linux-kernel - Re: [PATCH] fs: nfsd: Fix bug in compare

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Date:	Thu, 4 Dec 2014 11:46:25 -0500
From:	Jeff Layton <jeff.layton@...marydata.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: nfsd: Fix bug in compare_blob

On Thu,  4 Dec 2014 16:20:52 +0100
Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> Bugs similar to the one in acbbe6fbb240 (kcmp: fix standard comparison
> bug) are in rich supply.
> 
> In this variant, the problem is that struct xdr_netobj::len has type
> unsigned int, so the expression o1->len - o2->len _also_ has type
> unsigned int; it has completely well-defined semantics, and the result
> is some non-negative integer, which is always representable in a long
> long. But this means that if the conditional triggers, we are
> guaranteed to return a positive value from compare_blob.
> 
> In this case it could be fixed by
> 
> -       res = o1->len - o2->len;
> +       res = (long long)o1->len - (long long)o2->len;
> 
> but I'd rather eliminate the usually broken 'return a - b;' idiom.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
> 
> Notes:
>     How this could ever have worked is beyond me - compare_blob seems to
>     be used to maintain an rbtree, and I wouldn't expect rbtrees to behave
>     well if the comparison function doesn't satisfy the basic invariant
>     sign(cmp(a, b)) == -sign(cmp(b, a)).
> 
>  fs/nfsd/nfs4state.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index e9c3afe4b5d3..d504cd6927f8 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1711,15 +1711,14 @@ static int copy_cred(struct svc_cred *target, struct svc_cred *source)
>  	return 0;
>  }
>  
> -static long long
> +static int
>  compare_blob(const struct xdr_netobj *o1, const struct xdr_netobj *o2)
>  {
> -	long long res;
> -
> -	res = o1->len - o2->len;
> -	if (res)
> -		return res;
> -	return (long long)memcmp(o1->data, o2->data, o1->len);
> +	if (o1->len < o2->len)
> +		return -1;
> +	if (o1->len > o2->len)
> +		return 1;
> +	return memcmp(o1->data, o2->data, o1->len);
>  }
>  
>  static int same_name(const char *n1, const char *n2)
> @@ -1907,7 +1906,7 @@ add_clp_to_name_tree(struct nfs4_client *new_clp, struct rb_root *root)
>  static struct nfs4_client *
>  find_clp_in_name_tree(struct xdr_netobj *name, struct rb_root *root)
>  {
> -	long long cmp;
> +	int cmp;
>  	struct rb_node *node = root->rb_node;
>  	struct nfs4_client *clp;
>  

Ouch! Well spotted. I'd have probably stuck with just casting the
lengths to long longs, but this is not a particularly hot codepath and
so I'm ok with this.

This should probably also go to stable.

Reviewed-by: Jeff Layton <jlayton@...marydata.com>
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