linux-kernel - Re: [PATCH] fs/proc/kcore.c: remove check of list iterator against head past the loop body

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Date:   Thu, 31 Mar 2022 16:48:43 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Brian Johannesmeyer" <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH] fs/proc/kcore.c: remove check of list iterator against
 head past the loop body

On Fri,  1 Apr 2022 00:37:00 +0200 Jakob Koschel <jakobkoschel@...il.com> wrote:

> When list_for_each_entry() completes the iteration over the whole list
> without breaking the loop, the iterator value will be a bogus pointer
> computed based on the head element.
> 
> While it is safe to use the pointer to determine if it was computed
> based on the head element, either with list_entry_is_head() or
> &pos->member == head, using the iterator variable after the loop should
> be avoided.
> 
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer to point to the found element [1].
> 
> ...
>

Speaking of limiting scope...

--- a/fs/proc/kcore.c~fs-proc-kcorec-remove-check-of-list-iterator-against-head-past-the-loop-body-fix
+++ a/fs/proc/kcore.c
@@ -316,7 +316,6 @@ read_kcore(struct file *file, char __use
 	size_t page_offline_frozen = 1;
 	size_t phdrs_len, notes_len;
 	struct kcore_list *m;
-	struct kcore_list *iter;
 	size_t tsz;
 	int nphdr;
 	unsigned long start;
@@ -480,6 +479,8 @@ read_kcore(struct file *file, char __use
 		 * the previous entry, search for a matching entry.
 		 */
 		if (!m || start < m->addr || start >= m->addr + m->size) {
+			struct kcore_list *iter;
+
 			m = NULL;
 			list_for_each_entry(iter, &kclist_head, list) {
 				if (start >= iter->addr &&
_