Date:   Wed, 23 Jun 2021 13:28:44 +0000
From:   Justin He <Justin.He@....com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH 09/14] d_path: introduce struct prepend_buffer

Hi Al

> -----Original Message-----
> From: Al Viro <viro@....linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Justin He <Justin.He@....com>; Petr Mladek <pmladek@...e.com>; Steven
> Rostedt <rostedt@...dmis.org>; Sergey Senozhatsky
> <senozhatsky@...omium.org>; Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com>; Rasmus Villemoes
> <linux@...musvillemoes.dk>; Jonathan Corbet <corbet@....net>; Heiko
> Carstens <hca@...ux.ibm.com>; Vasily Gorbik <gor@...ux.ibm.com>; Christian
> Borntraeger <borntraeger@...ibm.com>; Eric W . Biederman
> <ebiederm@...ssion.com>; Darrick J. Wong <darrick.wong@...cle.com>; Peter
> Zijlstra (Intel) <peterz@...radead.org>; Ira Weiny <ira.weiny@...el.com>;
> Eric Biggers <ebiggers@...gle.com>; Ahmed S. Darwish
> <a.darwish@...utronix.de>; open list:DOCUMENTATION <linux-
> doc@...r.kernel.org>; Linux Kernel Mailing List <linux-
> kernel@...r.kernel.org>; linux-s390 <linux-s390@...r.kernel.org>; linux-
> fsdevel <linux-fsdevel@...r.kernel.org>
> Subject: [PATCH 09/14] d_path: introduce struct prepend_buffer
>
>         We've a lot of places where we have pairs of form (pointer to end
> of buffer, amount of space left in front of that).  These sit in pairs of
> variables located next to each other and usually passed by reference.
> Turn those into instances of new type (struct prepend_buffer) and pass
> reference to the pair instead of pairs of references to its fields.
>
> Declared and initialized by DECLARE_BUFFER(name, buf, buflen).
>
> extract_string(prepend_buffer) returns the buffer contents if
> no overflow has happened, ERR_PTR(ENAMETOOLONG) otherwise.
> All places where we used to have that boilerplate converted to use
> of that helper.
>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> ---
>  fs/d_path.c | 142 ++++++++++++++++++++++++++++++++-----------------------
> -----
>  1 file changed, 75 insertions(+), 67 deletions(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index 83db83446afd..06e93dd031bf 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -8,12 +8,26 @@
>  #include <linux/prefetch.h>
>  #include "mount.h"
>
> -static void prepend(char **buffer, int *buflen, const char *str, int
> namelen)
> +struct prepend_buffer {
> +     char *buf;
> +     int len;
> +};
> +#define DECLARE_BUFFER(__name, __buf, __len) \
> +     struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}
> +
> +static char *extract_string(struct prepend_buffer *p)
>  {
> -     *buflen -= namelen;
> -     if (likely(*buflen >= 0)) {
> -             *buffer -= namelen;
> -             memcpy(*buffer, str, namelen);
> +     if (likely(p->len >= 0))
> +             return p->buf;
> +     return ERR_PTR(-ENAMETOOLONG);
> +}
> +
> +static void prepend(struct prepend_buffer *p, const char *str, int
> namelen)
> +{
> +     p->len -= namelen;
> +     if (likely(p->len >= 0)) {
> +             p->buf -= namelen;
> +             memcpy(p->buf, str, namelen);
>       }
>  }
>
> @@ -34,22 +48,22 @@ static void prepend(char **buffer, int *buflen, const
> char *str, int namelen)
>   *
>   * Load acquire is needed to make sure that we see that terminating NUL.
>   */
> -static bool prepend_name(char **buffer, int *buflen, const struct qstr
> *name)
> +static bool prepend_name(struct prepend_buffer *p, const struct qstr
> *name)

Please also change the parameter description in the comments of
prepend_name(), otherwise "make C=1 W=1" will report warnings.


--
Cheers,
Justin (Jia He)


>  {
>       const char *dname = smp_load_acquire(&name->name); /* ^^^ */
>       u32 dlen = READ_ONCE(name->len);
> -     char *p;
> +     char *s;
>
> -     *buflen -= dlen + 1;
> -     if (unlikely(*buflen < 0))
> +     p->len -= dlen + 1;
> +     if (unlikely(p->len < 0))
>               return false;
> -     p = *buffer -= dlen + 1;
> -     *p++ = '/';
> +     s = p->buf -= dlen + 1;
> +     *s++ = '/';
>       while (dlen--) {
>               char c = *dname++;
>               if (!c)
>                       break;
> -             *p++ = c;
> +             *s++ = c;
>       }
>       return true;
>  }
> @@ -73,15 +87,14 @@ static bool prepend_name(char **buffer, int *buflen,
> const struct qstr *name)
>   */
>  static int prepend_path(const struct path *path,
>                       const struct path *root,
> -                     char **buffer, int *buflen)
> +                     struct prepend_buffer *p)
>  {
>       struct dentry *dentry;
>       struct vfsmount *vfsmnt;
>       struct mount *mnt;
>       int error = 0;
>       unsigned seq, m_seq = 0;
> -     char *bptr;
> -     int blen;
> +     struct prepend_buffer b;
>
>       rcu_read_lock();
>  restart_mnt:
> @@ -89,8 +102,7 @@ static int prepend_path(const struct path *path,
>       seq = 0;
>       rcu_read_lock();
>  restart:
> -     bptr = *buffer;
> -     blen = *buflen;
> +     b = *p;
>       error = 0;
>       dentry = path->dentry;
>       vfsmnt = path->mnt;
> @@ -105,8 +117,7 @@ static int prepend_path(const struct path *path,
>
>                       /* Escaped? */
>                       if (dentry != vfsmnt->mnt_root) {
> -                             bptr = *buffer;
> -                             blen = *buflen;
> +                             b = *p;
>                               error = 3;
>                               break;
>                       }
> @@ -127,7 +138,7 @@ static int prepend_path(const struct path *path,
>               }
>               parent = dentry->d_parent;
>               prefetch(parent);
> -             if (!prepend_name(&bptr, &blen, &dentry->d_name))
> +             if (!prepend_name(&b, &dentry->d_name))
>                       break;
>
>               dentry = parent;
> @@ -148,11 +159,10 @@ static int prepend_path(const struct path *path,
>       }
>       done_seqretry(&mount_lock, m_seq);
>
> -     if (blen == *buflen)
> -             prepend(&bptr, &blen, "/", 1);
> +     if (b.len == p->len)
> +             prepend(&b, "/", 1);
>
> -     *buffer = bptr;
> -     *buflen = blen;
> +     *p = b;
>       return error;
>  }
>
> @@ -176,24 +186,24 @@ char *__d_path(const struct path *path,
>              const struct path *root,
>              char *buf, int buflen)
>  {
> -     char *res = buf + buflen;
> +     DECLARE_BUFFER(b, buf, buflen);
>
> -     prepend(&res, &buflen, "", 1);
> -     if (prepend_path(path, root, &res, &buflen) > 0)
> +     prepend(&b, "", 1);
> +     if (prepend_path(path, root, &b) > 0)
>               return NULL;
> -     return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> +     return extract_string(&b);
>  }
>
>  char *d_absolute_path(const struct path *path,
>              char *buf, int buflen)
>  {
>       struct path root = {};
> -     char *res = buf + buflen;
> +     DECLARE_BUFFER(b, buf, buflen);
>
> -     prepend(&res, &buflen, "", 1);
> -     if (prepend_path(path, &root, &res, &buflen) > 1)
> +     prepend(&b, "", 1);
> +     if (prepend_path(path, &root, &b) > 1)
>               return ERR_PTR(-EINVAL);
> -     return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> +     return extract_string(&b);
>  }
>
>  static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
> @@ -224,7 +234,7 @@ static void get_fs_root_rcu(struct fs_struct *fs,
> struct path *root)
>   */
>  char *d_path(const struct path *path, char *buf, int buflen)
>  {
> -     char *res = buf + buflen;
> +     DECLARE_BUFFER(b, buf, buflen);
>       struct path root;
>
>       /*
> @@ -245,13 +255,13 @@ char *d_path(const struct path *path, char *buf, int
> buflen)
>       rcu_read_lock();
>       get_fs_root_rcu(current->fs, &root);
>       if (unlikely(d_unlinked(path->dentry)))
> -             prepend(&res, &buflen, " (deleted)", 11);
> +             prepend(&b, " (deleted)", 11);
>       else
> -             prepend(&res, &buflen, "", 1);
> -     prepend_path(path, &root, &res, &buflen);
> +             prepend(&b, "", 1);
> +     prepend_path(path, &root, &b);
>       rcu_read_unlock();
>
> -     return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
> +     return extract_string(&b);
>  }
>  EXPORT_SYMBOL(d_path);
>
> @@ -278,36 +288,34 @@ char *dynamic_dname(struct dentry *dentry, char
> *buffer, int buflen,
>
>  char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
>  {
> -     char *end = buffer + buflen;
> +     DECLARE_BUFFER(b, buffer, buflen);
>       /* these dentries are never renamed, so d_lock is not needed */
> -     prepend(&end, &buflen, " (deleted)", 11);
> -     prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len);
> -     prepend(&end, &buflen, "/", 1);
> -     return buflen >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
> +     prepend(&b, " (deleted)", 11);
> +     prepend(&b, dentry->d_name.name, dentry->d_name.len);
> +     prepend(&b, "/", 1);
> +     return extract_string(&b);
>  }
>
>  /*
>   * Write full pathname from the root of the filesystem into the buffer.
>   */
> -static char *__dentry_path(const struct dentry *d, char *p, int buflen)
> +static char *__dentry_path(const struct dentry *d, struct prepend_buffer
> *p)
>  {
>       const struct dentry *dentry;
> -     char *end;
> -     int len, seq = 0;
> +     struct prepend_buffer b;
> +     int seq = 0;
>
>       rcu_read_lock();
>  restart:
>       dentry = d;
> -     end = p;
> -     len = buflen;
> +     b = *p;
>       read_seqbegin_or_lock(&rename_lock, &seq);
>       while (!IS_ROOT(dentry)) {
>               const struct dentry *parent = dentry->d_parent;
>
>               prefetch(parent);
> -             if (!prepend_name(&end, &len, &dentry->d_name))
> +             if (!prepend_name(&b, &dentry->d_name))
>                       break;
> -
>               dentry = parent;
>       }
>       if (!(seq & 1))
> @@ -317,28 +325,29 @@ static char *__dentry_path(const struct dentry *d,
> char *p, int buflen)
>               goto restart;
>       }
>       done_seqretry(&rename_lock, seq);
> -     if (len == buflen)
> -             prepend(&end, &len, "/", 1);
> -     return len >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
> +     if (b.len == p->len)
> +             prepend(&b, "/", 1);
> +     return extract_string(&b);
>  }
>
>  char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
>  {
> -     char *p = buf + buflen;
> -     prepend(&p, &buflen, "", 1);
> -     return __dentry_path(dentry, p, buflen);
> +     DECLARE_BUFFER(b, buf, buflen);
> +
> +     prepend(&b, "", 1);
> +     return __dentry_path(dentry, &b);
>  }
>  EXPORT_SYMBOL(dentry_path_raw);
>
>  char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
>  {
> -     char *p = buf + buflen;
> +     DECLARE_BUFFER(b, buf, buflen);
>
>       if (unlikely(d_unlinked(dentry)))
> -             prepend(&p, &buflen, "//deleted", 10);
> +             prepend(&b, "//deleted", 10);
>       else
> -             prepend(&p, &buflen, "", 1);
> -     return __dentry_path(dentry, p, buflen);
> +             prepend(&b, "", 1);
> +     return __dentry_path(dentry, &b);
>  }
>
>  static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path
> *root,
> @@ -386,24 +395,23 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned
> long, size)
>       error = -ENOENT;
>       if (!d_unlinked(pwd.dentry)) {
>               unsigned long len;
> -             char *cwd = page + PATH_MAX;
> -             int buflen = PATH_MAX;
> +             DECLARE_BUFFER(b, page, PATH_MAX);
>
> -             prepend(&cwd, &buflen, "", 1);
> -             if (prepend_path(&pwd, &root, &cwd, &buflen) > 0)
> -                     prepend(&cwd, &buflen, "(unreachable)", 13);
> +             prepend(&b, "", 1);
> +             if (prepend_path(&pwd, &root, &b) > 0)
> +                     prepend(&b, "(unreachable)", 13);
>               rcu_read_unlock();
>
> -             if (buflen < 0) {
> +             if (b.len < 0) {
>                       error = -ENAMETOOLONG;
>                       goto out;
>               }
>
>               error = -ERANGE;
> -             len = PATH_MAX + page - cwd;
> +             len = PATH_MAX - b.len;
>               if (len <= size) {
>                       error = len;
> -                     if (copy_to_user(buf, cwd, len))
> +                     if (copy_to_user(buf, b.buf, len))
>                               error = -EFAULT;
>               }
>       } else {
> --
> 2.11.0

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