Why __force sparse annotation needed in copy_from_user() to vmalloc'ed area

[Thomas Petazzoni]

Hello,

In the course of reviewing a new framebuffer driver [1], I discovered
the following bit of code:

+	dst = (void __force *) (info->screen_base + p);
+
+	if (copy_from_user(dst, buf, count))
+		err = -EFAULT;

Where info->screen_base is a vmalloc'ed memory area. Many other
framebuffer drivers have used this __force sparse annotation: see [2]
or [3] for example.

I am confused as to why sparse requires the __force annotation for what
seems to be a regular user to kernel copy. The __force annotation is
typically not needed for user to kernel copy when the destination
kernel buffer has been kmalloc'ed.

Why does sparse handles vmalloc'ed buffer differently, and gives a
warning in this case?

Thanks,

Thomas

[1] http://marc.info/?l=linux-fbdev&m=134683775709128&w=2
[2] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=28cdf76bf0bce757428f84161e3aa510028d47b4
[3] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d2e8d369cd92a8bb856ff530fa1e4c03b0e0ff55
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com