netdev - Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

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Message-ID: <b2d20961dbb7533f380827a7fcc313ff849875c1.camel@HansenPartnership.com>
Date:   Mon, 28 Feb 2022 15:42:53 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christian König <christian.koenig@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jakob Koschel <jakobkoschel@...il.com>,
        alsa-devel@...a-project.org, linux-aspeed@...ts.ozlabs.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop
 body as a ptr

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@....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