Re: [PATCH v5 1/2] readdir: Introduce dirent_size()

[David Laight]

On Tue, 24 Mar 2026 12:41:15 +0100
"Christophe Leroy (CS GROUP)" <chleroy@xxxxxxxxxx> wrote:

> In several places in readdir.c there are calculations of the total size
> of a dirent, which contains a few fixed fields plus a name field with
> variable size. To add fun every dirent is of a slightly different type:
> - struct old_linux_dirent
> - struct linux_dirent
> - struct linux_dirent64
> - struct compat_old_linux_dirent
> - struct compat_linux_dirent
> 
> Replace ugly size calculation by a macro called dirent_size() which
> calculates the size of the structure based on the pointed type and
> the name field len.
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@xxxxxxxxxx>

Reviewed-by: David Laight <david.laight.linux@xxxxxxxxx>

> ---
>  fs/readdir.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/readdir.c b/fs/readdir.c
> index 73707b6816e9..fb910dc2f52b 100644
> --- a/fs/readdir.c
> +++ b/fs/readdir.c
> @@ -22,6 +22,8 @@
>  #include <linux/compat.h>
>  #include <linux/uaccess.h>
>  
> +#define dirent_size(dirent, len) offsetof(typeof(*(dirent)), d_name[len])
> +
>  /*
>   * Some filesystems were never converted to '->iterate_shared()'
>   * and their directory iterators want the inode lock held for
> @@ -198,9 +200,7 @@ static bool fillonedir(struct dir_context *ctx, const char *name, int namlen,
>  	}
>  	buf->result++;
>  	dirent = buf->dirent;
> -	if (!user_write_access_begin(dirent,
> -			(unsigned long)(dirent->d_name + namlen + 1) -
> -				(unsigned long)dirent))
> +	if (!user_write_access_begin(dirent, dirent_size(dirent, namlen + 1)))
>  		goto efault;
>  	unsafe_put_user(d_ino, &dirent->d_ino, efault_end);
>  	unsafe_put_user(offset, &dirent->d_offset, efault_end);
> @@ -263,8 +263,7 @@ static bool filldir(struct dir_context *ctx, const char *name, int namlen,
>  	struct getdents_callback *buf =
>  		container_of(ctx, struct getdents_callback, ctx);
>  	unsigned long d_ino;
> -	int reclen = ALIGN(offsetof(struct linux_dirent, d_name) + namlen + 2,
> -		sizeof(long));
> +	int reclen = ALIGN(dirent_size(dirent, namlen + 2), sizeof(long));
>  	int prev_reclen;
>  	unsigned int flags = d_type;
>  
> @@ -352,8 +351,7 @@ static bool filldir64(struct dir_context *ctx, const char *name, int namlen,
>  	struct linux_dirent64 __user *dirent, *prev;
>  	struct getdents_callback64 *buf =
>  		container_of(ctx, struct getdents_callback64, ctx);
> -	int reclen = ALIGN(offsetof(struct linux_dirent64, d_name) + namlen + 1,
> -		sizeof(u64));
> +	int reclen = ALIGN(dirent_size(dirent, namlen + 1), sizeof(u64));
>  	int prev_reclen;
>  	unsigned int flags = d_type;
>  
> @@ -460,9 +458,7 @@ static bool compat_fillonedir(struct dir_context *ctx, const char *name,
>  	}
>  	buf->result++;
>  	dirent = buf->dirent;
> -	if (!user_write_access_begin(dirent,
> -			(unsigned long)(dirent->d_name + namlen + 1) -
> -				(unsigned long)dirent))
> +	if (!user_write_access_begin(dirent, dirent_size(dirent, namlen + 1)))
>  		goto efault;
>  	unsafe_put_user(d_ino, &dirent->d_ino, efault_end);
>  	unsafe_put_user(offset, &dirent->d_offset, efault_end);
> @@ -519,8 +515,7 @@ static bool compat_filldir(struct dir_context *ctx, const char *name, int namlen
>  	struct compat_getdents_callback *buf =
>  		container_of(ctx, struct compat_getdents_callback, ctx);
>  	compat_ulong_t d_ino;
> -	int reclen = ALIGN(offsetof(struct compat_linux_dirent, d_name) +
> -		namlen + 2, sizeof(compat_long_t));
> +	int reclen = ALIGN(dirent_size(dirent, namlen + 2), sizeof(compat_long_t));
>  	int prev_reclen;
>  	unsigned int flags = d_type;
>