linux-kernel - Re: [PATCH v14 01/13] sk_run_filter: add BPF_S_ANC_SECCOMP_LD

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Message-ID: <1331658828.4449.16.camel@edumazet-glaptop>
Date:	Tue, 13 Mar 2012 10:13:48 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Indan Zupancic <indan@....nu>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net, hpa@...or.com,
	mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
	rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de,
	luto@....edu, eparis@...hat.com, serge.hallyn@...onical.com,
	djm@...drot.org, scarybeasts@...il.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com, keescook@...omium.org
Subject: Re: [PATCH v14 01/13] sk_run_filter: add BPF_S_ANC_SECCOMP_LD_W

On Tue, 2012-03-13 at 11:04 +0100, Indan Zupancic wrote:
> Hello,
> 
> I made a quick pseudo-patch for BPF JIT support. As far as I can tell,
> the actual code itself is very simple, just:
> 

> -	bpf_jit_compile(fp);
> +	jit = bpf_jit_compile(fp->insns, fp->len, 1);
> +	if (jit) {
> +		fp->bpf_func = jit;
> +		/* Free unused insns memory */
> +		newfp = krealloc(fp, sizeof(*fp), GFP_KERNEL);
> +		if (newfp)
> +			fp = newfp;
> +	}
> 
>  	old_fp = rcu_dereference_protected(sk->sk_filter,
>  					   sock_owned_by_user(sk));

Dont mix unrelated changes in a single patch.

Current krealloc() doesnt alloc a new area if the current one is bigger
than 'new_size', but this might change in next kernel versions.

So this part of your patch does nothing.

If it did, this kind of 'optimization' can actually be not good, because
sizeof(*fp) is small enough (less than half cache line size) to trigger
a possible false sharing issue. (other part of the cache line could be
used to hold a often dirtied object)



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