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

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Date:   Wed, 13 Apr 2022 19:05:01 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Jakob Koschel <jakobkoschel@...il.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Peter Jones <pjones@...hat.com>
Cc:     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 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.

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.


> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
>  drivers/firmware/efi/vars.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
> index cae590bd08f2..3994aad38661 100644
> --- a/drivers/firmware/efi/vars.c
> +++ b/drivers/firmware/efi/vars.c
> @@ -1081,14 +1081,16 @@ 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;
> +       struct efivar_entry *entry = NULL, *iter, *n;
>         int err = 0;
>
>         if (!prev || !*prev) {
> -               list_for_each_entry_safe(entry, n, head, list) {
> -                       err = func(entry, data);
> -                       if (err)
> +               list_for_each_entry_safe(iter, n, head, list) {
> +                       err = func(iter, data);
> +                       if (err) {
> +                               entry = iter;
>                                 break;
> +                       }
>                 }
>
>                 if (prev)
>
> base-commit: d888c83fcec75194a8a48ccd283953bdba7b2550
> --
> 2.25.1
>