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Date: Mon, 18 Nov 2013 13:34:03 +0100
From: Jan Kara <jack@...e.cz>
To: Johannes Berg <johannes@...solutions.net>
Cc: netdev@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>,
Jan Kara <jack@...e.cz>
Subject: Re: [RFC 3/8] quota/genetlink: use proper genetlink multicast APIs
On Sat 16-11-13 18:03:44, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
>
> The quota code is abusing the genetlink API and is using
> its family ID as the multicast group ID, which is invalid
> and may belong to somebody else (and likely will.)
>
> Make the quota code use the correct API, but since this
> is already used as-is by userspace, reserve a family ID
> for this code and also reserve that group ID to not break
> userspace assumptions.
Ah, sorry for messing that up. When I was writing this, libnl didn't have
support for multicast groups and so I didn't figure out from its documentation
there are any multicast groups at all... I'd also mention that
documentation of generic netlink at
http://www.linuxfoundation.org/collaborate/workgroups/networking/generic_netlink_howto
(pointed to from Documenation/networking/generic_netlink.txt) doesn't
mention anything about multicast groups either. So chances for messing
things up are pretty high...
Anyway, I'll fix quota_nld (userspace daemon consuming quota netlink
messages) to use multicast groups properly with a fallback to previous
behavior in case that fails (to keep compatibility with older kernels) and
since I don't believe anyone else is using this interface (and even
quota_nld isn't widely used), I think we can remove this hack from generic
netlink layer in a couple of years (hurray to a paragraph long sentence
;).
Thanks for fixing this and you can add:
Acked-by: Jan Kara <jack@...e.cz>
Honza
>
> Cc: Jan Kara <jack@...e.cz>
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> ---
> fs/quota/netlink.c | 17 +++++++++++++++--
> include/uapi/linux/genetlink.h | 1 +
> net/netlink/genetlink.c | 10 ++++++++--
> 3 files changed, 24 insertions(+), 4 deletions(-)
>
> diff --git a/fs/quota/netlink.c b/fs/quota/netlink.c
> index 16e8abb..b261ce4 100644
> --- a/fs/quota/netlink.c
> +++ b/fs/quota/netlink.c
> @@ -11,13 +11,23 @@
>
> /* Netlink family structure for quota */
> static struct genl_family quota_genl_family = {
> - .id = GENL_ID_GENERATE,
> + /*
> + * Needed due to multicast group ID abuse - old code assumed
> + * the family ID was also a valid multicast group ID (which
> + * isn't true) and userspace might thus rely on it. Assign a
> + * static ID for this group to make dealing with that easier.
> + */
> + .id = GENL_ID_VFS_DQUOT,
> .hdrsize = 0,
> .name = "VFS_DQUOT",
> .version = 1,
> .maxattr = QUOTA_NL_A_MAX,
> };
>
> +static struct genl_multicast_group quota_mcgrp = {
> + .name = "VFS_DQUOT", /* not really used */
> +};
> +
> /**
> * quota_send_warning - Send warning to userspace about exceeded quota
> * @type: The quota type: USRQQUOTA, GRPQUOTA,...
> @@ -78,7 +88,7 @@ void quota_send_warning(struct kqid qid, dev_t dev,
> goto attr_err_out;
> genlmsg_end(skb, msg_head);
>
> - genlmsg_multicast(skb, 0, quota_genl_family.id, GFP_NOFS);
> + genlmsg_multicast(skb, 0, quota_mcgrp.id, GFP_NOFS);
> return;
> attr_err_out:
> printk(KERN_ERR "VFS: Not enough space to compose quota message!\n");
> @@ -92,6 +102,9 @@ static int __init quota_init(void)
> if (genl_register_family("a_genl_family) != 0)
> printk(KERN_ERR
> "VFS: Failed to create quota netlink interface.\n");
> + if (genl_register_mc_group("a_genl_family, "a_mcgrp))
> + printk(KERN_ERR
> + "VFS: Failed to register quota mcast group.\n");
> return 0;
> };
>
> diff --git a/include/uapi/linux/genetlink.h b/include/uapi/linux/genetlink.h
> index c880a41..1af72d82 100644
> --- a/include/uapi/linux/genetlink.h
> +++ b/include/uapi/linux/genetlink.h
> @@ -27,6 +27,7 @@ struct genlmsghdr {
> */
> #define GENL_ID_GENERATE 0
> #define GENL_ID_CTRL NLMSG_MIN_TYPE
> +#define GENL_ID_VFS_DQUOT (NLMSG_MIN_TYPE + 1)
>
> /**************************************************************************
> * Controller
> diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
> index 8a2ed2c..d0757c6 100644
> --- a/net/netlink/genetlink.c
> +++ b/net/netlink/genetlink.c
> @@ -69,8 +69,11 @@ static struct list_head family_ht[GENL_FAM_TAB_SIZE];
> * abuses the API and thinks it can statically use group 1.
> * That group will typically conflict with other groups that
> * any proper users use.
> + * Bit 17 is marked as already used since the VFS quota code
> + * also abused this API and relied on family == group ID, we
> + * cater to that by giving it a static family and group ID.
> */
> -static unsigned long mc_group_start = 0x3;
> +static unsigned long mc_group_start = 0x3 | BIT(GENL_ID_VFS_DQUOT);
> static unsigned long *mc_groups = &mc_group_start;
> static unsigned long mc_groups_longs = 1;
>
> @@ -130,7 +133,8 @@ static u16 genl_generate_id(void)
> int i;
>
> for (i = 0; i <= GENL_MAX_ID - GENL_MIN_ID; i++) {
> - if (!genl_family_find_byid(id_gen_idx))
> + if (id_gen_idx != GENL_ID_VFS_DQUOT &&
> + !genl_family_find_byid(id_gen_idx))
> return id_gen_idx;
> if (++id_gen_idx > GENL_MAX_ID)
> id_gen_idx = GENL_MIN_ID;
> @@ -169,6 +173,8 @@ int genl_register_mc_group(struct genl_family *family,
> id = GENL_ID_CTRL;
> else if (strcmp(family->name, "NET_DM") == 0)
> id = 1;
> + else if (strcmp(family->name, "VFS_DQUOT") == 0)
> + id = GENL_ID_VFS_DQUOT;
> else
> id = find_first_zero_bit(mc_groups,
> mc_groups_longs * BITS_PER_LONG);
> --
> 1.8.4.rc3
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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