RE: [PATCH 02/14] d_path: saner calling conventions for __dentry_path()

[Justin He <Justin.He@arm.com>]

Hi Al

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Justin He <Justin.He@arm.com>; Petr Mladek <pmladek@suse.com>; Steven
> Rostedt <rostedt@goodmis.org>; Sergey Senozhatsky
> <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
> <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Heiko
> Carstens <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christian
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; open list:DOCUMENTATION <linux-
> doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
> kernel@vger.kernel.org>; linux-s390 <linux-s390@vger.kernel.org>; linux-
> fsdevel <linux-fsdevel@vger.kernel.org>
> Subject: [PATCH 02/14] d_path: saner calling conventions for __dentry_path()
>
> 1) lift NUL-termination into the callers
> 2) pass pointer to the end of buffer instead of that to beginning.
>
> (1) allows to simplify dentry_path() - we don't need to play silly
> games with restoring the leading / of "//deleted" after __dentry_path()
> would've overwritten it with NUL.
>
> We also do not need to check if (either) prepend() in there fails -
> if the buffer is not large enough, we'll end with negative buflen
> after prepend() and __dentry_path() will return the right value
> (ERR_PTR(-ENAMETOOLONG)) just fine.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/d_path.c | 33 +++++++++++++--------------------
>  1 file changed, 13 insertions(+), 20 deletions(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index 01df5dfa1f88..1a1cf05e7780 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -326,22 +326,21 @@ char *simple_dname(struct dentry *dentry, char
> *buffer, int buflen)
>  /*
>   * Write full pathname from the root of the filesystem into the buffer.
>   */
I suggest adding the comments to remind NUL terminator should be prepended
before invoking __dentry_path()


--
Cheers,
Justin (Jia He)


> -static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
> +static char *__dentry_path(const struct dentry *d, char *p, int buflen)
>  {
>       const struct dentry *dentry;
>       char *end, *retval;
>       int len, seq = 0;
>       int error = 0;
>
> -     if (buflen < 2)
> +     if (buflen < 1)
>               goto Elong;
>
>       rcu_read_lock();
>  restart:
>       dentry = d;
> -     end = buf + buflen;
> +     end = p;
>       len = buflen;
> -     prepend(&end, &len, "", 1);
>       /* Get '/' right */
>       retval = end-1;
>       *retval = '/';
> @@ -373,27 +372,21 @@ static char *__dentry_path(const struct dentry *d,
> char *buf, int buflen)
>
>  char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
>  {
> -     return __dentry_path(dentry, buf, buflen);
> +     char *p = buf + buflen;
> +     prepend(&p, &buflen, "", 1);
> +     return __dentry_path(dentry, p, buflen);
>  }
>  EXPORT_SYMBOL(dentry_path_raw);
>
>  char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
>  {
> -     char *p = NULL;
> -     char *retval;
> -
> -     if (d_unlinked(dentry)) {
> -             p = buf + buflen;
> -             if (prepend(&p, &buflen, "//deleted", 10) != 0)
> -                     goto Elong;
> -             buflen++;
> -     }
> -     retval = __dentry_path(dentry, buf, buflen);
> -     if (!IS_ERR(retval) && p)
> -             *p = '/';       /* restore '/' overriden with '\0' */
> -     return retval;
> -Elong:
> -     return ERR_PTR(-ENAMETOOLONG);
> +     char *p = buf + buflen;
> +
> +     if (unlikely(d_unlinked(dentry)))
> +             prepend(&p, &buflen, "//deleted", 10);
> +     else
> +             prepend(&p, &buflen, "", 1);
> +     return __dentry_path(dentry, p, buflen);
>  }
>
>  static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path
> *root,
> --
> 2.11.0

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