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Tue, 9 Nov 2021 20:41:23 +0100 (CET) Date: Tue, 9 Nov 2021 20:41:23 +0100 From: Peter Zijlstra To: Ard Biesheuvel Cc: Linux Kernel Mailing List , Linux ARM , Josh Poimboeuf , Jason Baron , Steven Rostedt , Mark Rutland , Kees Cook Subject: Re: [RFC PATCH 2/7] static_call: deal with unexported keys without cluttering up the API Message-ID: References: <20211109164549.1724710-1-ardb@kernel.org> <20211109164549.1724710-3-ardb@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Nov 09, 2021 at 07:53:33PM +0100, Ard Biesheuvel wrote: > On Tue, 9 Nov 2021 at 19:49, Peter Zijlstra wrote: > > > > On Tue, Nov 09, 2021 at 05:45:44PM +0100, Ard Biesheuvel wrote: > > > > > diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h > > > index 5a00b8b2cf9f..0bb36294cce7 100644 > > > --- a/include/linux/static_call_types.h > > > +++ b/include/linux/static_call_types.h > > > @@ -32,15 +32,20 @@ > > > struct static_call_site { > > > s32 addr; > > > s32 key; > > > + s32 tramp; > > > }; > > > > I can't say I'm thrilled at growing this thing, but the cleanup is nice. > > Perhaps we can increase alignment on struct static_call_key and instead > > frob it in .key still? > > > > This is already a place-relative field, and one points into the data > section and the other into text. So I don't see how we can squeeze > enough bits out of it to make this fit. Well, the actual address will be: ((unsigned long)&site->key + site->key) either way around, right? Now, if we align struct static_call_key to 8, that means the low 3 bits of that address will be 0 and free for us to muck about with. That is, we already use the low 2 bits of that (because natural alignment etc.). If we got 3 bits, we could shift the existing two bits one up and free up bit0, then say that if bit0 is set, it's a trampoline address. The only additional requirement would be that trampolines are (at least) aligned 2 (they're at 4 currently). Except.... it would be the linker having to create that field, and there's no way that thing is smart like that :-( Ooh well. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel