Re: [PATCH 09/11] gdb: add objfile -> solib backlink

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

On 4/28/26 12:28 PM, Tom Tromey wrote:
>>>>>> "Simon" == Simon Marchi <simon.marchi@efficios.com> writes:
> 
> Simon> However, I learned that there may be more than one solib linked to one
> Simon> objfile, so this backlink is actually a vector.
> 
> How / when can this happen?  It seems surprising to me, except maybe for
> separate debug info.

The only case I'm aware of is the dynamic linker itself.  It appears in
all namespaces, with the same load address.  We get one solib for each
"view" of it, but all these solibs point to the same objfile.

I think I learned that around this time:

https://gitlab.com/gnutools/binutils-gdb/-/commit/d33a66a31134bd63c4945d0d570e7296aaac3574

I will make this sentence more precise.

> 
> Simon> @@ -732,10 +733,14 @@ remove_solib (program_space *pspace,
> Simon>    notify_solib_unloaded (pspace, *solib_it, still_in_use, false);
>  
> Simon>    /* Unless the user loaded it explicitly, free SO's objfile.  */
> Simon> -  if (solib_it->objfile != nullptr
> Simon> -      && !(solib_it->objfile->flags & OBJF_USERLOADED)
> Simon> -      && !still_in_use)
> Simon> -    solib_it->objfile->unlink ();
> Simon> +  if (solib_it->objfile != nullptr)
> Simon> +    {
> Simon> +      /* Remove the objfile -> solib backlink.  */
> Simon> +      solib_it->objfile->remove_solib (*solib_it);
> Simon> +
> Simon> +      if (!(solib_it->objfile->flags & OBJF_USERLOADED) && !still_in_use)
> Simon> +	solib_it->objfile->unlink ();
> Simon> +    }
>  
> Doesn't this mean that if one solib is removed, then other solibs with
> the same objfile will have a dangling objfile pointer?

I don't think so because of the still_in_use check, which is initialized
with:

  bool still_in_use
    = solib_it->objfile != nullptr && solib_used (pspace, *solib_it);

solib_used is:

  /* Return true if KNOWN->objfile is used by any other solib object
     in PSPACE's list of shared libraries.  Return false otherwise.  */

  static bool
  solib_used (program_space *pspace, const solib &known)
  {
    for (const solib &pivot : pspace->solibs ())
      if (&pivot != &known && pivot.objfile == known.objfile)
        return true;

    return false;
  }

If another solib points to the same objfile, we won't unlink the
objfile.  "solib_used" is not the best name for this function.

Simon