tracing: do not return EFAULT if read copied anything

Impact: fix trace read to conform to standards

Andrew Morton, Theodore Tso and H. Peter Anvin brought to my attention
that a userspace read should not return -EFAULT if it succeeded in
copying anything. It should only return -EFAULT if it failed to copy
at all.

This patch modifies the check of copy_from_user and updates the return
code appropriately.

I also used H. Peter Anvin's short cut rule to just test ret == count.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ab5cbcae43a1..57155dc53530 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -346,6 +346,9 @@ ssize_t trace_seq_to_user(struct trace_seq *s, char __user *ubuf, size_t cnt)
         int len;
         int ret;
 
+        if (!cnt)
+                return 0;
+
         if (s->len <= s->readpos)
                 return -EBUSY;
 
@@ -353,9 +356,11 @@ ssize_t trace_seq_to_user(struct trace_seq *s, char __user *ubuf, size_t cnt)
         if (cnt > len)
                 cnt = len;
         ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);
-        if (ret)
+        if (ret == cnt)
                 return -EFAULT;
 
+        cnt -= ret;
+
         s->readpos += len;
         return cnt;
 }
@@ -3049,6 +3054,9 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
         ssize_t ret;
         size_t size;
 
+        if (!count)
+                return 0;
+
         /* Do we have previous read data to read? */
         if (info->read < PAGE_SIZE)
                 goto read;
@@ -3073,8 +3081,10 @@ read:
                 size = count;
 
         ret = copy_to_user(ubuf, info->spare + info->read, size);
-        if (ret)
+        if (ret == size)
                 return -EFAULT;
+        size -= ret;
+
         *ppos += size;
         info->read += size;