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

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Message-ID: <20201001174256.GV739899@swahl-home.5wahls.com>
Date:   Thu, 1 Oct 2020 12:42:56 -0500
From:   Steve Wahl <steve.wahl@....com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Steve Wahl <steve.wahl@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] x86/uv/time: Replace one-element array and save
 heap space

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@....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@...omium.org>
> Build-tested-by: kernel test robot <lkp@...el.com>
> Link: https://lore.kernel.org/lkml/5f75bc0a.rQcNS6620b2eA74S%25lkp@intel.com/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.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
>