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From:   =?UTF-8?Q?=C5=81ukasz_Bartosik?= <lb@semihalf.com>
Date:   Fri, 24 Nov 2023 15:38:51 +0100
Message-ID: <CAK8ByeK8mVQbmYsbCdZGf4F=RHPbGguDcKfhtbWF3-MpVDNkbg@mail.gmail.com>
Subject: Re: [PATCH v1 07/12] dyndbg: repack struct _ddebug
To:     jim.cromie@gmail.com
Cc:     Jason Baron <jbaron@akamai.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Guenter Roeck <groeck@google.com>,
        Yaniv Tzoreff <yanivt@google.com>,
        Benson Leung <bleung@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vincent Whitchurch <vincent.whitchurch@axis.com>,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Sean Paul <seanpaul@chromium.org>,
        Daniel Vetter <daniel@ffwll.ch>, linux-kernel@vger.kernel.org,
        upstream@semihalf.com
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niedz., 12 lis 2023 o 17:28 =C5=81ukasz Bartosik <lb@semihalf.com> napisa=
=C5=82(a):
>
> pt., 10 lis 2023 o 22:01 <jim.cromie@gmail.com> napisa=C5=82(a):
> >
> > On Fri, Nov 10, 2023 at 7:51=E2=80=AFAM =C5=81ukasz Bartosik <lb@semiha=
lf.com> wrote:
> > >
> > > sob., 4 lis 2023 o 02:49 <jim.cromie@gmail.com> napisa=C5=82(a):
> > > >
> > > > On Fri, Nov 3, 2023 at 7:10=E2=80=AFAM =C5=81ukasz Bartosik <lb@sem=
ihalf.com> wrote:
> > > > >
> > > > > From: Jim Cromie <jim.cromie@gmail.com>
> > > > >
> > > > > Move the JUMP_LABEL to the top of the struct, since theyre both
> > > > > align(8) and this closes a pahole (unfortunately trading for padd=
ing,
> > > > > but still).
> > > > >
> > > > > Signed-off-by: Jim Cromie <jim.cromie@gmail.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 ke=
ys.
> > 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 a=
rm64.
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