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Date: Tue, 22 Jun 2021 22:06:31 +0800
From: Jia He <justin.he@....com>
To: 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>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
Eric Biggers <ebiggers@...gle.com>,
"Ahmed S. Darwish" <a.darwish@...utronix.de>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Matthew Wilcox <willy@...radead.org>,
Christoph Hellwig <hch@...radead.org>, nd@....com,
Jia He <justin.he@....com>
Subject: [PATCH v5 1/4] fs: introduce helper d_path_unsafe()
This helper is similar to d_path() except that it doesn't take any
seqlock/spinlock. It is typical for debugging purposes. Besides,
an additional return value *prenpend_len* is used to get the full
path length of the dentry, ingoring the tail '\0'.
the full path length = end - buf - prepend_length - 1
Previously it will skip the prepend_name() loop at once in
__prepen_path() when the buffer length is not enough or even negative.
prepend_name_with_len() will get the full length of dentry name
together with the parent recursively regardless of the buffer length.
If someone invokes snprintf() with small but positive space,
prepend_name_with_len() moves and copies the string partially.
More than that, kasprintf() will pass NULL _buf_ and _end_ as the
parameters. Hence return 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 | 104 +++++++++++++++++++++++++++++++++++++++--
include/linux/dcache.h | 1 +
2 files changed, 101 insertions(+), 4 deletions(-)
diff --git a/fs/d_path.c b/fs/d_path.c
index 23a53f7b5c71..84a738375698 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c
@@ -33,8 +33,7 @@ static void prepend(struct prepend_buffer *p, const char *str, int namelen)
/**
* prepend_name - prepend a pathname in front of current buffer pointer
- * @buffer: buffer pointer
- * @buflen: allocated length of the buffer
+ * @p: prepend buffer which contains buffer pointer and allocated length
* @name: name string and length qstr structure
*
* With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
@@ -68,9 +67,84 @@ static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
return true;
}
+/**
+ * prepend_name_with_len - prepend a pathname in front of current buffer
+ * pointer with limited orig_buflen.
+ * @p: prepend buffer which contains buffer pointer and allocated length
+ * @name: name string and length qstr structure
+ * @orig_buflen original length of the buffer.
+ *
+ * With the original length of the buffer (p.ptr is changable), the dentry
+ * name string will be filled into the prepending buffer. Given the orginal
+ * length might be less than name string, the dentry name can be moved or
+ * truncated.
+ *
+ * Load acquire is needed to make sure that we see that terminating NUL,
+ * which is similar to prepend_name().
+ */
+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);
+
+ /* memcpy src */
+ 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) {
+ /* 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) {
+ /* Can be partially filled, and drop last dentry */
+ 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 +171,7 @@ 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 +336,29 @@ char *d_path(const struct path *path, char *buf, int buflen)
}
EXPORT_SYMBOL(d_path);
+/**
+ * d_path_unsafe - return the full path of a dentry without taking
+ * any seqlock/spinlock. This helper is typical for debugging purposes.
+ */
+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
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