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From: Jia He <justin.he@arm.com>
To: 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>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Eric Biggers <ebiggers@google.com>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>, Jia He <justin.he@arm.com>
Subject: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()
Date: Tue, 15 Jun 2021 23:49:49 +0800	[thread overview]
Message-ID: <20210615154952.2744-2-justin.he@arm.com> (raw)
In-Reply-To: <20210615154952.2744-1-justin.he@arm.com>

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(-)

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);
 
-- 
2.17.1


  reply	other threads:[~2021-06-15 15:51 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-15 15:49 [PATCH RFCv4 0/4] make '%pD' print full path for file Jia He
2021-06-15 15:49 ` Jia He [this message]
2021-06-15 20:40   ` [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe() Andy Shevchenko
2021-06-16  0:54     ` Justin He
2021-06-15 15:49 ` [PATCH RFCv4 2/4] lib/vsprintf.c: make '%pD' print full path for file Jia He
2021-06-15 20:44   ` Andy Shevchenko
2021-06-17 14:09   ` Petr Mladek
2021-06-22  2:20     ` Justin He
2021-06-15 15:49 ` [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer check in two Jia He
2021-06-17 14:17   ` Petr Mladek
2021-06-15 15:49 ` [PATCH RFCv4 4/4] lib/test_printf.c: add test cases for '%pD' Jia He
2021-06-15 20:47   ` Andy Shevchenko
2021-06-17 14:52     ` Petr Mladek
2021-06-22  2:21       ` Justin He
2021-06-15 20:42 ` [PATCH RFCv4 0/4] make '%pD' print full path for file Andy Shevchenko
2021-06-16  5:09 ` Christoph Hellwig
2021-06-16  5:16   ` Justin He

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