Commit 1aa54bca authored Jul 28, 2010 by Marcin Slusarz Committed by Steven Rostedt Aug 13, 2010

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>

parent 2a37a3df

kernel/trace/trace.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -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)

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