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

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Message-ID: <CAK8ByeK8mVQbmYsbCdZGf4F=RHPbGguDcKfhtbWF3-MpVDNkbg@mail.gmail.com>
Date:   Fri, 24 Nov 2023 15:38:51 +0100
From:   Łukasz Bartosik <lb@...ihalf.com>
To:     jim.cromie@...il.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

niedz., 12 lis 2023 o 17:28 Łukasz Bartosik <lb@...ihalf.com> napisał(a):
>
> pt., 10 lis 2023 o 22:01 <jim.cromie@...il.com> napisał(a):
> >
> > 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 */
>
> I will test it on arm64.

Hi Jim,

I verified that relocation of JUMP_LABEL to the top of the _ddebug
struct does not brak dynamic debug functionality on arm64.
I double checked I had CONFIG_JUMP_LABEL enabled in the kernel config for arm64.
I was able to enable/disable callsites and see debug logs being written.

But if you're concerned there might be issue related to that
relocation on other architectures then let's drop this patch
and I will use pahole instead of padding for location of flags and
trace destination fields.
What do you think ?

Thanks,
Lukasz