vfs: seq_file: add helpers for data filling

Add two helpers that allow access to the seq_file's own buffer, but
hide the internal details of seq_files.

This allows easier implementation of special purpose filling
functions.  It also cleans up some existing functions which duplicated
the seq_file logic.

Make these inline functions in seq_file.h, as suggested by Al.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 77 insertions, 36 deletions

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 66efd0aa8fb3..eae7d9dbf3ff 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -429,20 +429,21 @@ EXPORT_SYMBOL(mangle_path);
  */
 int seq_path(struct seq_file *m, struct path *path, char *esc)
 {
-        if (m->count < m->size) {
-                char *s = m->buf + m->count;
-                char *p = d_path(path, s, m->size - m->count);
+        char *buf;
+        size_t size = seq_get_buf(m, &buf);
+        int res = -1;
+
+        if (size) {
+                char *p = d_path(path, buf, size);
                 if (!IS_ERR(p)) {
-                        s = mangle_path(s, p, esc);
-                        if (s) {
-                                p = m->buf + m->count;
-                                m->count = s - m->buf;
-                                return s - p;
-                        }
+                        char *end = mangle_path(buf, p, esc);
+                        if (end)
+                                res = end - buf;
                 }
         }
-        m->count = m->size;
-        return -1;
+        seq_commit(m, res);
+
+        return res;
 }
 EXPORT_SYMBOL(seq_path);
 
@@ -454,27 +455,28 @@ EXPORT_SYMBOL(seq_path);
 int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
                   char *esc)
 {
-        int err = -ENAMETOOLONG;
-        if (m->count < m->size) {
-                char *s = m->buf + m->count;
+        char *buf;
+        size_t size = seq_get_buf(m, &buf);
+        int res = -ENAMETOOLONG;
+
+        if (size) {
                 char *p;
 
                 spin_lock(&dcache_lock);
-                p = __d_path(path, root, s, m->size - m->count);
+                p = __d_path(path, root, buf, size);
                 spin_unlock(&dcache_lock);
-                err = PTR_ERR(p);
+                res = PTR_ERR(p);
                 if (!IS_ERR(p)) {
-                        s = mangle_path(s, p, esc);
-                        if (s) {
-                                p = m->buf + m->count;
-                                m->count = s - m->buf;
-                                return 0;
-                        }
-                        err = -ENAMETOOLONG;
+                        char *end = mangle_path(buf, p, esc);
+                        if (end)
+                                res = end - buf;
+                        else
+                                res = -ENAMETOOLONG;
                 }
         }
-        m->count = m->size;
-        return err;
+        seq_commit(m, res);
+
+        return res < 0 ? res : 0;
 }
 
 /*
@@ -482,20 +484,21 @@ int seq_path_root(struct seq_file *m, struct path *path, struct path *root,
  */
 int seq_dentry(struct seq_file *m, struct dentry *dentry, char *esc)
 {
-        if (m->count < m->size) {
-                char *s = m->buf + m->count;
-                char *p = dentry_path(dentry, s, m->size - m->count);
+        char *buf;
+        size_t size = seq_get_buf(m, &buf);
+        int res = -1;
+
+        if (size) {
+                char *p = dentry_path(dentry, buf, size);
                 if (!IS_ERR(p)) {
-                        s = mangle_path(s, p, esc);
-                        if (s) {
-                                p = m->buf + m->count;
-                                m->count = s - m->buf;
-                                return s - p;
-                        }
+                        char *end = mangle_path(buf, p, esc);
+                        if (end)
+                                res = end - buf;
                 }
         }
-        m->count = m->size;
-        return -1;
+        seq_commit(m, res);
+
+        return res;
 }
 
 int seq_bitmap(struct seq_file *m, const unsigned long *bits,
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 0c6a86b79596..8366d8f12e53 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -35,6 +35,44 @@ struct seq_operations {
 
 #define SEQ_SKIP 1
 
+/**
+ * seq_get_buf - get buffer to write arbitrary data to
+ * @m: the seq_file handle
+ * @bufp: the beginning of the buffer is stored here
+ *
+ * Return the number of bytes available in the buffer, or zero if
+ * there's no space.
+ */
+static inline size_t seq_get_buf(struct seq_file *m, char **bufp)
+{
+        BUG_ON(m->count > m->size);
+        if (m->count < m->size)
+                *bufp = m->buf + m->count;
+        else
+                *bufp = NULL;
+
+        return m->size - m->count;
+}
+
+/**
+ * seq_commit - commit data to the buffer
+ * @m: the seq_file handle
+ * @num: the number of bytes to commit
+ *
+ * Commit @num bytes of data written to a buffer previously acquired
+ * by seq_buf_get.  To signal an error condition, or that the data
+ * didn't fit in the available space, pass a negative @num value.
+ */
+static inline void seq_commit(struct seq_file *m, int num)
+{
+        if (num < 0) {
+                m->count = m->size;
+        } else {
+                BUG_ON(m->count + num > m->size);
+                m->count += num;
+        }
+}
+
 char *mangle_path(char *s, char *p, char *esc);
 int seq_open(struct file *, const struct seq_operations *);
 ssize_t seq_read(struct file *, char __user *, size_t, loff_t *);