Re: [PATCH v4] ceph: bound encrypted snapshot suffix formatting

[Ilya Dryomov]

On Fri, Apr 10, 2026 at 10:46 PM Viacheslav Dubeyko
<Slava.Dubeyko@xxxxxxx> wrote:
>
> On Fri, 2026-04-10 at 20:40 +0000, Viacheslav Dubeyko wrote:
> > On Thu, 2026-04-09 at 18:09 +0000, Viacheslav Dubeyko wrote:
> > > On Thu, 2026-04-09 at 10:39 +0800, Pengpeng Hou wrote:
> > > > ceph_encode_encrypted_dname() base64-encodes the encrypted snapshot
> > > > name into the caller buffer and then, for long snapshot names, appends
> > > > _<ino> with sprintf(p + elen, ...).
> > > >
> > > > Some callers only provide NAME_MAX bytes. For long snapshot names, a
> > > > large inode suffix can push the final encoded name past NAME_MAX even
> > > > though the encrypted prefix stayed within the documented 240-byte
> > > > budget.
> > > >
> > > > Format the suffix into a small local buffer first and reject names
> > > > whose suffix would exceed the caller's NAME_MAX output buffer.
> > > >
> > > > Signed-off-by: Pengpeng Hou <pengpeng@xxxxxxxxxxx>
> > > > ---
> > > > Changes since v3:
> > > > - reject `elen > 240` explicitly instead of relying only on the earlier
> > > >   `WARN_ON()`
> > > > - rewrite the NAME_MAX bound check in terms of the final total length
> > > >   instead of `NAME_MAX - prefix_len - elen`
> > > >
> > > >  fs/ceph/crypto.c | 31 +++++++++++++++++++++++++++++--
> > > >  1 file changed, 29 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/ceph/crypto.c b/fs/ceph/crypto.c
> > > > index f3de43ccb470..42e3fff34697 100644
> > > > --- a/fs/ceph/crypto.c
> > > > +++ b/fs/ceph/crypto.c
> > > > @@ -15,6 +15,12 @@
> > > >  #include "mds_client.h"
> > > >  #include "crypto.h"
> > > >
> > > > +/*
> > > > + * Reserve room for '_' + decimal 64-bit inode number + trailing NUL.
> > > > + * ceph_encode_encrypted_dname() copies only the visible suffix bytes.
> > > > + */
> > > > +#define CEPH_ENCRYPTED_SNAP_INO_SUFFIX_MAX       sizeof("_18446744073709551615")
> > > > +
> > > >  static int ceph_crypt_get_context(struct inode *inode, void *ctx, size_t len)
> > > >  {
> > > >   struct ceph_inode_info *ci = ceph_inode(inode);
> > > > @@ -209,6 +215,7 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
> > > >   struct inode *dir = parent;
> > > >   char *p = buf;
> > > >   u32 len;
> > > > + int prefix_len = 0;
> > > >   int name_len = elen;
> > > >   int ret;
> > > >   u8 *cryptbuf = NULL;
> > > > @@ -219,6 +226,7 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
> > > >           if (IS_ERR(dir))
> > > >                   return PTR_ERR(dir);
> > > >           p++; /* skip initial '_' */
> > > > +         prefix_len = 1;
> > > >   }
> > > >
> > > >   if (!fscrypt_has_encryption_key(dir))
> > > > @@ -271,8 +279,27 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
> > > >
> > > >   /* To understand the 240 limit, see CEPH_NOHASH_NAME_MAX comments */
> > > >   WARN_ON(elen > 240);
> > > > - if (dir != parent) // leading _ is already there; append _<inum>
> > > > -         elen += 1 + sprintf(p + elen, "_%ld", dir->i_ino);
> > > > + if (elen > 240) {
> > > > +         elen = -ENAMETOOLONG;
> > > > +         goto out;
> > > > + }
> > > > +
> > > > + if (dir != parent) {
> > > > +         int total_len;
> > > > +         /* leading '_' is already there; append _<inum> */
> > > > +         char suffix[CEPH_ENCRYPTED_SNAP_INO_SUFFIX_MAX];
> > > > +
> > > > +         ret = snprintf(suffix, sizeof(suffix), "_%lu", dir->i_ino);
> > > > +         total_len = prefix_len + elen + ret;
> > > > +         if (total_len > NAME_MAX) {
> > > > +                 elen = -ENAMETOOLONG;
> > > > +                 goto out;
> > > > +         }
> > > > +
> > > > +         memcpy(p + elen, suffix, ret);
> > > > +         /* Include the leading '_' skipped by p. */
> > > > +         elen = total_len;
> > > > + }
> > > >
> > > >  out:
> > > >   kfree(cryptbuf);
> > >
> > > Looks good.
> > >
> > > Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@xxxxxxx>
> > >
> > > Let me run xfstests for the patch to double check that everything is OK. I'll
> > > share the result ASAP.
> > >
> >
> > The xfstests run was successful. I don't see any issues with the patch.
> >
> > Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@xxxxxxx>
> >
> >
>
> Applied on testing branch of CephFS kernel client git tree.

Hi Pengpeng, Slava,

This patch raised my attention because my understanding was that the
entire CEPH_NOHASH_NAME_MAX + sha256() was put in place precisely to
handle longer names nicely and make them fit into NAME_MAX-sized buffer.
Simply rejecting longer names seemed to be in direct contradiction with
that and yet the patch on its own was clearly merited given

 * (240 bytes is the maximum size allowed for snapshot names to take into
 *  account the format: '_<SNAPSHOT-NAME>_<INODE-NUMBER>'.)

comment on CEPH_NOHASH_NAME_MAX definition.

I dug a bit deeper and started a discussion in [1].  The preliminary
conclusion is that the 240 bytes assumption was a mistake -- somehow
the minimum number of characters needed for <inum> ended up being used
instead of the maximum.  CEPH_NOHASH_NAME_MAX value is likely incorrect
and should have been smaller -- something along the lines of 174 -
SHA256_DIGEST_SIZE instead of 180 - SHA256_DIGEST_SIZE.

I kept this patch in the testing branch but not including it for 7.1
pending further investigation.

[1] https://github.com/ceph/ceph/pull/45312

Thanks,

                Ilya