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
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3498,6 +3498,7 @@
size_t cnt, loff_t *fpos)
{
char *buf;
+ size_t written;
if (tracing_disabled)
return -EINVAL;
@@ -3519,11 +3520,15 @@
} 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)
