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

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Date:   Sat, 25 Nov 2023 23:00:11 -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 24, 2023 at 7:39 AM Łukasz Bartosik <lb@...ihalf.com> wrote:
>
> 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 ?
>


On balance, I think it should go in.
0 - my bias was towards abundance of paranoia
1 - youve done real work to evaluate the actual risk
2 - Jason is on thread, hasnt said WHOA
3 - actual patches have seen some testing (lkp-robot included)
4 - static-keys/jump-labels have been around a long time

One new topic:

Do you have any thoughts or plans wrt self-testing ?

the addition of private instances,
that can be opened & closed, and written to by +T:private_1

would benefit greatly from a test harness to validate it.
so far all Ive done is demo scripts

:-) thanks

> Thanks,
> Lukasz