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

From: Greg Kroah-Hartman
Date: Thu Apr 14 2022 - 11:23:19 EST


From: Waiman Long <longman@xxxxxxxxxx>

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@xxxxxxxxxx
Fixes: 3e347261a80b ("sparsemem extreme implementation")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Reported-by: Justin Forbes <jforbes@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Rafael Aquini <aquini@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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);