tracing: Sanitize value returned from write(trace_marker, "...", len)

When userspace code writes non-new-line-terminated string to trace_marker
file, write handler appends new-line and returns number of bytes written
to trace buffer, so
write(fd, "abc", 3) will return 4

That's unexpected and unfortunately it confuses glibc's fprintf function.

Example:
int main() {
  fprintf(stderr, "abc");
  return 0;
}

$ gcc test.c -o test
$ echo mmiotrace > /sys/kernel/debug/tracing/current_tracer
$ ./test 2>/sys/kernel/debug/tracing/trace_marker

results in infinite loop:
write(fd, "abc", 3) = 4
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
(...)

...and kernel trace buffer full of empty markers.

Fix it by sanitizing write return value.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
LKML-Reference: <20100727231801.GB2826@joi.lan>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 086d363..88b42d1 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3498,6 +3498,7 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
                                        size_t cnt, loff_t *fpos)
 {
        char *buf;
+       size_t written;
 
        if (tracing_disabled)
                return -EINVAL;
@@ -3519,11 +3520,15 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
        } else
                buf[cnt] = '\0';
 
-       cnt = mark_printk("%s", buf);
+       written = mark_printk("%s", buf);
        kfree(buf);
-       *fpos += cnt;
+       *fpos += written;
 
-       return cnt;
+       /* don't tell userspace we wrote more - it might confuse them */
+       if (written > cnt)
+               written = cnt;
+
+       return written;
 }
 
 static int tracing_clock_show(struct seq_file *m, void *v)