RE: [PATCH 07/13] fs/kernel_read_file: Switch buffer size arg to size_t

[David Laight <David.Laight@ACULAB.COM>]

From: Kees Cook
> Sent: 17 July 2020 18:43
> In preparation for further refactoring of kernel_read_file*(), rename
> the "max_size" argument to the more accurate "buf_size", and correct
> its type to size_t. Add kerndoc to explain the specifics of how the
> arguments will be used. Note that with buf_size now size_t, it can no
> longer be negative (and was never called with a negative value). Adjust
> callers to use it as a "maximum size" when *buf is NULL.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  fs/kernel_read_file.c            | 34 +++++++++++++++++++++++---------
>  include/linux/kernel_read_file.h |  8 ++++----
>  security/integrity/digsig.c      |  2 +-
>  security/integrity/ima/ima_fs.c  |  2 +-
>  4 files changed, 31 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/kernel_read_file.c b/fs/kernel_read_file.c
> index dc28a8def597..e21a76001fff 100644
> --- a/fs/kernel_read_file.c
> +++ b/fs/kernel_read_file.c
> @@ -5,15 +5,31 @@
>  #include <linux/security.h>
>  #include <linux/vmalloc.h>
> 
> +/**
> + * kernel_read_file() - read file contents into a kernel buffer
> + *
> + * @file	file to read from
> + * @buf		pointer to a "void *" buffer for reading into (if
> + *		*@buf is NULL, a buffer will be allocated, and
> + *		@buf_size will be ignored)
> + * @buf_size	size of buf, if already allocated. If @buf not
> + *		allocated, this is the largest size to allocate.
> + * @id		the kernel_read_file_id identifying the type of
> + *		file contents being read (for LSMs to examine)
> + *
> + * Returns number of bytes read (no single read will be bigger
> + * than INT_MAX), or negative on error.
> + *
> + */

That seems to be self-inconsistent.
If '*buf' is NULL is both says that buf_size is ignored and
is treated as a limit.
To make life easier, zero should probably be treated as no-limit.

	David

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