Re: [PATCH] seq_file: seq_show_option_n() is used for precise sizes

[Andy Shevchenko]

On Wed, Jul 26, 2023 at 02:59:57PM -0700, Kees Cook wrote:
> When seq_show_option_n() is used, it is for non-string memory that
> happens to be printable bytes. As such, we must use memcpy() to copy the
> bytes and then explicitly NUL-terminate the result.

FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Muchun Song <muchun.song@xxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/seq_file.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
> index bd023dd38ae6..386ab580b839 100644
> --- a/include/linux/seq_file.h
> +++ b/include/linux/seq_file.h
> @@ -249,18 +249,19 @@ static inline void seq_show_option(struct seq_file *m, const char *name,
>  
>  /**
>   * seq_show_option_n - display mount options with appropriate escapes
> - *		       where @value must be a specific length.
> + *		       where @value must be a specific length (i.e.
> + *		       not NUL-terminated).
>   * @m: the seq_file handle
>   * @name: the mount option name
>   * @value: the mount option name's value, cannot be NULL
> - * @length: the length of @value to display
> + * @length: the exact length of @value to display, must be constant expression
>   *
>   * This is a macro since this uses "length" to define the size of the
>   * stack buffer.
>   */
>  #define seq_show_option_n(m, name, value, length) {	\
>  	char val_buf[length + 1];			\
> -	strncpy(val_buf, value, length);		\
> +	memcpy(val_buf, value, length);			\
>  	val_buf[length] = '\0';				\
>  	seq_show_option(m, name, val_buf);		\
>  }
> -- 
> 2.34.1
> 

-- 
With Best Regards,
Andy Shevchenko