Messages in this thread | | | Date | Mon, 28 Feb 2022 21:07:03 +0100 | Subject | Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr | From | Christian König <> |
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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);
So the solution should probably not be to change all those use cases to use more temporary variables, but rather to add a list_find_entry(..., condition) macro and consistently use that one instead.
Regards, Christian.
> > Linus
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