Re: [PATCH 1/2] ceph: pass fscrypt `tname` buffers directly

[Sam Edwards]

On Wed, May 27, 2026 at 5:00 AM David Laight
<david.laight.linux@xxxxxxxxx> wrote:
>
> On Tue, 26 May 2026 19:58:27 -0700
> Sam Edwards <cfsworks@xxxxxxxxx> wrote:
>
> > ceph_fname_to_usr() needs a temporary buffer for some operations
> > (currently only base64-decoding ciphertext) and it is convenient to
> > allow the caller to specify this buffer to avoid a heap allocation, so
> > it has a (nullable) `tname` argument. Until now, this argument was a
> > `struct fscrypt_str`; however, this is unnecessary for two reasons:
> >
> > 1. `tname->len` isn't used anywhere: ceph_fname_to_usr() assumes a
> >    buffer large enough to hold the ciphertext, and
> >    parse_reply_info_readdir() -- the only caller to use tname -- doesn't
> >    set it.
> > 2. While the `tname` parameter is documented "may be NULL,"
> >    parse_reply_info_readdir() always passes it but with `tname->name`
> >    sometimes NULL in violation of the contract, indicating that the
> >    unnecessary container creates actual confusion.
> >
> > Therefore, change the type to `unsigned char *` and pass the buffer
> > directly.
> >
> > Signed-off-by: Sam Edwards <CFSworks@xxxxxxxxx>
> > ---
> >  fs/ceph/crypto.c     | 10 +++++-----
> >  fs/ceph/crypto.h     |  4 ++--
> >  fs/ceph/mds_client.c |  6 +++---
> >  3 files changed, 10 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/ceph/crypto.c b/fs/ceph/crypto.c
> > index 64d240759277..7515cb251226 100644
> > --- a/fs/ceph/crypto.c
> > +++ b/fs/ceph/crypto.c
> > @@ -300,7 +300,7 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
> >   *
> >   * Returns 0 on success or negative error code on error.
> >   */
> > -int ceph_fname_to_usr(const struct ceph_fname *fname, struct fscrypt_str *tname,
> > +int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
>
> I can't help feeling that the buffer length should also be passed.
> Either explicitly or, if constant, implicitly by embedding the array
> in a structure.

It isn't constant; the specific requirement (unchanged in patch 2) is
that the buffer be at least large enough to hold the ciphertext. The
only caller to pass tname has a comment explaining how it meets the
size requirement, so this is currently safe.

Or is your feeling more about general robustness, ensuring that the
function prototype of ceph_fname_to_usr() makes it hard for future
patches to ignore the length requirement? If so, the issue is
ultimately that the base64_*() functions don't accept a `dstlen` that
ceph_fname_to_usr() could use to (meaningfully) enforce the size
requirement.

Cheers,
Sam

>
> -- David
>
>
> >                     struct fscrypt_str *oname, bool *is_nokey)
> >  {
> >       struct inode *dir = fname->dir;
> > @@ -357,16 +357,16 @@ int ceph_fname_to_usr(const struct ceph_fname *fname, struct fscrypt_str *tname,
> >                       ret = fscrypt_fname_alloc_buffer(NAME_MAX, &_tname);
> >                       if (ret)
> >                               goto out_inode;
> > -                     tname = &_tname;
> > +                     tname = _tname.name;
> >               }
> >
> > -             declen = base64_decode(name, name_len,
> > -                                    tname->name, false, BASE64_IMAP);
> > +             declen = base64_decode(name, name_len, tname, false,
> > +                                    BASE64_IMAP);
> >               if (declen <= 0) {
> >                       ret = -EIO;
> >                       goto out;
> >               }
> > -             iname.name = tname->name;
> > +             iname.name = tname;
> >               iname.len = declen;
> >       } else {
> >               iname.name = fname->ctext;
> > diff --git a/fs/ceph/crypto.h b/fs/ceph/crypto.h
> > index b748e2060bc9..79cb563fd887 100644
> > --- a/fs/ceph/crypto.h
> > +++ b/fs/ceph/crypto.h
> > @@ -115,7 +115,7 @@ static inline void ceph_fname_free_buffer(struct inode *parent,
> >               fscrypt_fname_free_buffer(fname);
> >  }
> >
> > -int ceph_fname_to_usr(const struct ceph_fname *fname, struct fscrypt_str *tname,
> > +int ceph_fname_to_usr(const struct ceph_fname *fname, unsigned char *tname,
> >                     struct fscrypt_str *oname, bool *is_nokey);
> >  int ceph_fscrypt_prepare_readdir(struct inode *dir);
> >
> > @@ -204,7 +204,7 @@ static inline void ceph_fname_free_buffer(struct inode *parent,
> >  }
> >
> >  static inline int ceph_fname_to_usr(const struct ceph_fname *fname,
> > -                                 struct fscrypt_str *tname,
> > +                                 unsigned char *tname,
> >                                   struct fscrypt_str *oname, bool *is_nokey)
> >  {
> >       oname->name = fname->name;
> > diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> > index ed17e0023705..aa6730b48e97 100644
> > --- a/fs/ceph/mds_client.c
> > +++ b/fs/ceph/mds_client.c
> > @@ -488,11 +488,11 @@ static int parse_reply_info_readdir(void **p, void *end,
> >               struct inode *inode = d_inode(req->r_dentry);
> >               struct ceph_inode_info *ci = ceph_inode(inode);
> >               struct ceph_mds_reply_dir_entry *rde = info->dir_entries + i;
> > -             struct fscrypt_str tname = FSTR_INIT(NULL, 0);
> >               struct fscrypt_str oname = FSTR_INIT(NULL, 0);
> >               struct ceph_fname fname;
> >               u32 altname_len, _name_len;
> >               u8 *altname, *_name;
> > +             u8 *tname = NULL;
> >
> >               /* dentry */
> >               ceph_decode_32_safe(p, end, _name_len, bad);
> > @@ -540,7 +540,7 @@ static int parse_reply_info_readdir(void **p, void *end,
> >                        * always be shorter, which is 3/4 of origin
> >                        * string.
> >                        */
> > -                     tname.name = _name;
> > +                     tname = _name;
> >
> >                       /*
> >                        * Set oname to _name too, and this will be
> > @@ -557,7 +557,7 @@ static int parse_reply_info_readdir(void **p, void *end,
> >                       oname.len = altname_len;
> >               }
> >               rde->is_nokey = false;
> > -             err = ceph_fname_to_usr(&fname, &tname, &oname, &rde->is_nokey);
> > +             err = ceph_fname_to_usr(&fname, tname, &oname, &rde->is_nokey);
> >               if (err) {
> >                       pr_err_client(cl, "unable to decode %.*s, got %d\n",
> >                                     _name_len, _name, err);
>