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

[David Laight <david.laight.linux@gmail.com>]

On Wed, 27 May 2026 11:06:08 -0700
Sam Edwards <cfsworks@gmail.com> wrote:

> On Wed, May 27, 2026 at 5:00 AM David Laight
> <david.laight.linux@gmail.com> wrote:
> >
> > On Tue, 26 May 2026 19:58:27 -0700
> > Sam Edwards <cfsworks@gmail.com> 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@gmail.com>
> > > ---
> > >  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.

Ugg...
That is just an accident waiting to happen.

> 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.

The output for the base64 functions depends only on the size of the
input - so is easy to get right.
And for decode is always shorter.

I've just looked at what happens when tname is NULL - that looks
broken as well - why not just kmalloc() a buffer that is the right
size instead of using a wrapper function that might return a
different length entirely.
Maybe it should be too long, but bugs happen.

There are seem to be random overwrites of buffers of pointers
to buffers - more code that is badly fragile.

-- David

> 
> 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);  
> >