Re: [PATCH linux-next] tracing: use strscpy() to instead of strncpy()

Hi Steve,

Can we revisit this patch? see below.

On Tue, Jan 24, 2023 at 12:17:03PM GMT, Steven Rostedt wrote:

On Mon, 9 Jan 2023 19:39:21 +0800 (CST)
[off-list ref] wrote:

quoted

From: Xu Panda <redacted>

The implementation of strscpy() is more robust and safer.
That's now the recommended way to copy NUL-terminated strings.

But the string being copied is *not* NUL-terminated! And this change causes
a bug.

This is the 3rd patch I've seen that blindly converts strncpy() to
strscpy() and causes a bug in doing so. Not very safe if you ask me.

quoted

Signed-off-by: Xu Panda <redacted>
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
---
 kernel/trace/trace_events_synth.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index 67592eed0be8..cd636edd045e 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c

@@ -195,8 +195,7 @@ static int synth_field_string_size(char *type)
 	if (len == 0)
 		return 0; /* variable-length string */

-	strncpy(buf, start, len);
-	buf[len] = '\0';
+	strscpy(buf, start, len + 1);

 	err = kstrtouint(buf, 0, &size);
 	if (err)

Here's the code being affected:

static int synth_field_string_size(char *type)
{
	char buf[4], *end, *start;
	unsigned int len;
	int size, err;

	start = strstr(type, "char[");
	if (start == NULL)
		return -EINVAL;
	start += sizeof("char[") - 1;

	end = strchr(type, ']');
	if (!end || end < start || type + strlen(type) > end + 1)
		return -EINVAL;

	len = end - start;
	if (len > 3)
		return -EINVAL;

	if (len == 0)
		return 0; /* variable-length string */

	strncpy(buf, start, len);
	buf[len] = '\0';

And you are replacing the above two lines with just:

	strscpy(buf, start, len + 1);


If you noticed, the string being placed into buf is:

  "char[123]"

Where we want to copy that "123" into buf.

strscpy() expects the source to be nul terminated, or it will return -E2BIG.

So the above will *always* return -E2BIG *and* not end buf[] with '\0' as
if strscpy() returns -E2BIG, then buf[] is not guaranteed to be
NUL-terminated.

@buf should still be NUL-terminated while returning -E2BIG in this
instance. For context, here's the implementation of strscpy():
  ...
  /* Hit buffer length without finding a NUL; force NUL-termination. */
  if (res)
    dest[res-1] = '\0';

  return -E2BIG;

... and the only other spot where we can return E2BIG is way earlier in
the function where we check the count.
  if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
    return -E2BIG;

So it seems we should be NUL-terminating @buf in all cases, considering
count is greater than 0 and certainly not larger than INT_MAX. And, with
the `len + 1` we shouldn't be seeing any data loss either.

  NACK!

-- Steve

I'm keen on replacing this instance of strncpy towards the goal of [1].
If we don't want to use strscpy() I think memcpy() is another viable
alternative (of course leaving the manual NUL-byte assignment as-is).

[1]: https://github.com/KSPP/linux/issues/90

Thanks
Justin