From: Junio C Hamano Date: Sun, 22 Feb 2026 07:07:08 GMT Subject: Re: [PATCH] object-file: use `container_of()` to convert from base types Message-ID: In-Reply-To: <20260218210120.1146078-1-jltobler@gmail.com> Justin Tobler writes: > static void prepare_loose_object_transaction(struct odb_transaction *base) > { > - struct odb_transaction_files *transaction = (struct odb_transaction_files *)base; > + struct odb_transaction_files *transaction = > + container_of(base, struct odb_transaction_files, base); > > /* > * We lazily create the temporary object directory This conversion triggers undefined behaviour sanitizer. We see in the post-context: if (!transaction || transaction->objdir) return; which means the caller can feed NULL as base. Taking 0 offset is unfortunately a no-no for a NULL pointer. Unfortunately, this patch is already part of 'next' as of 7a30cb26 (Merge branch 'jt/object-file-use-container-of' into next, 2026-02-20). Perhaps a fix-up patch on top of the topic branch like this? ----- >8 ----- Subject: [PATCH] object-file.c: avoid container_of() of a NULL container Even though the "struct odb_transaction" member is at the beginning of the containing "struct odb_transaction_files", i.e., with offset 0, using container_of() to add offset 0 to a NULL pointer would be flagged as a bad behaviour under SANITIZE=undefined. Signed-off-by: Junio C Hamano --- object-file.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git c/object-file.c w/object-file.c index 1a24f08978..d69cb9b7e2 100644 --- c/object-file.c +++ w/object-file.c @@ -719,8 +719,11 @@ struct odb_transaction_files { static void prepare_loose_object_transaction(struct odb_transaction *base) { - struct odb_transaction_files *transaction = - container_of(base, struct odb_transaction_files, base); + struct odb_transaction_files *transaction = NULL; + + if (base) + transaction = + container_of(base, struct odb_transaction_files, base); /* * We lazily create the temporary object directory @@ -739,8 +742,11 @@ static void prepare_loose_object_transaction(struct odb_transaction *base) static void fsync_loose_object_transaction(struct odb_transaction *base, int fd, const char *filename) { - struct odb_transaction_files *transaction = - container_of(base, struct odb_transaction_files, base); + struct odb_transaction_files *transaction = NULL; + + if (base) + transaction = + container_of(base, struct odb_transaction_files, base); /* * If we have an active ODB transaction, we issue a call that