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From: Jeff Layton on 9 Aug 2010 12:20
On Mon, 09 Aug 2010 16:23:22 +0100
David Howells <dhowells(a)redhat.com> wrote:

> From: Wang Lei <wang840925(a)gmail.com>
>
> If the DNS server returns an error, allow that to be cached in the DNS resolver
> key in lieu of a value. Userspace passes the desired error number as an option
> in the payload:
>
> "#dnserror=<number>"
>
> Userspace must map h_errno from the name resolution routines to an appropriate
> Linux error before passing it up. Something like the following mapping is
> recommended:
>
> [HOST_NOT_FOUND] = ENODATA,
> [TRY_AGAIN] = EAGAIN,
> [NO_RECOVERY] = ECONNREFUSED,
> [NO_DATA] = ENODATA,
>
> in lieu of Linux errors specifically for representing name service errors. The
> filesystem must map these errors appropropriately before passing them to
> userspace. AFS is made to map ENODATA and EAGAIN to EDESTADDRREQ for the
> return to userspace; ECONNREFUSED is allowed to stand as is.
>
> The error can be seen in /proc/keys as a negative number after the description
> of the key. Compare, for example, the following key entries:
>
> 2f97238c I--Q-- 1 53s 3f010000 0 0 dns_resol afsdb:grand.centrall.org: -61
> 338bfbbe I--Q-- 1 59m 3f010000 0 0 dns_resol afsdb:grand.central.org: 37
>
>
> If the error option is supplied in the payload, the main part of the payload is
> discarded. The key should have an expiry time set by userspace.
>
> Signed-off-by: Wang Lei <wang840925(a)gmail.com>
> Signed-off-by: David Howells <dhowells(a)redhat.com>
> ---
>
> fs/afs/cell.c | 4 ++
> net/dns_resolver/dns_key.c | 92 ++++++++++++++++++++++++++++++++++++++++--
> net/dns_resolver/dns_query.c | 5 ++
> 3 files changed, 96 insertions(+), 5 deletions(-)
>
> diff --git a/fs/afs/cell.c b/fs/afs/cell.c
> index ffea35c..d076588 100644
> --- a/fs/afs/cell.c
> +++ b/fs/afs/cell.c
> @@ -73,6 +73,10 @@ static struct afs_cell *afs_cell_alloc(const char *name, char *vllist)
> if (!vllist || strlen(vllist) < 7) {
> ret = dns_query("afsdb", name, namelen, "ipv4", &dvllist, NULL);
> if (ret < 0) {
> + if (ret == -ENODATA || ret == -EAGAIN || ret == -ENOKEY)
> + /* translate these errors into something
> + * userspace might understand */
> + ret = -EDESTADDRREQ;
> _leave(" = %d", ret);
> return ERR_PTR(ret);
> }
> diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
> index 400a04d..739435a 100644
> --- a/net/dns_resolver/dns_key.c
> +++ b/net/dns_resolver/dns_key.c
> @@ -29,6 +29,7 @@
> #include <linux/kernel.h>
> #include <linux/keyctl.h>
> #include <linux/err.h>
> +#include <linux/seq_file.h>
> #include <keys/dns_resolver-type.h>
> #include <keys/user-type.h>
> #include "internal.h"
> @@ -43,6 +44,8 @@ MODULE_PARM_DESC(debug, "DNS Resolver debugging mask");
>
> const struct cred *dns_resolver_cache;
>
> +#define DNS_ERRORNO_OPTION "dnserror"
> +
> /*
> * Instantiate a user defined key for dns_resolver.
> *
> @@ -59,9 +62,10 @@ static int
> dns_resolver_instantiate(struct key *key, const void *_data, size_t datalen)
> {
> struct user_key_payload *upayload;
> + unsigned long derrno;
> int ret;
> size_t result_len = 0;
> - const char *data = _data, *opt;
> + const char *data = _data, *end, *opt;
>
> kenter("%%%d,%s,'%s',%zu",
> key->serial, key->description, data, datalen);
> @@ -71,13 +75,77 @@ dns_resolver_instantiate(struct key *key, const void *_data, size_t datalen)
> datalen--;
>
> /* deal with any options embedded in the data */
> + end = data + datalen;
> opt = memchr(data, '#', datalen);
> if (!opt) {
> - kdebug("no options currently supported");
> - return -EINVAL;
> + /* no options: the entire data is the result */
> + kdebug("no options");
> + result_len = datalen;
> + } else {
> + const char *next_opt;
> +
> + result_len = opt - data;
> + opt++;
> + kdebug("options: '%s'", opt);
> + do {
> + const char *eq;
> + int opt_len, opt_nlen, opt_vlen, tmp;
> +
> + next_opt = memchr(opt, '#', end - opt) ?: end;
> + opt_len = next_opt - opt;
> + if (!opt_len) {
> + printk(KERN_WARNING
> + "Empty option to dns_resolver key %d\n",
> + key->serial);
> + return -EINVAL;
> + }
> +
> + eq = memchr(opt, '=', opt_len) ?: end;
> + opt_nlen = eq - opt;
> + eq++;
> + opt_vlen = next_opt - eq; /* will be -1 if no value */
> +
> + tmp = opt_vlen >= 0 ? opt_vlen : 0;
> + kdebug("option '%*.*s' val '%*.*s'",
> + opt_nlen, opt_nlen, opt, tmp, tmp, eq);
> +
> + /* see if it's an error number representing a DNS error
> + * that's to be recorded as the result in this key */
> + if (opt_nlen == sizeof(DNS_ERRORNO_OPTION) - 1 &&
> + memcmp(opt, DNS_ERRORNO_OPTION, opt_nlen) == 0) {
> + kdebug("dns error number option");
> + if (opt_vlen <= 0)
> + goto bad_option_value;
> +
> + ret = strict_strtoul(eq, 10, &derrno);
> + if (ret < 0)
> + goto bad_option_value;
> +
> + if (derrno < 1 || derrno > 511)
> + goto bad_option_value;
> +
> + kdebug("dns error no. = %lu", derrno);
> + key->type_data.x[0] = -derrno;
> + continue;
> + }
> +
> + bad_option_value:
> + printk(KERN_WARNING
> + "Option '%*.*s' to dns_resolver key %d:"
> + " bad/missing value\n",
> + opt_nlen, opt_nlen, opt, key->serial);
> + return -EINVAL;
> + } while (opt = next_opt + 1, opt < end);
> + }
> +
> + /* don't cache the result if we're caching an error saying there's no
> + * result */
> + if (key->type_data.x[0]) {
> + kleave(" = 0 [h_error %ld]", key->type_data.x[0]);
> + return 0;
> }
>
> - result_len = datalen;
> + kdebug("store result");
> ret = key_payload_reserve(key, result_len);
> if (ret < 0)
> return -EINVAL;
> @@ -135,13 +203,27 @@ no_match:
> return ret;
> }
>
> +/*
> + * Describe a DNS key
> + */
> +static void dns_resolver_describe(const struct key *key, struct seq_file *m)
> +{
> + int err = key->type_data.x[0];
> +
> + seq_puts(m, key->description);
> + if (err)
> + seq_printf(m, ": %d", err);
> + else
> + seq_printf(m, ": %u", key->datalen);
> +}
> +
> struct key_type key_type_dns_resolver = {
> .name = "dns_resolver",
> .instantiate = dns_resolver_instantiate,
> .match = dns_resolver_match,
> .revoke = user_revoke,
> .destroy = user_destroy,
> - .describe = user_describe,
> + .describe = dns_resolver_describe,
> .read = user_read,
> };
>
> diff --git a/net/dns_resolver/dns_query.c b/net/dns_resolver/dns_query.c
> index 03d5255..c32be29 100644
> --- a/net/dns_resolver/dns_query.c
> +++ b/net/dns_resolver/dns_query.c
> @@ -136,6 +136,11 @@ int dns_query(const char *type, const char *name, size_t namelen,
> if (ret < 0)
> goto put;
>
> + /* If the DNS server gave an error, return that to the caller */
> + ret = rkey->type_data.x[0];
> + if (ret)
> + goto put;
> +
> upayload = rcu_dereference_protected(rkey->payload.data,
> lockdep_is_held(&rkey->sem));
> len = upayload->datalen;
>

Looks good to me. I guess with this, callers need to be prepared to
deal with any error from userspace from 1-511, but that seems
reasonable.

--
Jeff Layton <jlayton(a)redhat.com>
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