[PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

[PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

[Waiman Long]

The gcc 12 compiler reports a 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
       :

With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is

    extern struct mem_section **mem_section;

Obviously, mem_section cannot be NULL, but *mem_section can be if memory
hasn't been allocated for the dynamic mem_section[] array yet. Fix this
warning by checking for "!*mem_section" instead.

Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/mmzone.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index aed44e9b5d89..bd1b19925f3b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
 static inline struct mem_section *__nr_to_section(unsigned long nr)
 {
 #ifdef CONFIG_SPARSEMEM_EXTREME
-	if (!mem_section)
+	if (!*mem_section)
 		return NULL;
 #endif
 	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
-- 
2.27.0

Re: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

[Waiman Long]

On 2/1/22 14:29, Waiman Long wrote:
> The gcc 12 compiler reports a 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
>         :
>
> With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is
>
>      extern struct mem_section **mem_section;
>
> Obviously, mem_section cannot be NULL, but *mem_section can be if memory
> hasn't been allocated for the dynamic mem_section[] array yet. Fix this
> warning by checking for "!*mem_section" instead.
>
> Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")

Sorry, I forgot to add the line

Reported-by: Justin Forbes <jforbes@redhat.com>

> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   include/linux/mmzone.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aed44e9b5d89..bd1b19925f3b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
>   static inline struct mem_section *__nr_to_section(unsigned long nr)
>   {
>   #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!mem_section)
> +	if (!*mem_section)
>   		return NULL;
>   #endif
>   	if (!mem_section[SECTION_NR_TO_ROOT(nr)])

Re: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

[Mike Rapoport]

On Tue, Feb 01, 2022 at 02:29:24PM -0500, Waiman Long wrote:
> The gcc 12 compiler reports a 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
>        :
> 
> With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is
> 
>     extern struct mem_section **mem_section;
> 
> Obviously, mem_section cannot be NULL, but *mem_section can be if memory
> hasn't been allocated for the dynamic mem_section[] array yet. Fix this
> warning by checking for "!*mem_section" instead.

Hmm, if mem_section cannot be NULL, it means if will be never allocated,
no?

static void __init memory_present(int nid, unsigned long start, unsigned long end)
{
	unsigned long pfn;

#ifdef CONFIG_SPARSEMEM_EXTREME
	if (unlikely(!mem_section)) {
		unsigned long size, align;

		size = sizeof(struct mem_section *) * NR_SECTION_ROOTS;
		align = 1 << (INTERNODE_CACHE_SHIFT);
		mem_section = memblock_alloc(size, align);
		if (!mem_section)
			panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
			      __func__, size, align);
	}
#endif

...

> 
> Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/mmzone.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aed44e9b5d89..bd1b19925f3b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
>  static inline struct mem_section *__nr_to_section(unsigned long nr)
>  {
>  #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!mem_section)
> +	if (!*mem_section)
>  		return NULL;
>  #endif
>  	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
> -- 
> 2.27.0
> 
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

[Hector Martin]

On 02/02/2022 04.29, Waiman Long wrote:
> The gcc 12 compiler reports a 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
>        :
> 
> With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is
> 
>     extern struct mem_section **mem_section;
> 
> Obviously, mem_section cannot be NULL, but *mem_section can be if memory
> hasn't been allocated for the dynamic mem_section[] array yet. Fix this
> warning by checking for "!*mem_section" instead.
> 
> Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/mmzone.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aed44e9b5d89..bd1b19925f3b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
>  static inline struct mem_section *__nr_to_section(unsigned long nr)
>  {
>  #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!mem_section)
> +	if (!*mem_section)
>  		return NULL;
>  #endif
>  	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
> 

This broke booting on Apple T6000 (M1 Pro; support not yet mainlined)
and it is obviously incorrect. !*mem_section is the same thing as
!mem_section[0], which is always checking element 0 for NULL instead of
the element we're interested in. These machines don't have memory at 0,
fail the spurious check, and crash on early boot.

[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x612f0240]
[    0.000000] Linux version
5.17.0-rc4-asahi-next-20220217-00141-g3eb6fdba1573 (marcan@raider)
(aarch64-linux-gnu-gcc (Gentoo 11.2.0 p1) 11.2.0, GNU ld (Gentoo 2.36.1
p1) 2.36.1) #1112 SMP PREEMPT Sat Feb 19 18:01:31 JST 2022
[    0.000000] random: fast init done
[    0.000000] Machine model: Apple MacBook Pro (14-inch, M1 Pro, 2021)
[    0.000000] efi: UEFI not found.
[    0.000000] earlycon: s5l0 at MMIO32 0x000000039b200000 (options '')
[    0.000000] printk: bootconsole [s5l0] enabled
[    0.000000] Unable to handle kernel NULL pointer dereference at
virtual address 0000000000000000
<snip>
[    0.000000] pc : sparse_init+0x150/0x268
<snip>

Please revert fff3b2a167db5 and edecc06b4d34e.

-- 
Hector Martin (marcan@marcan.st)
Public Key: https://mrcn.st/pub