From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: "Christian König" <christian.koenig@amd.com>,
"Linus Torvalds" <torvalds@linux-foundation.org>
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr
Date: Mon, 28 Feb 2022 15:42:53 -0500 [thread overview]
Message-ID: <b2d20961dbb7533f380827a7fcc313ff849875c1.camel@HansenPartnership.com> (raw)
In-Reply-To: <282f0f8d-f491-26fc-6ae0-604b367a5a1a@amd.com>
On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote:
> Am 28.02.22 um 20:56 schrieb Linus Torvalds:
> > On Mon, Feb 28, 2022 at 4:19 AM Christian König
> > <christian.koenig@amd.com> wrote:
> > > I don't think that using the extra variable makes the code in any
> > > way
> > > more reliable or easier to read.
> > So I think the next step is to do the attached patch (which
> > requires
> > that "-std=gnu11" that was discussed in the original thread).
> >
> > 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.
> >
> > 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. Except
> >
> > (a) that code in sgx/encl.c currently initializes 'tmp' to NULL
> > for
> > inexplicable reasons - possibly because it already expected this
> > behavior
> >
> > (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.
> >
> > Oh well.
> >
> > Anyway, give this patch a look, and at least if it's expanded to do
> > "(pos) = NULL" in the entry statement for the for-loop, it will
> > avoid the HEAD type confusion that Jakob is working on. And I think
> > in a cleaner way than the horrid games he plays.
> >
> > (But it won't avoid possible CPU speculation of such type
> > confusion. That, in my opinion, is a completely different issue)
>
> Yes, completely agree.
>
> > 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.
> >
> > Anybody have any ideas?
>
> I think we should look at the use cases why code is touching (pos)
> after the loop.
>
> 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:
>
> 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.
James
next prev parent reply other threads:[~2022-02-28 20:54 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-28 11:08 [PATCH 0/6] Remove usage of list iterator past the loop body Jakob Koschel
2022-02-28 11:08 ` [PATCH 1/6] drivers: usb: remove " Jakob Koschel
2022-02-28 11:24 ` Dan Carpenter
2022-02-28 12:03 ` Jakob Koschel
2022-02-28 13:18 ` Dan Carpenter
2022-02-28 18:20 ` Joe Perches
2022-03-01 5:52 ` Dan Carpenter
2022-02-28 11:08 ` [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr Jakob Koschel
2022-02-28 11:20 ` Greg KH
2022-02-28 12:06 ` Jakob Koschel
2022-03-01 17:37 ` Greg KH
2022-02-28 12:19 ` Christian König
2022-02-28 19:56 ` Linus Torvalds
2022-02-28 20:03 ` Linus Torvalds
2022-02-28 20:10 ` Linus Torvalds
2022-02-28 20:14 ` Linus Torvalds
2022-02-28 20:53 ` Segher Boessenkool
2022-02-28 20:16 ` Matthew Wilcox
2022-02-28 20:27 ` Johannes Berg
2022-02-28 20:41 ` Linus Torvalds
2022-02-28 20:37 ` Linus Torvalds
2022-02-28 23:26 ` Matthew Wilcox
2022-03-01 0:45 ` Linus Torvalds
2022-03-01 0:57 ` Linus Torvalds
2022-03-01 18:14 ` Kees Cook
2022-03-01 18:47 ` Linus Torvalds
2022-03-01 19:01 ` Matthew Wilcox
2022-03-01 3:03 ` David Laight
2022-02-28 21:47 ` Jakob Koschel
2022-03-01 0:41 ` Linus Torvalds
2022-03-01 6:32 ` Jakub Kicinski
2022-03-01 11:28 ` Jakob Koschel
2022-03-01 17:36 ` Greg KH
2022-03-01 17:40 ` Jakob Koschel
2022-03-01 17:58 ` Greg KH
2022-03-01 18:21 ` Kees Cook
2022-03-02 9:31 ` Xiaomeng Tong
2022-03-02 14:04 ` David Laight
2022-03-03 2:27 ` Xiaomeng Tong
2022-03-03 4:58 ` David Laight
2022-03-03 7:26 ` Xiaomeng Tong
2022-03-03 9:30 ` David Laight
2022-03-03 12:37 ` Xiaomeng Tong
2022-03-03 12:18 ` [Kgdb-bugreport] " Daniel Thompson
2022-03-04 6:59 ` Xiaomeng Tong
2022-03-03 7:32 ` Jakob Koschel
2022-03-03 8:30 ` Xiaomeng Tong
2022-03-03 8:38 ` Xiaomeng Tong
2022-02-28 20:07 ` Christian König
2022-02-28 20:42 ` James Bottomley [this message]
2022-02-28 20:56 ` Christian König
2022-02-28 21:13 ` James Bottomley
2022-03-01 7:03 ` Christian König
2022-02-28 22:05 ` Jakob Koschel
2022-02-28 21:18 ` Jeffrey Walton
2022-02-28 21:59 ` Mike Rapoport
2022-02-28 22:28 ` James Bottomley
2022-02-28 22:50 ` Barnabás Pőcze
2022-03-01 0:30 ` Segher Boessenkool
2022-03-01 0:54 ` Linus Torvalds
2022-03-01 19:06 ` Linus Torvalds
2022-03-01 19:42 ` Linus Torvalds
2022-03-01 22:58 ` David Laight
2022-03-01 23:03 ` Linus Torvalds
2022-03-01 23:19 ` David Laight
2022-03-01 23:55 ` Linus Torvalds
2022-03-02 9:29 ` Rasmus Villemoes
2022-03-02 20:07 ` Kees Cook
2022-03-02 20:18 ` Linus Torvalds
2022-03-02 20:59 ` Kees Cook
2022-03-03 8:37 ` Dan Carpenter
2022-03-03 10:56 ` Dan Carpenter
2022-03-01 2:15 ` David Laight
2022-02-28 13:13 ` Dan Carpenter
2022-02-28 11:08 ` [PATCH 3/6] treewide: fix incorrect use to determine if list is empty Jakob Koschel
2022-02-28 11:38 ` Dan Carpenter
2022-02-28 11:08 ` [PATCH 4/6] drivers: remove unnecessary use of list iterator variable Jakob Koschel
2022-02-28 11:08 ` [PATCH 5/6] treewide: remove dereference of list iterator after loop body Jakob Koschel
2022-02-28 11:08 ` [PATCH 6/6] treewide: remove check of list iterator against head past the " Jakob Koschel
2022-02-28 11:22 ` Dominique Martinet
2022-02-28 13:12 ` Dan Carpenter
2022-03-01 20:36 ` Linus Torvalds
2022-03-02 17:14 ` [Intel-gfx] " Tvrtko Ursulin
2022-03-07 15:00 ` [PATCH 0/6] Remove usage of list iterator " Dan Carpenter
2022-03-07 15:26 ` David Laight
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