linux-kernel - Re: [PATCH] fs: make d_path-like functions all have unsigned size

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Date:   Tue, 27 Jul 2021 12:19:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jordy Zomer <jordy@...ing.systems>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] fs: make d_path-like functions all have unsigned size

On Tue, Jul 27, 2021 at 12:36:25PM +0200, Greg Kroah-Hartman wrote:
> When running static analysis tools to find where signed values could
> potentially wrap the family of d_path() functions turn out to trigger a
> lot of mess.  In evaluating the code, all of these usages seem safe, but
> pointer math is involved so if a negative number is ever somehow passed
> into these functions, memory can be traversed backwards in ways not
> intended.

> diff --git a/fs/d_path.c b/fs/d_path.c
> index 23a53f7b5c71..7876b741a47e 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -182,7 +182,7 @@ static int prepend_path(const struct path *path,
>   */
>  char *__d_path(const struct path *path,
>  	       const struct path *root,
> -	       char *buf, int buflen)
> +	       char *buf, unsigned int buflen)
>  {
>  	DECLARE_BUFFER(b, buf, buflen);

I have questions about the quality of the analysis tool you're using.

struct prepend_buffer {
        char *buf;
        int len;
};
#define DECLARE_BUFFER(__name, __buf, __len) \
        struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}

Why is it not flagging the assignment of an unsigned int buflen to
a signed int len?

> +char *__d_path(const struct path *, const struct path *, char *, unsigned int);
> +char *d_absolute_path(const struct path *, char *, unsigned int);
> +char *d_path(const struct path *, char *, unsigned int);
> +char *dentry_path_raw(const struct dentry *, char *, unsigned int);
> +char *dentry_path(const struct dentry *, char *, unsigned int);

While you're touching these declarations, please name the 'unsigned int'
parameter.  I don't care about the others; they are obvious, but an
unsigned int might be flags, a length, or a small grey walrus.