linux-kernel - [PATCH 5.4 464/475] mm/sparsemem: fix mem_section will never be NULL gcc 12 warning

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Date:   Thu, 14 Apr 2022 15:14:09 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Waiman Long <longman@...hat.com>,
        Justin Forbes <jforbes@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Rafael Aquini <aquini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.4 464/475] mm/sparsemem: fix mem_section will never be NULL gcc 12 warning

From: Waiman Long <longman@...hat.com>

commit a431dbbc540532b7465eae4fc8b56a85a9fc7d17 upstream.

The gcc 12 compiler reports a "'mem_section' will never be NULL" warning
on the following code:

    static inline struct mem_section *__nr_to_section(unsigned long nr)
    {
    #ifdef CONFIG_SPARSEMEM_EXTREME
        if (!mem_section)
                return NULL;
    #endif
        if (!mem_section[SECTION_NR_TO_ROOT(nr)])
                return NULL;
       :

It happens with CONFIG_SPARSEMEM_EXTREME off.  The mem_section definition
is

    #ifdef CONFIG_SPARSEMEM_EXTREME
    extern struct mem_section **mem_section;
    #else
    extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
    #endif

In the !CONFIG_SPARSEMEM_EXTREME case, mem_section is a static
2-dimensional array and so the check "!mem_section[SECTION_NR_TO_ROOT(nr)]"
doesn't make sense.

Fix this warning by moving the "!mem_section[SECTION_NR_TO_ROOT(nr)]"
check up inside the CONFIG_SPARSEMEM_EXTREME block and adding an
explicit NR_SECTION_ROOTS check to make sure that there is no
out-of-bound array access.

Link: https://lkml.kernel.org/r/20220331180246.2746210-1-longman@redhat.com
Fixes: 3e347261a80b ("sparsemem extreme implementation")
Signed-off-by: Waiman Long <longman@...hat.com>
Reported-by: Justin Forbes <jforbes@...hat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Rafael Aquini <aquini@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 include/linux/mmzone.h |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1250,13 +1250,16 @@ static inline unsigned long *section_to_
 
 static inline struct mem_section *__nr_to_section(unsigned long nr)
 {
+	unsigned long root = SECTION_NR_TO_ROOT(nr);
+
+	if (unlikely(root >= NR_SECTION_ROOTS))
+		return NULL;
+
 #ifdef CONFIG_SPARSEMEM_EXTREME
-	if (!mem_section)
+	if (!mem_section || !mem_section[root])
 		return NULL;
 #endif
-	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
-		return NULL;
-	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
+	return &mem_section[root][nr & SECTION_ROOT_MASK];
 }
 extern unsigned long __section_nr(struct mem_section *ms);
 extern size_t mem_section_usage_size(void);