Re: [RFC PATCH v3] ceph: prevent a client from exceeding the MDS maximum xattr size

Xiubo Li
2022-06-02T02:33:45Z

On 6/2/22 12:29 AM, Luís Henriques wrote:
> The MDS tries to enforce a limit on the total key/values in extended
> attributes. However, this limit is enforced only if doing a synchronous
> operation (MDS_OP_SETXATTR) -- if we're buffering the xattrs, the MDS
> doesn't have a chance to enforce these limits.
>
> This patch adds support for decoding the xattrs maximum size setting that is
> distributed in the mdsmap. Then, when setting an xattr, the kernel client
> will revert to do a synchronous operation if that maximum size is exceeded.
>
> While there, fix a dout() that would trigger a printk warning:
>
> [ 98.718078] ------------[ cut here ]------------
> [ 98.719012] precision 65536 too large
> [ 98.719039] WARNING: CPU: 1 PID: 3755 at lib/vsprintf.c:2703 vsnprintf+0x5e3/0x600
> ...
>
> URL: https://tracker.ceph.com/issues/55725
> Signed-off-by: Luís Henriques <lhenriques@suse.de>
> ---
> fs/ceph/mdsmap.c | 27 +++++++++++++++++++++++----
> fs/ceph/xattr.c | 12 ++++++++----
> include/linux/ceph/mdsmap.h | 1 +
> 3 files changed, 32 insertions(+), 8 deletions(-)
>
> * Changes since v2
>
> Well, a lot has changed since v2! Now the xattr max value setting is
> obtained through the mdsmap, which needs to be decoded, and the feature
> that was used in the previous revision was dropped. The drawback is that
> the MDS isn't unable to know in advance if a client is aware of this xattr
> max value.
>
> * Changes since v1
>
> Added support for new feature bit to get the MDS max_xattr_pairs_size
> setting.
>
> Also note that this patch relies on a patch that hasn't been merged yet
> ("ceph: use correct index when encoding client supported features"),
> otherwise the new feature bit won't be correctly encoded.
>
> diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
> index 30387733765d..36b2bc18ca2a 100644
> --- a/fs/ceph/mdsmap.c
> +++ b/fs/ceph/mdsmap.c
> @@ -13,6 +13,12 @@
>
> #include "super.h"
>
> +/*
> + * Maximum size of xattrs the MDS can handle per inode by default. This
> + * includes the attribute name and 4+4 bytes for the key/value sizes.
> + */
> +#define MDS_MAX_XATTR_SIZE (1<<16) /* 64K */
> +
> #define CEPH_MDS_IS_READY(i, ignore_laggy) \
> (m->m_info[i].state > 0 && ignore_laggy ? true : !m->m_info[i].laggy)
>
> @@ -352,12 +358,10 @@ struct ceph_mdsmap *ceph_mdsmap_decode(void **p, void *end, bool msgr2)
> __decode_and_drop_type(p, end, u8, bad_ext);
> }
> if (mdsmap_ev >= 8) {
> - u32 name_len;
> /* enabled */
> ceph_decode_8_safe(p, end, m->m_enabled, bad_ext);
> - ceph_decode_32_safe(p, end, name_len, bad_ext);
> - ceph_decode_need(p, end, name_len, bad_ext);
> - *p += name_len;
> + /* fs_name */
> + ceph_decode_skip_string(p, end, bad_ext);
> }
> /* damaged */
> if (mdsmap_ev >= 9) {
> @@ -370,6 +374,21 @@ struct ceph_mdsmap *ceph_mdsmap_decode(void **p, void *end, bool msgr2)
> } else {
> m->m_damaged = false;
> }
> + if (mdsmap_ev >= 17) {
> + /* balancer */
> + ceph_decode_skip_string(p, end, bad_ext);
> + /* standby_count_wanted */
> + ceph_decode_skip_32(p, end, bad_ext);
> + /* old_max_mds */
> + ceph_decode_skip_32(p, end, bad_ext);
> + /* min_compat_client */
> + ceph_decode_skip_8(p, end, bad_ext);

This is incorrect.

If mdsmap_ev == 15 the min_compat_client will be a feature_bitset_t
instead of int8_t.

> + /* required_client_features */
> + ceph_decode_skip_set(p, end, 64, bad_ext);
> + ceph_decode_64_safe(p, end, m->m_max_xattr_size, bad_ext);
> + } else {
> + m->m_max_xattr_size = MDS_MAX_XATTR_SIZE;
> + }
> bad_ext:
> dout("mdsmap_decode m_enabled: %d, m_damaged: %d, m_num_laggy: %d\n",
> !!m->m_enabled, !!m->m_damaged, m->m_num_laggy);
> diff --git a/fs/ceph/xattr.c b/fs/ceph/xattr.c
> index 8c2dc2c762a4..67f046dac35c 100644
> --- a/fs/ceph/xattr.c
> +++ b/fs/ceph/xattr.c
> @@ -1086,7 +1086,7 @@ static int ceph_sync_setxattr(struct inode *inode, const char *name,
> flags |= CEPH_XATTR_REMOVE;
> }
>
> - dout("setxattr value=%.*s\n", (int)size, value);
> + dout("setxattr value size: %ld\n", size);
>
> /* do request */
> req = ceph_mdsc_create_request(mdsc, op, USE_AUTH_MDS);
> @@ -1184,8 +1184,14 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> spin_lock(&ci->i_ceph_lock);
> retry:
> issued = __ceph_caps_issued(ci, NULL);
> - if (ci->i_xattrs.version == 0 || !(issued & CEPH_CAP_XATTR_EXCL))
> + required_blob_size = __get_required_blob_size(ci, name_len, val_len);
> + if ((ci->i_xattrs.version == 0) || !(issued & CEPH_CAP_XATTR_EXCL) ||
> + (required_blob_size >= mdsc->mdsmap->m_max_xattr_size)) {

Shouldn't it be '>' instead ?

We'd better always force to do a sync request with old ceph. Just check
if the mdsmap_ev < 17. It's not safe to buffer it because it maybe
discarded as your ceph PR does.

-- Xiubo

> + dout("%s do sync setxattr: version: %llu size: %d max: %llu\n",
> + __func__, ci->i_xattrs.version, required_blob_size,
> + mdsc->mdsmap->m_max_xattr_size);
> goto do_sync;
> + }
>
> if (!lock_snap_rwsem && !ci->i_head_snapc) {
> lock_snap_rwsem = true;
> @@ -1201,8 +1207,6 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> ceph_cap_string(issued));
> __build_xattrs(inode);
>
> - required_blob_size = __get_required_blob_size(ci, name_len, val_len);
> -
> if (!ci->i_xattrs.prealloc_blob ||
> required_blob_size > ci->i_xattrs.prealloc_blob->alloc_len) {
> struct ceph_buffer *blob;
> diff --git a/include/linux/ceph/mdsmap.h b/include/linux/ceph/mdsmap.h
> index 523fd0452856..4c3e0648dc27 100644
> --- a/include/linux/ceph/mdsmap.h
> +++ b/include/linux/ceph/mdsmap.h
> @@ -25,6 +25,7 @@ struct ceph_mdsmap {
> u32 m_session_timeout; /* seconds */
> u32 m_session_autoclose; /* seconds */
> u64 m_max_file_size;
> + u64 m_max_xattr_size; /* maximum size for xattrs blob */
> u32 m_max_mds; /* expected up:active mds number */
> u32 m_num_active_mds; /* actual up:active mds number */
> u32 possible_max_rank; /* possible max rank index */
>