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Date:   Mon, 28 Feb 2022 23:59:07 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     James Bottomley <James.Bottomley@HansenPartnership.com>,
        =?ISO-8859-1?Q?Christian_K=F6nig?= <christian.koenig@amd.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Jakob Koschel <jakobkoschel@gmail.com>,
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References: <20220228110822.491923-1-jakobkoschel@gmail.com> <20220228110822.491923-3-jakobkoschel@gmail.com> <2e4e95d6-f6c9-a188-e1cd-b1eae465562a@amd.com> <CAHk-=wgQps58DPEOe4y5cTh5oE9EdNTWRLXzgMiETc+mFX7jzw@mail.gmail.com> <282f0f8d-f491-26fc-6ae0-604b367a5a1a@amd.com> <b2d20961dbb7533f380827a7fcc313ff849875c1.camel@HansenPartnership.com>
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On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley <James=2EBotto=
mley@HansenPartnership=2Ecom> wrote:
>On Mon, 2022-02-28 at 21:07 +0100, Christian K=C3=B6nig wrote:
>> Am 28=2E02=2E22 um 20:56 schrieb Linus Torvalds:
>> > On Mon, Feb 28, 2022 at 4:19 AM Christian K=C3=B6nig
>> > <christian=2Ekoenig@amd=2Ecom> wrote:
>> > > I don't think that using the extra variable makes the code in any
>> > > way
>> > > more reliable or easier to read=2E
>> > So I think the next step is to do the attached patch (which
>> > requires
>> > that "-std=3Dgnu11" that was discussed in the original thread)=2E
>> >=20
>> > That will guarantee that the 'pos' parameter of
>> > list_for_each_entry()
>> > is only updated INSIDE the for_each_list_entry() loop, and can
>> > never
>> > point to the (wrongly typed) head entry=2E
>> >=20
>> > And I would actually hope that it should actually cause compiler
>> > warnings about possibly uninitialized variables if people then use
>> > the
>> > 'pos' pointer outside the loop=2E Except
>> >=20
>> >   (a) that code in sgx/encl=2Ec currently initializes 'tmp' to NULL
>> > for
>> > inexplicable reasons - possibly because it already expected this
>> > behavior
>> >=20
>> >   (b) when I remove that NULL initializer, I still don't get a
>> > warning,
>> > because we've disabled -Wno-maybe-uninitialized since it results in
>> > so
>> > many false positives=2E
>> >=20
>> > Oh well=2E
>> >=20
>> > Anyway, give this patch a look, and at least if it's expanded to do
>> > "(pos) =3D NULL" in the entry statement for the for-loop, it will
>> > avoid the HEAD type confusion that Jakob is working on=2E And I think
>> > in a cleaner way than the horrid games he plays=2E
>> >=20
>> > (But it won't avoid possible CPU speculation of such type
>> > confusion=2E That, in my opinion, is a completely different issue)
>>=20
>> Yes, completely agree=2E
>>=20
>> > I do wish we could actually poison the 'pos' value after the loop
>> > somehow - but clearly the "might be uninitialized" I was hoping for
>> > isn't the way to do it=2E
>> >=20
>> > Anybody have any ideas?
>>=20
>> I think we should look at the use cases why code is touching (pos)
>> after the loop=2E
>>=20
>> Just from skimming over the patches to change this and experience
>> with the drivers/subsystems I help to maintain I think the primary
>> pattern looks something like this:
>>=20
>> list_for_each_entry(entry, head, member) {
>>      if (some_condition_checking(entry))
>>          break;
>> }
>> do_something_with(entry);
>
>
>Actually, we usually have a check to see if the loop found anything,
>but in that case it should something like
>
>if (list_entry_is_head(entry, head, member)) {
>    return with error;
>}
>do_somethin_with(entry);
>
>Suffice?  The list_entry_is_head() macro is designed to cope with the
>bogus entry on head problem=2E

Won't suffice because the end goal of this work is to limit scope of entry=
 only to loop=2E Hence the need for additional variable=2E

Besides, there are no guarantees that people won't do_something_with(entry=
) without the check or won't compare entry to NULL to check if the loop fin=
ished with break or not=2E

>James


--=20
Sincerely yours,
Mike