[PATCH RFCv3 1/3] fs: introduce helper d_path_unsafe()

Jia He
2021-06-11T16:00:20Z

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