Re: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()
From: Andy Shevchenko <hidden>
Date: 2021-06-15 20:41:00
Also in:
linux-fsdevel, lkml
On Tue, Jun 15, 2021 at 6:56 PM Jia He [off-list ref] 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()
quoted hunk ↗ jump to hunk
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) 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.
quoted hunk ↗ jump to hunk
+ } + 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.
quoted hunk ↗ jump to hunk
+ */ +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