linux-kernel - Re: [PATCH] fs/befs/linuxvfs.c: need signed cast for variable 'block'

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Date:	Thu, 31 Oct 2013 09:53:59 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs/befs/linuxvfs.c: need signed cast for variable 'block'

On Wed, Oct 30, 2013 at 7:52 PM, Chen Gang <gang.chen@...anux.com> wrote:
> Need signed cast for it, the original author assume the type of 'block'
> is long, so just cast to long. The related warnings (with allmodconfig):
>
>   fs/befs/linuxvfs.c:136:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
>
> Signed-off-by: Chen Gang <gang.chen@...anux.com>
> ---
>  fs/befs/linuxvfs.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/fs/befs/linuxvfs.c b/fs/befs/linuxvfs.c
> index daa15d6..27e5179 100644
> --- a/fs/befs/linuxvfs.c
> +++ b/fs/befs/linuxvfs.c
> @@ -133,7 +133,7 @@ befs_get_block(struct inode *inode, sector_t block,
>         befs_debug(sb, "---> befs_get_block() for inode %lu, block %ld",
>                    inode->i_ino, block);
>
> -       if (block < 0) {
> +       if ((long)block < 0) {
>                 befs_error(sb, "befs_get_block() was asked for a block "
>                            "number less than zero: block %ld in inode %lu",
>                            block, inode->i_ino);

If block (type sector_t) is unsigned, we shouldn't cast it signed.
This entire code path should be removed. What is BEFS's expected
maximum block size? (Looks like even befs_blocknr_t is u64, so nothing
seems trivially in danger of wrapping.) I would also note that all the
format strings are wrong too (%ld instead of %lu).

-Kees

-- 
Kees Cook
Chrome OS Security
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