2 files changed, 33 insertions, 4 deletions

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 5267098532bf..a1a4cfe19210 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -48,8 +48,16 @@ int seq_open(struct file *file, const struct seq_operations *op)
          */
         file->f_version = 0;
 
-        /* SEQ files support lseek, but not pread/pwrite */
-        file->f_mode &= ~(FMODE_PREAD | FMODE_PWRITE);
+        /*
+         * seq_files support lseek() and pread().  They do not implement
+         * write() at all, but we clear FMODE_PWRITE here for historical
+         * reasons.
+         *
+         * If a client of seq_files a) implements file.write() and b) wishes to
+         * support pwrite() then that client will need to implement its own
+         * file.open() which calls seq_open() and then sets FMODE_PWRITE.
+         */
+        file->f_mode &= ~FMODE_PWRITE;
         return 0;
 }
 EXPORT_SYMBOL(seq_open);
@@ -131,6 +139,22 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
         int err = 0;
 
         mutex_lock(&m->lock);
+
+        /* Don't assume *ppos is where we left it */
+        if (unlikely(*ppos != m->read_pos)) {
+                m->read_pos = *ppos;
+                while ((err = traverse(m, *ppos)) == -EAGAIN)
+                        ;
+                if (err) {
+                        /* With prejudice... */
+                        m->read_pos = 0;
+                        m->version = 0;
+                        m->index = 0;
+                        m->count = 0;
+                        goto Done;
+                }
+        }
+
         /*
          * seq_file->op->..m_start/m_stop/m_next may do special actions
          * or optimisations based on the file->f_version, so we want to
@@ -230,8 +254,10 @@ Fill:
 Done:
         if (!copied)
                 copied = err;
-        else
+        else {
                 *ppos += copied;
+                m->read_pos += copied;
+        }
         file->f_version = m->version;
         mutex_unlock(&m->lock);
         return copied;
@@ -266,16 +292,18 @@ loff_t seq_lseek(struct file *file, loff_t offset, int origin)
                         if (offset < 0)
                                 break;
                         retval = offset;
-                        if (offset != file->f_pos) {
+                        if (offset != m->read_pos) {
                                 while ((retval=traverse(m, offset)) == -EAGAIN)
                                         ;
                                 if (retval) {
                                         /* with extreme prejudice... */
                                         file->f_pos = 0;
+                                        m->read_pos = 0;
                                         m->version = 0;
                                         m->index = 0;
                                         m->count = 0;
                                 } else {
+                                        m->read_pos = offset;
                                         retval = file->f_pos = offset;
                                 }
                         }
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 40ea5058c2ec..f616f31576d7 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -19,6 +19,7 @@ struct seq_file {
         size_t from;
         size_t count;
         loff_t index;
+        loff_t read_pos;
         u64 version;
         struct mutex lock;
         const struct seq_operations *op;