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Date:   Thu, 05 Apr 2018 16:21:23 +0000
From:   James Y Knight <jyknight@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        gregkh@...uxfoundation.org,
        Nick Desaulniers <ndesaulniers@...gle.com>, mka@...omium.org,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        tglx@...utronix.de, Andrew Morton <akpm@...ux-foundation.org>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Kees Cook <keescook@...gle.com>, groeck@...omium.org,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [GIT PULL] x86/build changes for v4.17

On Thu, Apr 5, 2018 at 3:08 AM Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Apr 04, 2018 at 10:21:05PM +0000, James Y Knight wrote:
> > But allowing random pointer arithmetic, and pointer arithmetic
wraparound,
> > is still different than asserting that an object _field access_ can
> > overflow. Clang does not believe that can happen -- it assumes that an
> > object will still be contiguous. And that's why the llist stuff used to
be
> > broken, before it was corrected to do simply do math on a uintptr_t
(which
> > is a nice and simple and sane fix!).

> That 'fix' wasn't anything simple, I recently ran into that
> member_address_is_nonnull() trainwreck and had to think real hard wtf it
> was about.

I agree the comment there could be clearer. You could replace it with
something like this (apologies: this patch is likely going to be mangled by
gmail's plaintext mode hard-wrapping it.)

diff --git a/include/linux/llist.h b/include/linux/llist.h
index 85abc2915e8d..04e972a0bbe8 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -99,12 +99,15 @@ static inline void init_llist_head(struct llist_head
*list)
   *
   * This macro is conceptually the same as
   * &ptr->member != NULL
- * but it works around the fact that compilers can decide that taking a
member
- * address is never a NULL pointer.
- *
- * Real objects that start at a high address and have a member at NULL are
- * unlikely to exist, but such pointers may be returned e.g. by the
- * container_of() macro.
+ * except that it uses addition on a uintptr_t instead of member
+ * access syntax. This avoids running into a compiler assumption that
+ * objects must be contiguous in memory, and therefore that member
+ * address lookup cannot wrap, and therefore that a field with a
+ * positive offset within an object can never be at address 0.
+ *
+ * Real objects which start at a high address and have a member at
+ * NULL do not exist, but such a pointer is the result of applying
+ * container_of() to NULL, which llist_for_each_entry does.
   */
  #define member_address_is_nonnull(ptr, member) \
   ((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)

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