[PATCH 2/2] jit: remove bp locations when unregistering jit code

[Strasuns, Mihails]

Hello,

I will focus on the design related question for now:

> -----Original Message-----
> From: Simon Marchi <simark@simark.ca>
> Sent: Sunday, May 24, 2020 5:02 PM
> To: Strasuns, Mihails <mihails.strasuns@intel.com>; gdb-
> patches@sourceware.org
> Subject: Re: [PATCH 2/2] jit: remove bp locations when unregistering jit code
> 
> I'm not very familiar with how breakpoint locations are computed and
> updated (I just know it's complicated). But I confirm that the test shows the
> problem without the fix applied and passes with the fix applied.  I'd like if
> some other maintainer more familiar with this area could review this part.
> 
> I'd be really curious to understand how GDB handles this when it comes to
> shared libraries.  Let's imagine the same case, but using shared libraries.
> When a shared library is unloaded, the breakpoint locations that were in that
> library must be forgotten somehow?  If so through which hook / mechanism?

This is something I have initially looked into myself didn't find a clear answer in GDB sources :-)

bp_location defines this field:

---
  /* This location's address is in an unloaded solib, and so this
     location should not be inserted.  It will be automatically
     enabled when that solib is loaded.  */
  bool shlib_disabled = false;
---

What I don't understand though is that once set to true/1, this field is never reset back to 0. My guess is that this is intended to work in a similar way to my fix, i.e. that a new breakpoint location gets created for the new load rather than the old one being recreated. However I am not entirely sure.

There is a fair amount of differences between semantics of jit and shared libraries, so I decided to start with an independent fix and ask in this mail list for someone who knows better.

Two important notes:

- there is a `disable_breakpoints_in_freed_objfile` hook in breakpoint.c which looks like it should have covered this case but somehow it is not enough.
- the issue in the test case only manifests if reloaded object file uses exactly the same base address and thus breakpoint location looks the same to GDB

BR,
Mihails

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