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

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Message-ID: <20230816093917.GC980931@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Aug 2023 11:39:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.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 Tue, Aug 15, 2023 at 05:10:32PM -0700, Josh Poimboeuf wrote:
> 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 ?

Right, so what I meant was the 3 bytes after the return, which is 5+3,
but yeah, that can be said better.

> 
> > +	 * 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.

Oh, right, in those few configs where it doesn't use the tree. Sure can
do.