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

Luís Henriques
2022-06-02T09:26:40Z

Xiubo Li <xiubli@redhat.com> writes:

> 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.

Hmm... can you point me at where that's done in the code? As usual, I'm
confused with that code and simply can't see that.

Also, if that happens only when mdsmap_ev == 15, then there's no problem
because that branch is only taken if it's >= 17.

>
>
>> + /* 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 ?

Ok, I'll fix that.

> 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.

Right, that can be done. So, I can simply set the m_max_xattr_size to '0'
if mdsmap_ev < 17. Then, this 'if' condition will always be evaluated to
true because required_blob_size will be > 0. Does that sound OK?

Cheers,
--
Luís

> -- 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 */
>>