video: fbdev: fix error handling for get_user_pages_fast()

Dealing with the return value of get_user_pages*() variants has a few
classic pitfalls, and this driver found one of them: the return value
might be zero, positive, or -errno. And if positive, it might be fewer
pages than were requested. And if fewer pages than requested, then
the caller should return (via put_page()) the pages that *were*
pinned.

This driver was doing that *except* that it had a problem with the
-errno case, which was being stored in an unsigned int, and which
would case an interesting mess if it ever happened: nr_pages would be
interpreted as a spectacularly huge unsigned value, rather than a
small negative value. Also, it was unnecessarily overriding a
potentially informative -errno, with -EINVAL, in some cases.

Instead: clamp the nr_pages to zero or positive, so that the error
handling works. And return the -errno value from get_user_pages*(),
unchanged, if we get one. And explain this with comments, seeing as
how it is error-prone.

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200522041506.39638-2-jhubbard@nvidia.com

drivers/video/fbdev/pvr2fb.c

@@ -654,8 +654,22 @@ static ssize_t pvr2fb_write(struct fb_info *info, const char *buf,
 
 	ret = get_user_pages_fast((unsigned long)buf, nr_pages, FOLL_WRITE, pages);
 	if (ret < nr_pages) {
+		if (ret < 0) {
+			/*
+			 *  Clamp the unsigned nr_pages to zero so that the
+			 *  error handling works. And leave ret at whatever
+			 *  -errno value was returned from GUP.
+			 */
+			nr_pages = 0;
+		} else {
 			nr_pages = ret;
+			/*
+			 * Use -EINVAL to represent a mildly desperate guess at
+			 * why we got fewer pages (maybe even zero pages) than
+			 * requested.
+			 */
 			ret = -EINVAL;
+		}
 		goto out_unmap;
 	}