> 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.
This sounds like the main goal of the patch is to avoid an ASan error,
but I think it's more important to avoid (and to be more explicit
about avoiding) the undefined behavior. I.e. along the lines of
s/In theory it's wrong/It's undefined behavior/ and
s/ASan error/undefined behavior/
Furthermore, fsck.c:fsck_walk_tree() does the same "immediately
reference the object member in lookup_blob()'s and lookup_tree()'s
return value" thing. I think those should receive the same treatment
as well.
> Signed-off-by: Rene Scharfe <l.s.r@xxxxxx>
> ---
> 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;
> --
> 2.14.2
>
