@@ -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) {
- nr_pages = ret;
- ret = -EINVAL;
+ 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;
}
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> --- drivers/video/fbdev/pvr2fb.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)