Re: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()

[Andy Shevchenko]

On Tue, Jun 15, 2021 at 6:56 PM Jia He <justin.he@xxxxxxx> wrote:
>
> This helper is similar to d_path except that it doesn't take any
> seqlock/spinlock. It is typical for debugging purpose. Besides,

purposes

> an additional return value *prenpend_len* is used to get the full
> path length of the dentry.
>
> prepend_name_with_len() enhances the behavior of prepend_name().
> Previously it will skip the loop at once in __prepen_path() when the
> space is not enough. __prepend_path() gets the full length of dentry
> together with the parent recusively.

recursively

>
> Besides, if someone invokes snprintf with small but positive space,
> prepend_name_with() needs to move and copy the string partially.
>
> More than that, kasnprintf will pass NULL _buf_ and _end_, hence

kasprintf()

> it returns at the very beginning with false in this case;
>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
>  fs/d_path.c            | 83 +++++++++++++++++++++++++++++++++++++++++-
>  include/linux/dcache.h |  1 +
>  2 files changed, 82 insertions(+), 2 deletions(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index 23a53f7b5c71..4fc224eadf58 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -68,9 +68,66 @@ static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
>         return true;
>  }
>
> +static bool prepend_name_with_len(struct prepend_buffer *p, const struct qstr *name,
> +                        int orig_buflen)
> +{
> +       const char *dname = smp_load_acquire(&name->name); /* ^^^ */

What does this funny comment mean?

> +       int dlen = READ_ONCE(name->len);
> +       char *s;
> +       int last_len = p->len;
> +
> +       p->len -= dlen + 1;
> +
> +       if (unlikely(!p->buf))
> +               return false;
> +
> +       if (orig_buflen <= 0)
> +               return false;
> +       /*
> +        * The first time we overflow the buffer. Then fill the string
> +        * partially from the beginning
> +        */
> +       if (unlikely(p->len < 0)) {
> +               int buflen = strlen(p->buf);
> +
> +               s = p->buf;
> +
> +               /* Still have small space to fill partially */
> +               if (last_len > 0) {
> +                       p->buf -= last_len;
> +                       buflen += last_len;
> +               }
> +
> +               if (buflen > dlen + 1) {
> +                       /* This dentry name can be fully filled */
> +                       memmove(p->buf + dlen + 1, s, buflen - dlen - 1);
> +                       p->buf[0] = '/';
> +                       memcpy(p->buf + 1, dname, dlen);
> +               } else if (buflen > 0) {
> +                       /* Partially filled, and drop last dentry name */
> +                       p->buf[0] = '/';
> +                       memcpy(p->buf + 1, dname, buflen - 1);
> +               }
> +
> +               return false;
> +       }
> +
> +       s = p->buf -= dlen + 1;
> +       *s++ = '/';

> +       while (dlen--) {
> +               char c = *dname++;
> +
> +               if (!c)
> +                       break;
> +               *s++ = c;

I'm wondering why can't memcpy() be used here as well.

> +       }
> +       return true;
> +}
>  static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
>                           const struct path *root, struct prepend_buffer *p)
>  {
> +       int orig_buflen = p->len;
> +
>         while (dentry != root->dentry || &mnt->mnt != root->mnt) {
>                 const struct dentry *parent = READ_ONCE(dentry->d_parent);
>
> @@ -97,8 +154,7 @@ static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
>                         return 3;
>
>                 prefetch(parent);
> -               if (!prepend_name(p, &dentry->d_name))
> -                       break;
> +               prepend_name_with_len(p, &dentry->d_name, orig_buflen);
>                 dentry = parent;
>         }
>         return 0;
> @@ -263,6 +319,29 @@ char *d_path(const struct path *path, char *buf, int buflen)
>  }
>  EXPORT_SYMBOL(d_path);
>
> +/**
> + * d_path_unsafe - fast return the full path of a dentry without taking
> + * any seqlock/spinlock. This helper is typical for debugging purpose.

purposes

Haven't you got kernel doc validation warnings? Please, describe
parameters as well.

> + */
> +char *d_path_unsafe(const struct path *path, char *buf, int buflen,
> +                   int *prepend_len)
> +{
> +       struct path root;
> +       struct mount *mnt = real_mount(path->mnt);
> +       DECLARE_BUFFER(b, buf, buflen);
> +
> +       rcu_read_lock();
> +       get_fs_root_rcu(current->fs, &root);
> +
> +       prepend(&b, "", 1);
> +       __prepend_path(path->dentry, mnt, &root, &b);
> +       rcu_read_unlock();
> +
> +       *prepend_len = b.len;
> +
> +       return b.buf;
> +}
> +
>  /*
>   * Helper function for dentry_operations.d_dname() members
>   */
> diff --git a/include/linux/dcache.h b/include/linux/dcache.h
> index 9e23d33bb6f1..ec118b684055 100644
> --- a/include/linux/dcache.h
> +++ b/include/linux/dcache.h
> @@ -301,6 +301,7 @@ char *dynamic_dname(struct dentry *, char *, int, const char *, ...);
>  extern char *__d_path(const struct path *, const struct path *, char *, int);
>  extern char *d_absolute_path(const struct path *, char *, int);
>  extern char *d_path(const struct path *, char *, int);
> +extern char *d_path_unsafe(const struct path *, char *, int, int*);
>  extern char *dentry_path_raw(const struct dentry *, char *, int);
>  extern char *dentry_path(const struct dentry *, char *, int);
>
> --
> 2.17.1
>


-- 
With Best Regards,
Andy Shevchenko