Add the __counted_by compiler attribute to the flexible array member
inodes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Remove the now obsolete comment on the count field.

Refactor ext4_expand_inode_array() by assigning count before copying any
data using memcpy().

Use struct_size() instead of offsetof() and remove the local variable
count.

Increment count before adding a new inode to the inodes array.

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
---
Changes in v2:
- Adjust ext4_expand_inode_array() as suggested by Gustavo A. R. Silva
- Use struct_size() and struct_size_t() instead of offsetof()
- Link to v1: https://lore.kernel.org/linux-kernel/20240729110454.346918-3-thorsten.blum@toblux.com/

Changes in v3:
- Use struct_size() instead of struct_size_t() as suggested by Kees Cook
- Remove the local variable count as suggested by Gustavo A. R. Silva
- Increment count before adding a new inode as suggested by Gustavo
- Link to v2: https://lore.kernel.org/linux-kernel/20240730172301.231867-4-thorsten.blum@toblux.com/
---
 fs/ext4/xattr.c | 24 +++++++++++-------------
 fs/ext4/xattr.h |  4 ++--
 2 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 46ce2f21fef9..4e20ef7cc502 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -2879,33 +2879,31 @@ ext4_expand_inode_array(struct ext4_xattr_inode_array **ea_inode_array,
 	if (*ea_inode_array == NULL) {
 		/*
 		 * Start with 15 inodes, so it fits into a power-of-two size.
-		 * If *ea_inode_array is NULL, this is essentially offsetof()
 		 */
-		(*ea_inode_array) =
-			kmalloc(offsetof(struct ext4_xattr_inode_array,
-					 inodes[EIA_MASK]),
-				GFP_NOFS);
+		(*ea_inode_array) = kmalloc(
+			struct_size(*ea_inode_array, inodes, EIA_MASK),
+			GFP_NOFS);
 		if (*ea_inode_array == NULL)
 			return -ENOMEM;
 		(*ea_inode_array)->count = 0;
 	} else if (((*ea_inode_array)->count & EIA_MASK) == EIA_MASK) {
 		/* expand the array once all 15 + n * 16 slots are full */
 		struct ext4_xattr_inode_array *new_array = NULL;
-		int count = (*ea_inode_array)->count;
 
-		/* if new_array is NULL, this is essentially offsetof() */
 		new_array = kmalloc(
-				offsetof(struct ext4_xattr_inode_array,
-					 inodes[count + EIA_INCR]),
-				GFP_NOFS);
+			struct_size(*ea_inode_array, inodes,
+				    (*ea_inode_array)->count + EIA_INCR),
+			GFP_NOFS);
 		if (new_array == NULL)
 			return -ENOMEM;
-		memcpy(new_array, *ea_inode_array,
-		       offsetof(struct ext4_xattr_inode_array, inodes[count]));
+		new_array->count = (*ea_inode_array)->count;
+		memcpy(new_array, *ea_inode_array,
+		       struct_size(new_array, inodes, new_array->count));
 		kfree(*ea_inode_array);
 		*ea_inode_array = new_array;
 	}
-	(*ea_inode_array)->inodes[(*ea_inode_array)->count++] = inode;
+	(*ea_inode_array)->count++;
+	(*ea_inode_array)->inodes[(*ea_inode_array)->count - 1] = inode;
 	return 0;
 }
 
diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
index bd97c4aa8177..e14fb19dc912 100644
--- a/fs/ext4/xattr.h
+++ b/fs/ext4/xattr.h
@@ -130,8 +130,8 @@ struct ext4_xattr_ibody_find {
 };
 
 struct ext4_xattr_inode_array {
-	unsigned int count;		/* # of used items in the array */
-	struct inode *inodes[];
+	unsigned int count;
+	struct inode *inodes[] __counted_by(count);
 };
 
 extern const struct xattr_handler ext4_xattr_user_handler;
-- 
2.45.2

On 30/07/24 14:55, Thorsten Blum wrote:
> Add the __counted_by compiler attribute to the flexible array member
> inodes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> CONFIG_FORTIFY_SOURCE.
> 
> Remove the now obsolete comment on the count field.
> 
> Refactor ext4_expand_inode_array() by assigning count before copying any
> data using memcpy().
> 
> Use struct_size() instead of offsetof() and remove the local variable
> count.
> 
> Increment count before adding a new inode to the inodes array.
> 
> Compile-tested only.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
> ---
> Changes in v2:
> - Adjust ext4_expand_inode_array() as suggested by Gustavo A. R. Silva
> - Use struct_size() and struct_size_t() instead of offsetof()
> - Link to v1: https://lore.kernel.org/linux-kernel/20240729110454.346918-3-thorsten.blum@toblux.com/
> 
> Changes in v3:
> - Use struct_size() instead of struct_size_t() as suggested by Kees Cook
> - Remove the local variable count as suggested by Gustavo A. R. Silva
> - Increment count before adding a new inode as suggested by Gustavo
> - Link to v2: https://lore.kernel.org/linux-kernel/20240730172301.231867-4-thorsten.blum@toblux.com/
> ---
>   fs/ext4/xattr.c | 24 +++++++++++-------------
>   fs/ext4/xattr.h |  4 ++--
>   2 files changed, 13 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 46ce2f21fef9..4e20ef7cc502 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2879,33 +2879,31 @@ ext4_expand_inode_array(struct ext4_xattr_inode_array **ea_inode_array,
>   	if (*ea_inode_array == NULL) {
>   		/*
>   		 * Start with 15 inodes, so it fits into a power-of-two size.
> -		 * If *ea_inode_array is NULL, this is essentially offsetof()
>   		 */
> -		(*ea_inode_array) =
> -			kmalloc(offsetof(struct ext4_xattr_inode_array,
> -					 inodes[EIA_MASK]),
> -				GFP_NOFS);
> +		(*ea_inode_array) = kmalloc(
> +			struct_size(*ea_inode_array, inodes, EIA_MASK),
> +			GFP_NOFS);
>   		if (*ea_inode_array == NULL)
>   			return -ENOMEM;
>   		(*ea_inode_array)->count = 0;
>   	} else if (((*ea_inode_array)->count & EIA_MASK) == EIA_MASK) {
>   		/* expand the array once all 15 + n * 16 slots are full */
>   		struct ext4_xattr_inode_array *new_array = NULL;
> -		int count = (*ea_inode_array)->count;
>   
> -		/* if new_array is NULL, this is essentially offsetof() */
>   		new_array = kmalloc(
> -				offsetof(struct ext4_xattr_inode_array,
> -					 inodes[count + EIA_INCR]),
> -				GFP_NOFS);
> +			struct_size(*ea_inode_array, inodes,
> +				    (*ea_inode_array)->count + EIA_INCR),
> +			GFP_NOFS);
>   		if (new_array == NULL)
>   			return -ENOMEM;
> -		memcpy(new_array, *ea_inode_array,
> -		       offsetof(struct ext4_xattr_inode_array, inodes[count]));
> +		new_array->count = (*ea_inode_array)->count;

The line above is not really necessary if you copy `*ea_inode_array` at once like

memcpy(... , struct_size(new_array, inodes, (*ea_inode_array)->count));

or like

memcpy(... , struct_size(*ea_inode_array, inodes, (*ea_inode_array)->count));

whichever works.

> +		memcpy(new_array, *ea_inode_array,
> +		       struct_size(new_array, inodes, new_array->count));
>   		kfree(*ea_inode_array);
>   		*ea_inode_array = new_array;
>   	}
> -	(*ea_inode_array)->inodes[(*ea_inode_array)->count++] = inode;
> +	(*ea_inode_array)->count++;
> +	(*ea_inode_array)->inodes[(*ea_inode_array)->count - 1] = inode;

This looks good, thanks!

>   	return 0;
>   }
>   
> diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
> index bd97c4aa8177..e14fb19dc912 100644
> --- a/fs/ext4/xattr.h
> +++ b/fs/ext4/xattr.h
> @@ -130,8 +130,8 @@ struct ext4_xattr_ibody_find {
>   };
>   
>   struct ext4_xattr_inode_array {
> -	unsigned int count;		/* # of used items in the array */
> -	struct inode *inodes[];
> +	unsigned int count;
> +	struct inode *inodes[] __counted_by(count);
>   };
>   
>   extern const struct xattr_handler ext4_xattr_user_handler;

--
Gustavo