linux-kernel - Re: [PATCH v1 07/12] dyndbg: repack struct

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Date:   Fri, 10 Nov 2023 14:00:37 -0700
From:   jim.cromie@...il.com
To:     Łukasz Bartosik <lb@...ihalf.com>
Cc:     Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...gle.com>,
        Yaniv Tzoreff <yanivt@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        upstream@...ihalf.com
Subject: Re: [PATCH v1 07/12] dyndbg: repack struct _ddebug

On Fri, Nov 10, 2023 at 7:51 AM Łukasz Bartosik <lb@...ihalf.com> wrote:
>
> sob., 4 lis 2023 o 02:49 <jim.cromie@...il.com> napisał(a):
> >
> > On Fri, Nov 3, 2023 at 7:10 AM Łukasz Bartosik <lb@...ihalf.com> wrote:
> > >
> > > From: Jim Cromie <jim.cromie@...il.com>
> > >
> > > Move the JUMP_LABEL to the top of the struct, since theyre both
> > > align(8) and this closes a pahole (unfortunately trading for padding,
> > > but still).
> > >
> > > Signed-off-by: Jim Cromie <jim.cromie@...il.com>
> >
> > let me add, I havent really tested this, nevermind thorough.
> > specifically, I didnt look for any offset dependence on the static-key
> > inside their container.
> > Conversely, maybe theres a free default or something in there.
> >
>
> Any idea how to properly test the relocation of the key ?

I was hoping Jason knew it from memory.

I have booted dd-kitchen-sink, which includes it, and it didnt melt the box.

I just checked `pahole vmlinux` output for the existence of 0-offset keys.
Its not conclusive, cuz im only looking at x86.

it does occur, but only for "sub-types".

struct static_key_true {
        struct static_key          key;                  /*     0    16 */

        /* size: 16, cachelines: 1, members: 1 */
        /* last cacheline: 16 bytes */
};
struct static_key_false {
        struct static_key          key;                  /*     0    16 */

        /* size: 16, cachelines: 1, members: 1 */
        /* last cacheline: 16 bytes */
};
struct static_key_false_deferred {
        struct static_key_false    key;                  /*     0    16 */
...};
struct static_key_mod {
        struct static_key_mod *    next;                 /*     0     8 */
...};
struct static_key_deferred {
        struct static_key          key;                  /*     0    16 */