Subject: Re: [PATCH] object-file: use `container_of()` to convert from base types

From: Junio C Hamano <[email-masked]>
Date: Sat, 21 Feb 2026 23:07:08 -0800

Justin Tobler <[email-masked]> 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 <[email-masked]>
---
 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