linux-kernel - Re: [PATCH] x86/static_call: Fix __static_call

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Message-ID: <20230816001032.n4jp2uek3gi654bq@treble>
Date:   Tue, 15 Aug 2023 17:10:32 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, baron@...mai.com, rostedt@...dmis.org,
        ardb@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        linux-kernel@...r.kernel.org, christian@...cart.de,
        song@...nel.org, mcgrof@...nel.org
Subject: Re: [PATCH] x86/static_call: Fix __static_call_fixup()

On Wed, Aug 16, 2023 at 01:08:09AM +0200, Peter Zijlstra wrote:
>  bool __static_call_fixup(void *tramp, u8 op, void *dest)
>  {
> +	/*
> +	 * Not all .return_sites are a static_call trampoline (most are not).
> +	 * Check if the next 3 bytes are still kernel text, if not, then this

s/3/7 ?

> +	 * definitely is not a trampoline and we need not worry further.
> +	 *
> +	 * This avoids the memcmp() below tripping over pagefaults etc..
> +	 */
> +	if (!kernel_text_address(tramp+7))
> +		return false;
> +
>  	if (memcmp(tramp+5, tramp_ud, 3)) {
>  		/* Not a trampoline site, not our problem. */
>  		return false;

kernel_text_address() can be quite heavyweight to call in a loop during
module loading.  Maybe that doesn't matter much.  But it would be a lot
faster to only call kernel_text_address() if tramp+7 is on the next
page.

-- 
Josh