2-bad5-b9850d2999b5@simark.ca>
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?
> Would this work, using make_scope_exit?
Ah, possibly. Let me try that. Thanks for the suggestion.
>
> /* 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. */
> auto reset_buffer = make_scope_exit
> ([buffer] ()
> {
> buffer->current_thread = nullptr;
> buffer->copy_insn_closure.reset ();
> });
>
> /* 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);
>
> reset_buffer.release ();
>
> Simon
next prev parent reply other threads:[~2022-11-02 18:06 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-26 8:41 [PATCH] " Luis Machado
2022-10-28 2:42 ` Simon Marchi
2022-10-28 9:53 ` Luis Machado
2022-10-28 10:20 ` Luis Machado
2022-11-02 14:33 ` [PATCH,v2] " Luis Machado
2022-11-02 17:44 ` [PATCH, v2] " Simon Marchi
2022-11-02 18:06 ` Luis Machado [this message]
2022-11-02 18:22 ` Simon Marchi
2022-11-02 19:15 ` Luis Machado
2022-11-11 9:32 ` [PATCH,v3] " Luis Machado
2022-11-11 12:39 ` Simon Marchi
2022-11-11 12:48 ` Luis Machado
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