linux-kernel - Re: [PATCH 1/2] efi: remove use of list iterator variable after loop

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Date:   Wed, 13 Apr 2022 20:08:09 +0200
From:   Jakob Koschel <jakobkoschel@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Matthew Garrett <mjg59@...f.ucam.org>,
        Peter Jones <pjones@...hat.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH 1/2] efi: remove use of list iterator variable after loop



> On 13. Apr 2022, at 19:05, Ard Biesheuvel <ardb@...nel.org> wrote:
> 
> On Fri, 1 Apr 2022 at 00:11, Jakob Koschel <jakobkoschel@...il.com> wrote:
>> 
>> In preparation to limiting the scope of a list iterator to the list
>> traversal loop, use a dedicated pointer to iterate through the list [1].
>> 
>> In the current state the list_for_each_entry() is guaranteed to
>> hit a break or goto in order to work properly. If the list iterator
>> executes completely or the list is empty the iterator variable contains
>> a type confused bogus value infered from the head of the list.
>> 
>> With this patch the variable used past the list iterator is only set
>> if the list exists early and is NULL otherwise. It should therefore
>> be safe to just set *prev = NULL (as it was before).
>> 
> 
> This generic boilerplate is fine to include, but it would help if you
> could point out why repainting the current logic with your new brush
> is appropriate here.

This makes sense, I can see that the commit message should be improved here.

> 
> In this particular case, I wonder whether updating *prev makes sense
> to begin with if we are returning an error, and if we fix that, the
> issue disappears as well.

Actually I'm rethinking this now. The only use of 'prev' that I can see is
in efi_pstore_erase_name(). It only uses it if found != 0
which would mean err != 0 in __efivar_entry_iter().

This would allow massively simplifying the entire function.
The valid case is updating *prev when there is an "error" as far as I can tell.

I've sketched up a rewritten function that should hopefully be more clear and
archive the same goal, I'm curious what you think:


	int __efivar_entry_iter(int (*func)(struct efivar_entry *, void *),
				struct list_head *head, void *data,
				struct efivar_entry **prev)
	{
		struct efivar_entry *entry, *n;
		int err = 0;

		/* If prev is set and *prev != NULL start iterating from there */
		if (prev)
			entry = list_prepare_entry(*prev, head, list);
		/* Otherwise start at the beginning */
		else
			entry = list_entry(head, typeof(*entry), list);
		list_for_each_entry_safe_continue(entry, n, head, list) {
			err = func(entry, data);
			if (err && prev)
				*prev = entry;
			if (err)
				return err;
		}

		return 0;
	}