Hi Andy
Thanks for the comments, I will resolve you mentioned typo/grammar issues

Some answer below

-----Original Message-----
From: Andy Shevchenko <redacted>
Sent: Wednesday, June 16, 2021 4:41 AM
To: Justin He <redacted>
Cc: Petr Mladek <pmladek@suse.com>; Steven Rostedt <rostedt@goodmis.org>;
Sergey Senozhatsky [off-list ref]; Andy Shevchenko
[off-list ref]; Rasmus Villemoes
[off-list ref]; Jonathan Corbet [off-list ref]; Alexander
Viro [off-list ref]; Linus Torvalds <torvalds@linux-
foundation.org>; Peter Zijlstra (Intel) [off-list ref]; Eric
Biggers [off-list ref]; Ahmed S. Darwish [off-list ref];
Linux Documentation List [off-list ref]; Linux Kernel Mailing
List [off-list ref]; Linux FS Devel <linux-
fsdevel@vger.kernel.org>; Matthew Wilcox [off-list ref]
Subject: Re: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()

On Tue, Jun 15, 2021 at 6:56 PM Jia He [off-list ref] wrote:

quoted

This helper is similar to d_path except that it doesn't take any
seqlock/spinlock. It is typical for debugging purpose. Besides,

purposes

quoted

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

quoted

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()

quoted

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

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jia He <redacted>
---
 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)

quoted

        return true;
 }

+static bool prepend_name_with_len(struct prepend_buffer *p, const struct

qstr *name,

quoted

+                        int orig_buflen)
+{
+       const char *dname = smp_load_acquire(&name->name); /* ^^^ */

What does this funny comment mean?

It is inherited from the prepend_name()

quoted

+       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++ = '/';

quoted

+       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

quoted

+       }
+       return true;
+}
 static int __prepend_path(const struct dentry *dentry, const struct

mount *mnt,

quoted

                          const struct path *root, struct prepend_buffer

*p)

quoted

 {
+       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,

quoted

                        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)

quoted

 }
 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)