Subject: Re: [PATCH] tag: avoid NULL pointer arithmetic

From: Junio C Hamano <[email-masked]>
Date: Mon, 02 Oct 2017 13:13:00 +0900

René Scharfe <[email-masked]> writes:

> lookup_blob() etc. can return NULL if the referenced object isn't of the
> expected type.  In theory it's wrong to reference the object member in
> that case.  In practice it's OK because it's located at offset 0 for all
> types, so the pointer arithmetic (NULL + 0) is optimized out by the
> compiler.  The issue is reported by Clang's AddressSanitizer, though.
>
> Avoid the ASan error by casting the results of the lookup functions to
> struct object pointers.  That works fine with NULL pointers as well.  We
> already rely on the object member being first in all object types in
> other places in the code.
>
> Signed-off-by: Rene Scharfe <[email-masked]>
> ---

Yikes, that is tricky.

Taking the address of .object field appears to be a lot cleaner and
more kosher than casting, but from the compiler's point of view it's
quite the opposite.  Sigh.

Thanks.  The patch itself looks like the right solution to me.

>  tag.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/tag.c b/tag.c
> index 7e10acfb6e..fcbe012f7a 100644
> --- a/tag.c
> +++ b/tag.c
> @@ -142,13 +142,13 @@ int parse_tag_buffer(struct tag *item, const void *data, unsigned long size)
>  	bufptr = nl + 1;
>  
>  	if (!strcmp(type, blob_type)) {
> -		item->tagged = &lookup_blob(&oid)->object;
> +		item->tagged = (struct object *)lookup_blob(&oid);
>  	} else if (!strcmp(type, tree_type)) {
> -		item->tagged = &lookup_tree(&oid)->object;
> +		item->tagged = (struct object *)lookup_tree(&oid);
>  	} else if (!strcmp(type, commit_type)) {
> -		item->tagged = &lookup_commit(&oid)->object;
> +		item->tagged = (struct object *)lookup_commit(&oid);
>  	} else if (!strcmp(type, tag_type)) {
> -		item->tagged = &lookup_tag(&oid)->object;
> +		item->tagged = (struct object *)lookup_tag(&oid);
>  	} else {
>  		error("Unknown type %s", type);
>  		item->tagged = NULL;