Re: [PATCH v5] fs: clear file privilege bits when mmap writing

From: Al Viro
Date: Thu Dec 10 2015 - 15:28:17 EST

On Thu, Dec 10, 2015 at 11:47:18AM -0800, Kees Cook wrote:

> In open, sure, but what about under mm/memory.c where we're trying to
> twiddle it from vma->file->f_flags as in my patch? That seemed like it
> would want atomic safety.

Sigh... Again, I'm not at all convinced that this is the right approach,
but generally you need ->f_lock. And in situations where the bit can
go only off->on, check it lockless, skip the whole thing entirely if it's
already set and grab the spinlock otherwise.
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