[PATCH v2][next] x86/uv/time: Replace one-element array and save heap space

[PATCH v2][next] x86/uv/time: Replace one-element array and save heap space

[Gustavo A. R. Silva]

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

struct uv_rtc_timer_head contains a one-element array cpu[1].

Switch it to a flexible array and use the struct_size() helper to
calculate the allocation size. Also, save some heap space in the
process[3].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
[3] https://lore.kernel.org/lkml/20200518190114.GA7757@embeddedor/

Reviewed-by: Kees Cook <keescook@chromium.org>
Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f75bc0a.rQcNS6620b2eA74S%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Modify changelog text (Thomas).
 - Add Reviewed-by and Build-tested-by tags.
 - Add a public link to the build-test results.

 arch/x86/platform/uv/uv_time.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
index f82a1337a608..6c348c2d0def 100644
--- a/arch/x86/platform/uv/uv_time.c
+++ b/arch/x86/platform/uv/uv_time.c
@@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
 	struct {
 		int	lcpu;		/* systemwide logical cpu number */
 		u64	expires;	/* next timer expiration for this cpu */
-	} cpu[1];
+	} cpu[];
 };
 
 /*
@@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
 		struct uv_rtc_timer_head *head = blade_info[bid];
 
 		if (!head) {
-			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
-				(uv_blade_nr_possible_cpus(bid) *
-					2 * sizeof(u64)),
+			head = kmalloc_node(struct_size(head, cpu,
+				uv_blade_nr_possible_cpus(bid)),
 				GFP_KERNEL, nid);
 			if (!head) {
 				uv_rtc_deallocate_timers();
-- 
2.27.0

Re: [PATCH v2][next] x86/uv/time: Replace one-element array and save heap space

[Joe Perches]

On Thu, 2020-10-01 at 09:56 -0500, Gustavo A. R. Silva wrote:
> There is a regular need in the kernel to provide a way to declare having
> a dynamically sized set of trailing elements in a structure. Kernel code
> should always use “flexible array members”[1] for these cases. The older
> style of one-element or zero-length arrays should no longer be used[2].
> 
> struct uv_rtc_timer_head contains a one-element array cpu[1].
> 
> Switch it to a flexible array and use the struct_size() helper to
> calculate the allocation size. Also, save some heap space in the
> process[3].

trivia:

> diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
[]
> @@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
>  		struct uv_rtc_timer_head *head = blade_info[bid];
>  
>  		if (!head) {
> -			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
> -				(uv_blade_nr_possible_cpus(bid) *
> -					2 * sizeof(u64)),
> +			head = kmalloc_node(struct_size(head, cpu,
> +				uv_blade_nr_possible_cpus(bid)),
>  				GFP_KERNEL, nid);
>  			if (!head) {
>  				uv_rtc_deallocate_timers();

Maybe save the value of uv_blade_nr_possible_cpus(bid)
to reduce duplication and make the sizeof_struct more
readable?

		if (!head) {
			int ncpus = uv_blade_nr_possible_cpus(bid);

			head = kmalloc_node(struct_size(head, cpu, ncpus),
					    GFP_KERNEL, nid);
			if (!head) {
				uv_rtc_deallocate_timers();
				return -ENOMEM;
			}
			spin_lock_init(&head->lock);
			head->ncpus = ncpus;
			head->next_cpu = -1;
			blade_info[bid] = head;
		}

[tip: x86/platform] x86/uv/time: Use a flexible array in struct uv_rtc_timer_head

[tip-bot2 for Gustavo A. R. Silva]

The following commit has been merged into the x86/platform branch of tip:

Commit-ID:     a0947081af2ac9549e6ba19877456730713bde23
Gitweb:        https://git.kernel.org/tip/a0947081af2ac9549e6ba19877456730713bde23
Author:        Gustavo A. R. Silva <gustavoars@kernel.org>
AuthorDate:    Thu, 01 Oct 2020 09:56:08 -05:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 01 Oct 2020 18:47:39 +02:00

x86/uv/time: Use a flexible array in struct uv_rtc_timer_head

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The
older style of one-element or zero-length arrays should no longer be
used[2].

struct uv_rtc_timer_head contains a one-element array cpu[1]. Switch it
to a flexible array and use the struct_size() helper to calculate the
allocation size. Also, save some heap space in the process[3].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
[3] https://lore.kernel.org/lkml/20200518190114.GA7757@embeddedor/

 [ bp: Massage a bit. ]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Steve Wahl <steve.wahl@hpe.com>
Link: https://lkml.kernel.org/r/20201001145608.GA10204@embeddedor
---
 arch/x86/platform/uv/uv_time.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
index f82a133..6c348c2 100644
--- a/arch/x86/platform/uv/uv_time.c
+++ b/arch/x86/platform/uv/uv_time.c
@@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
 	struct {
 		int	lcpu;		/* systemwide logical cpu number */
 		u64	expires;	/* next timer expiration for this cpu */
-	} cpu[1];
+	} cpu[];
 };
 
 /*
@@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
 		struct uv_rtc_timer_head *head = blade_info[bid];
 
 		if (!head) {
-			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
-				(uv_blade_nr_possible_cpus(bid) *
-					2 * sizeof(u64)),
+			head = kmalloc_node(struct_size(head, cpu,
+				uv_blade_nr_possible_cpus(bid)),
 				GFP_KERNEL, nid);
 			if (!head) {
 				uv_rtc_deallocate_timers();

Re: [PATCH v2][next] x86/uv/time: Replace one-element array and save heap space

[Steve Wahl]

On Thu, Oct 01, 2020 at 09:56:08AM -0500, Gustavo A. R. Silva wrote:
> There is a regular need in the kernel to provide a way to declare having
> a dynamically sized set of trailing elements in a structure. Kernel code
> should always use “flexible array members”[1] for these cases. The older
> style of one-element or zero-length arrays should no longer be used[2].

I see this has already gone in, so I'm late.  But for whatever it's
worth, it's a good change.  Thank you.

Reviewed-by: Steve Wahl <steve.wahl@hpe.com>

--> Steve wahl

> struct uv_rtc_timer_head contains a one-element array cpu[1].
> 
> Switch it to a flexible array and use the struct_size() helper to
> calculate the allocation size. Also, save some heap space in the
> process[3].
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays
> [3] https://lore.kernel.org/lkml/20200518190114.GA7757@embeddedor/
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Build-tested-by: kernel test robot <lkp@intel.com>
> Link: https://lore.kernel.org/lkml/5f75bc0a.rQcNS6620b2eA74S%25lkp@intel.com/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> Changes in v2:
>  - Modify changelog text (Thomas).
>  - Add Reviewed-by and Build-tested-by tags.
>  - Add a public link to the build-test results.
> 
>  arch/x86/platform/uv/uv_time.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
> index f82a1337a608..6c348c2d0def 100644
> --- a/arch/x86/platform/uv/uv_time.c
> +++ b/arch/x86/platform/uv/uv_time.c
> @@ -52,7 +52,7 @@ struct uv_rtc_timer_head {
>  	struct {
>  		int	lcpu;		/* systemwide logical cpu number */
>  		u64	expires;	/* next timer expiration for this cpu */
> -	} cpu[1];
> +	} cpu[];
>  };
>  
>  /*
> @@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
>  		struct uv_rtc_timer_head *head = blade_info[bid];
>  
>  		if (!head) {
> -			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
> -				(uv_blade_nr_possible_cpus(bid) *
> -					2 * sizeof(u64)),
> +			head = kmalloc_node(struct_size(head, cpu,
> +				uv_blade_nr_possible_cpus(bid)),
>  				GFP_KERNEL, nid);
>  			if (!head) {
>  				uv_rtc_deallocate_timers();
> -- 
> 2.27.0
>