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Date: Tue, 15 Jun 2021 23:40:40 +0300
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Jia He <justin.he@....com>
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>,
Alexander Viro <viro@...iv.linux.org.uk>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Eric Biggers <ebiggers@...gle.com>,
"Ahmed S. Darwish" <a.darwish@...utronix.de>,
Linux Documentation List <linux-doc@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
Matthew Wilcox <willy@...radead.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@....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@...radead.org>
> Signed-off-by: Jia He <justin.he@....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
>
--
With Best Regards,
Andy Shevchenko
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