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

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Message-ID: <20230815204112.1f7ab2bf@gandalf.local.home>
Date:   Tue, 15 Aug 2023 20:41:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     jpoimboe@...nel.org, x86@...nel.org, baron@...mai.com,
        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, 16 Aug 2023 01:08:09 +0200
Peter Zijlstra <peterz@...radead.org> wrote:


> diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> index b70670a98597..2e67512d7104 100644
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -186,6 +186,16 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
>   */
>  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
> +	 * 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))

The comment says "next 3 bytes" and the test is "tramp+7". Why the magic 7 number?

If the tramp is 5 bytes, shouldn't it be +8?

-- Steve


> +		return false;
> +
>  	if (memcmp(tramp+5, tramp_ud, 3)) {
>  		/* Not a trampoline site, not our problem. */
>  		return false;