Subject: Re: [PATCH 5/6] pack-bitmap-write.c: avoid uninitialized 'write_as' field

From: Patrick Steinhardt <[email-masked]>
Date: Wed, 15 May 2024 11:05:10 +0200

On Tue, May 14, 2024 at 03:57:03PM -0400, Taylor Blau wrote:
> Prepare to free() memory associated with bitmapped_commit structs by
> zero'ing the 'write_as' field.
> 
> In ideal cases, it is fine to do something like:
> 
>     for (i = 0; i < writer->selected_nr; i++) {
>         struct bitmapped_commit *bc = &writer->selected[i];
>         if (bc->write_as != bc->bitmap)
>             ewah_free(bc->write_as);
>         ewah_free(bc->bitmap);
>     }
> 
> but if not all of the 'write_as' fields were populated (e.g., because
> the packing_data given does not form a reachability closure), then we
> may attempt to free uninitialized memory.
> 
> Guard against this by preemptively zero'ing this field just in case.
> 
> Signed-off-by: Taylor Blau <[email-masked]>
> ---
>  pack-bitmap-write.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
> index c0087dab12..420f17c2e0 100644
> --- a/pack-bitmap-write.c
> +++ b/pack-bitmap-write.c
> @@ -112,6 +112,7 @@ static inline void push_bitmapped_commit(struct bitmap_writer *writer,
>  
>  	writer->selected[writer->selected_nr].commit = commit;
>  	writer->selected[writer->selected_nr].bitmap = NULL;
> +	writer->selected[writer->selected_nr].write_as = NULL;
>  	writer->selected[writer->selected_nr].flags = 0;

Instead of having to ensure that all fields are initialized we could
also set the whole structure to zero via `memset()`, which might be a
bit more sustainable in the future. That alone doesn't really warrant a
reroll though.

Patrick