Re: [PATCH V3 1/8] sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output

On Thu, Oct 01, 2020 at 10:50:29PM +0200, Greg Kroah-Hartman wrote:

On Wed, Sep 30, 2020 at 09:17:03PM -0700, Kees Cook wrote:

quoted

On Wed, Sep 30, 2020 at 01:57:40PM +0200, Greg Kroah-Hartman wrote:

quoted

Kees, and Rafael, I don't know if you saw this proposal from Joe for
sysfs files, questions below:

I'm a fan. I think the use of sprintf() in sysfs might have been one of
my earliest complaints about unsafe code patterns in the kernel. ;)

Ok, great.

quoted

quoted

quoted

+/**
+ *	sysfs_emit - scnprintf equivalent, aware of PAGE_SIZE buffer.
+ *	@buf:	start of PAGE_SIZE buffer.
+ *	@fmt:	format
+ *	@...:	optional arguments to @format
+ *
+ *
+ * Returns number of characters written to @buf.
+ */
+int sysfs_emit(char *buf, const char *fmt, ...)
+{
+	va_list args;
+	int len;
+
+	if (WARN(!buf || offset_in_page(buf),
+		 "invalid sysfs_emit: buf:%p\n", buf))

I don't want the %p here, but otherwise, sure. I'd also make it a _ONCE
variant:

	if (WARN_ONCE(!buf || offset_in_page(buf),
		 "invalid sysfs_emit: offset_in_page(buf):%zd\n",
		  buf ? offset_in_page(buf) : 0))

As Joe points out, _ONCE doesn't work because this happens from all
sysfs files, not just one.

Sure, it's just a question if you want log spamming vs how reachable you
think something might be. I would expect this to be uncommon to
encounter, but very repeatable for whatever system DOES hit it, so doing
_ONCE means they see the report and don't get completely flooded with
it.

I'm fine either way.

-- 
Kees Cook