Re: unrecognized node type while displaying a Path due to dangling pointer - Mailing list pgsql-hackers

On Tue, Jul 11, 2023 at 1:19 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

On 2023-Jul-11, Jeevan Chalke wrote:

> 4. However, 2nd path was already sorted and passed as is to the add_path().
> 5. add_path() decides to reject this new path on some metrics. However, in
> the end, it pfree() this passed in path. It seems wrong as its references
> do present elsewhere. For example, in the first path's parent rels path
> list.
> 6. So, while displaying the parent's path, we end up with these warnings.

In other words, this is use-after-free, with add_path freeing the
passed-in Path pointer, but one particular case in which this Path is
still used afterwards.

> I tried to get a fix for this but no luck so far.

I proposed to add an add_path_extended() function that adds 'bool
free_input_path' argument, and pass it false in that one place in
create_ordered_paths.

Yeah, this can be a way.

However, I am thinking the other way around now. What if we first added the unmodified input path as it is to the ordered_rel first?

If we do so, then while adding the next path, add_path() may decide to remove the older one as the newer path is the best one. The remove_old logic in add_path() will free the path (unsorted one), and we end up with the same error.

And if we conditionally remove that path (remove_old logic one), then we need to pass false in every add_path() call in create_ordered_paths().

Am I missing something?

Thanks

 

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Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/

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Jeevan Chalke
Senior Staff SDE, Database Architect, and Manager
Product Development



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