linux-kernel - Re: [RFC PATCH v5 03/12] __wr_after

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Date:   Thu, 14 Feb 2019 12:28:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Igor Stoppa <igor.stoppa@...wei.com>,
        Andy Lutomirski <luto@...capital.net>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ahmed Soliman <ahmedsoliman@...a.vt.edu>,
        linux-integrity@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 03/12] __wr_after_init: Core and default arch

On Thu, Feb 14, 2019 at 12:41:32AM +0200, Igor Stoppa wrote:
> +static inline void *wr_memset(void *p, int c, __kernel_size_t n)
> +{
> +	return memset(p, c, n);
> +}
> +
> +static inline void *wr_memcpy(void *p, const void *q, __kernel_size_t n)
> +{
> +	return memcpy(p, q, n);
> +}
> +
> +#define wr_assign(var, val)	((var) = (val))
> +#define wr_rcu_assign_pointer(p, v)	rcu_assign_pointer(p, v)
> +
> +#else
> +
> +void *wr_memset(void *p, int c, __kernel_size_t n);
> +void *wr_memcpy(void *p, const void *q, __kernel_size_t n);
> +
> +/**
> + * wr_assign() - sets a write-rare variable to a specified value
> + * @var: the variable to set
> + * @val: the new value
> + *
> + * Returns: the variable
> + */
> +
> +#define wr_assign(dst, val) ({			\
> +	typeof(dst) tmp = (typeof(dst))val;	\
> +						\
> +	wr_memcpy(&dst, &tmp, sizeof(dst));	\
> +	dst;					\
> +})
> +
> +/**
> + * wr_rcu_assign_pointer() - initialize a pointer in rcu mode
> + * @p: the rcu pointer - it MUST be aligned to a machine word
> + * @v: the new value
> + *
> + * Returns the value assigned to the rcu pointer.
> + *
> + * It is provided as macro, to match rcu_assign_pointer()
> + * The rcu_assign_pointer() is implemented as equivalent of:
> + *
> + * smp_mb();
> + * WRITE_ONCE();
> + */
> +#define wr_rcu_assign_pointer(p, v) ({	\
> +	smp_mb();			\
> +	wr_assign(p, v);		\
> +	p;				\
> +})

This requires that wr_memcpy() (through wr_assign) is single-copy-atomic
for native types. There is not a comment in sight that states this.

Also, is this true of x86/arm64 memcpy ?