diff mbox series

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

Message ID 20210615154952.2744-2-justin.he@arm.com (mailing list archive)
State New
Headers show
Series make '%pD' print full path for file | expand

Commit Message

Jia He June 15, 2021, 3:49 p.m. UTC
This helper is similar to d_path except that it doesn't take any
seqlock/spinlock. It is typical for debugging purpose. Besides,
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.

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
it returns at the very beginning with false in this case;

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jia He <justin.he@arm.com>
---
 fs/d_path.c            | 83 +++++++++++++++++++++++++++++++++++++++++-
 include/linux/dcache.h |  1 +
 2 files changed, 82 insertions(+), 2 deletions(-)

Comments

Andy Shevchenko June 15, 2021, 8:40 p.m. UTC | #1
On Tue, Jun 15, 2021 at 6:56 PM Jia He <justin.he@arm.com> 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@infradead.org>
> Signed-off-by: Jia He <justin.he@arm.com>
> ---
>  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
>
Jia He June 16, 2021, 12:54 a.m. UTC | #2
Hi Andy
Thanks for the comments, I will resolve you mentioned typo/grammar issues

Some answer below

> -----Original Message-----
> From: Andy Shevchenko <andy.shevchenko@gmail.com>
> Sent: Wednesday, June 16, 2021 4:41 AM
> To: Justin He <Justin.He@arm.com>
> Cc: 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>; Alexander
> Viro <viro@zeniv.linux.org.uk>; Linus Torvalds <torvalds@linux-
> foundation.org>; Peter Zijlstra (Intel) <peterz@infradead.org>; Eric
> Biggers <ebiggers@google.com>; Ahmed S. Darwish <a.darwish@linutronix.de>;
> Linux Documentation List <linux-doc@vger.kernel.org>; Linux Kernel Mailing
> List <linux-kernel@vger.kernel.org>; Linux FS Devel <linux-
> fsdevel@vger.kernel.org>; Matthew Wilcox <willy@infradead.org>
> Subject: Re: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()
> 
> On Tue, Jun 15, 2021 at 6:56 PM Jia He <justin.he@arm.com> 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@infradead.org>
> > Signed-off-by: Jia He <justin.he@arm.com>
> > ---
> >  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?

It is inherited from the prepend_name()

> 
> > +       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.


From the comments of commit 7a5cf791:

>However, there may be mismatch between length and pointer.
> * The length cannot be trusted, we need to copy it byte-by-byte until
> * the length is reached or a null byte is found.

Seems we shouldn't use memcpy/strcpy here

> 
> > +       }
> > +       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.

I will check it and update if there is a warning, thanks for the reminder.


--
Cheers,
Justin (Jia He)
diff mbox series

Patch

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); /* ^^^ */
+	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;
+	}
+	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.
+ */
+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);