Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack_frames() to use the new unwinder

[Josh Poimboeuf]

On Mon, Aug 22, 2016 at 03:11:32PM -0700, Linus Torvalds wrote:
> On Thu, Aug 18, 2016 at 6:06 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > Convert arch_within_stack_frames() to use the new unwinder.
> 
> Please don't do this.
> 
> There's no real reason to unwind the stack frame. If it's not on the
> current stack page, it shouldn't be a valid source anyway, so
> unwidning things just seems entirely pointless.
> 
> Quite frankly, I think the whole "look at the stack frames" logic
> should be removed from this. It's classic crap that external patches
> do. How many call-sites does it actually check, and how many of them
> aren't already checked by the existing static checks for constant
> addresses within existing objects?

I noticed the __compiletime_object_size() check is completely disabled
for gcc >= 4.6, thanks to:

  2fb0815c9ee6 ("gcc4: disable __compiletime_object_size for GCC 4.6+")

AFAICT, that change went too far: it disabled both the compile-time
*and* the runtime checks, so copy_from_user_overflow() is never called.

Working on a couple of patches to try to fix that.

-- 
Josh