d_path: make 'prepend()' fill up the buffer exactly on overflow

Instead of just marking the buffer as having overflowed, fill it up as
much as we can.  That will allow the overflow case to then return
whatever truncated result if it wants to.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/d_path.c

@@ -22,13 +22,57 @@ static char *extract_string(struct prepend_buffer *p)
 	return ERR_PTR(-ENAMETOOLONG);
 }
 
-static void prepend(struct prepend_buffer *p, const char *str, int namelen)
+static bool prepend_char(struct prepend_buffer *p, unsigned char c)
 {
-	p->len -= namelen;
-	if (likely(p->len >= 0)) {
-		p->buf -= namelen;
-		memcpy(p->buf, str, namelen);
+	if (likely(p->len > 0)) {
+		p->len--;
+		*--p->buf = c;
+		return true;
+	}
+	p->len = -1;
+	return false;
+}
+
+/*
+ * The source of the prepend data can be an optimistoc load
+ * of a dentry name and length. And because we don't hold any
+ * locks, the length and the pointer to the name may not be
+ * in sync if a concurrent rename happens, and the kernel
+ * copy might fault as a result.
+ *
+ * The end result will correct itself when we check the
+ * rename sequence count, but we need to be able to handle
+ * the fault gracefully.
+ */
+static bool prepend_copy(void *dst, const void *src, int len)
+{
+	if (unlikely(copy_from_kernel_nofault(dst, src, len))) {
+		memset(dst, 'x', len);
+		return false;
 	}
+	return true;
+}
+
+static bool prepend(struct prepend_buffer *p, const char *str, int namelen)
+{
+	// Already overflowed?
+	if (p->len < 0)
+		return false;
+
+	// Will overflow?
+	if (p->len < namelen) {
+		// Fill as much as possible from the end of the name
+		str += namelen - p->len;
+		p->buf -= p->len;
+		prepend_copy(p->buf, str, p->len);
+		p->len = -1;
+		return false;
+	}
+
+	// Fits fully
+	p->len -= namelen;
+	p->buf -= namelen;
+	return prepend_copy(p->buf, str, namelen);
 }
 
 /**
@@ -40,32 +84,21 @@ static void prepend(struct prepend_buffer *p, const char *str, int namelen)
  * With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
  * make sure that either the old or the new name pointer and length are
  * fetched. 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. It also prepends "/" at
+ * But since the length cannot be trusted, we need to copy the name very
+ * carefully when doing the prepend_copy(). It also prepends "/" at
  * the beginning of the name. The sequence number check at the caller will
  * retry it again when a d_move() does happen. So any garbage in the buffer
  * due to mismatched pointer and length will be discarded.
  *
- * Load acquire is needed to make sure that we see that terminating NUL.
+ * Load acquire is needed to make sure that we see the new name data even
+ * if we might get the length wrong.
  */
 static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
 {
 	const char *dname = smp_load_acquire(&name->name); /* ^^^ */
 	u32 dlen = READ_ONCE(name->len);
-	char *s;
 
-	p->len -= dlen + 1;
-	if (unlikely(p->len < 0))
-		return false;
-	s = p->buf -= dlen + 1;
-	*s++ = '/';
-	while (dlen--) {
-		char c = *dname++;
-		if (!c)
-			break;
-		*s++ = c;
-	}
-	return true;
+	return prepend(p, dname, dlen) && prepend_char(p, '/');
 }
 
 static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
@@ -158,7 +191,7 @@ static int prepend_path(const struct path *path,
 		b = *p;
 
 	if (b.len == p->len)
-		prepend(&b, "/", 1);
+		prepend_char(&b, '/');
 
 	*p = b;
 	return error;
@@ -186,7 +219,7 @@ char *__d_path(const struct path *path,
 {
 	DECLARE_BUFFER(b, buf, buflen);
 
-	prepend(&b, "", 1);
+	prepend_char(&b, 0);
 	if (unlikely(prepend_path(path, root, &b) > 0))
 		return NULL;
 	return extract_string(&b);
@@ -198,7 +231,7 @@ char *d_absolute_path(const struct path *path,
 	struct path root = {};
 	DECLARE_BUFFER(b, buf, buflen);
 
-	prepend(&b, "", 1);
+	prepend_char(&b, 0);
 	if (unlikely(prepend_path(path, &root, &b) > 1))
 		return ERR_PTR(-EINVAL);
 	return extract_string(&b);
@@ -255,7 +288,7 @@ char *d_path(const struct path *path, char *buf, int buflen)
 	if (unlikely(d_unlinked(path->dentry)))
 		prepend(&b, " (deleted)", 11);
 	else
-		prepend(&b, "", 1);
+		prepend_char(&b, 0);
 	prepend_path(path, &root, &b);
 	rcu_read_unlock();
 
@@ -290,7 +323,7 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 	/* these dentries are never renamed, so d_lock is not needed */
 	prepend(&b, " (deleted)", 11);
 	prepend(&b, dentry->d_name.name, dentry->d_name.len);
-	prepend(&b, "/", 1);
+	prepend_char(&b, '/');
 	return extract_string(&b);
 }
 
@@ -324,15 +357,15 @@ static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
 	}
 	done_seqretry(&rename_lock, seq);
 	if (b.len == p->len)
-		prepend(&b, "/", 1);
+		prepend_char(&b, '/');
 	return extract_string(&b);
 }
 
 char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
 {
 	DECLARE_BUFFER(b, buf, buflen);
 
-	prepend(&b, "", 1);
+	prepend_char(&b, 0);
 	return __dentry_path(dentry, &b);
 }
 EXPORT_SYMBOL(dentry_path_raw);
@@ -344,7 +377,7 @@ char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
 	if (unlikely(d_unlinked(dentry)))
 		prepend(&b, "//deleted", 10);
 	else
-		prepend(&b, "", 1);
+		prepend_char(&b, 0);
 	return __dentry_path(dentry, &b);
 }
 
@@ -397,7 +430,7 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 		unsigned len;
 		DECLARE_BUFFER(b, page, PATH_MAX);
 
-		prepend(&b, "", 1);
+		prepend_char(&b, 0);
 		if (unlikely(prepend_path(&pwd, &root, &b) > 0))
 			prepend(&b, "(unreachable)", 13);
 		rcu_read_unlock();