Re: [PATCH,v2] Only allow closure lookup by address if there are threads displaced-stepping

[Simon Marchi <simon.marchi@polymtl.ca>]

On 10/2/23 02:56, Luis Machado wrote:
> Updated on v2:
> 
> - Added missing license to test file.
> - Formatting fixes.
> - Made the testcase always run.
> 
> Since commit 1e5ccb9c5ff4fd8ade4a8694676f99f4abf2d679, we have an assertion in
> displaced_step_buffers::copy_insn_closure_by_addr that makes sure a closure
> is available whenever we have a match between the provided address argument and
> the buffer address.
> 
> That is fine, but the report in PR30872 shows this assertion triggering when
> it really shouldn't. After some investigation, here's what I found out.
> 
> The 32-bit Arm architecture is the only one that calls
> gdbarch_displaced_step_copy_insn_closure_by_addr directly, and that's because
> 32-bit Arm needs to figure out the thumb state of the original instruction
> that we displaced-stepped through the displaced-step buffer.
> 
> Before the assertion was put in place by commit
> 1e5ccb9c5ff4fd8ade4a8694676f99f4abf2d679, there was the possibility of
> getting nullptr back, which meant we were not doing a displaced-stepping
> operation.
> 
> Now, with the assertion in place, this is running into issues.
> 
> It looks like displaced_step_buffers::copy_insn_closure_by_addr is
> being used to return a couple different answers depending on the
> state we're in:
> 
> 1 - If we are actively displaced-stepping, then copy_insn_closure_by_addr
> is supposed to return a valid closure for us, so we can determine the
> thumb mode.
> 
> 2 - If we are not actively displaced-stepping, then copy_insn_closure_by_addr
> should return nullptr to signal that there isn't any displaced-step buffers
> in use, because we don't have a valid closure (but we should always have
> this).
> 
> Since the displaced-step buffers are always allocated, but not always used,
> that means the buffers will always contain data. In particular, the buffer
> addr field cannot be used to determine if the buffer is active or not.
> 
> For instance, we cannot set the buffer addr field to 0x0, as that can be a
> valid PC in some cases.
> 
> My understanding is that the current_thread field should be a good candidate
> to signal that a particular displaced-step buffer is active or not. If it is
> nullptr, we have no threads using that buffer to displaced-step.  Otherwise,
> it is an active buffer in use by a particular thread.
> 
> The following fix modifies the displaced_step_buffers::copy_insn_closure_by_addr
> function so we only attempt to return a closure if the buffer has an assigned
> current_thread and if the buffer address matches the address argument.
> 
> Alternatively, I think we could use a function to answer the question of
> whether we're actively displaced-stepping (so we have an active buffer) or
> not.
> 
> I've also added a testcase that exercises the problem. It should reproduce
> reliably on Arm, as that is the only architecture that faces this problem
> at the moment.
> 
> Regression-tested on Ubuntu 20.04. OK?
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30872

Approved-By: Simon Marchi <simon.marchi@efficios.com>

Thanks,

Simon