Re: [PATCH] [PR gdb/29272] Make sure a copy_insn_closure is available when we have a match in copy_insn_closure_by_addr

[Simon Marchi <simark@simark.ca>]

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

> 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).

Simon