From: Andy Lutomirski Subject: Re: [RFC PATCH 1/9] kernel: add support for patchable function pointers Date: Fri, 5 Oct 2018 09:58:23 -0700 Message-ID: References: <20181005081333.15018-1-ard.biesheuvel@linaro.org> <20181005081333.15018-2-ard.biesheuvel@linaro.org> <20181005141433.GS19272@hirez.programming.kicks-ass.net> <9E0E08C8-0DFC-4E50-A4FA-73208835EF9E@amacapital.net> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Cc: Peter Zijlstra , LKML , "Jason A. Donenfeld" , Eric Biggers , Samuel Neves , Andrew Lutomirski , Arnd Bergmann , Herbert Xu , "David S. Miller" , Catalin Marinas , Will Deacon , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Thomas Gleixner , Ingo Molnar , Kees Cook , "Martin K. Petersen" , Greg KH , Andrew Mort To: Ard Biesheuvel Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-crypto.vger.kernel.org On Fri, Oct 5, 2018 at 8:24 AM Ard Biesheuvel w= rote: > > On 5 October 2018 at 17:08, Andy Lutomirski wrote: > > > > > >> On Oct 5, 2018, at 7:14 AM, Peter Zijlstra wrot= e: > >> > >>> On Fri, Oct 05, 2018 at 10:13:25AM +0200, Ard Biesheuvel wrote: > >>> diff --git a/include/linux/ffp.h b/include/linux/ffp.h > >>> new file mode 100644 > >>> index 000000000000..8fc3b4c9b38f > >>> --- /dev/null > >>> +++ b/include/linux/ffp.h > >>> @@ -0,0 +1,43 @@ > >>> +/* SPDX-License-Identifier: GPL-2.0 */ > >>> + > >>> +#ifndef __LINUX_FFP_H > >>> +#define __LINUX_FFP_H > >>> + > >>> +#include > >>> +#include > >>> + > >>> +#ifdef CONFIG_HAVE_ARCH_FFP > >>> +#include > >>> +#else > >>> + > >>> +struct ffp { > >>> + void (**fn)(void); > >>> + void (*default_fn)(void); > >>> +}; > >>> + > >>> +#define DECLARE_FFP(_fn, _def) \ > >>> + extern typeof(_def) *_fn; \ > >>> + extern struct ffp const __ffp_ ## _fn > >>> + > >>> +#define DEFINE_FFP(_fn, _def) \ > >>> + typeof(_def) *_fn =3D &_def; \ > >>> + struct ffp const __ffp_ ## _fn \ > >>> + =3D { (void(**)(void))&_fn, (void(*)(void))&_def }; \ > >>> + EXPORT_SYMBOL(__ffp_ ## _fn) > >>> + > >>> +static inline void ffp_set_target(const struct ffp *m, void *new_fn) > >>> +{ > >>> + WRITE_ONCE(*m->fn, new_fn); > >>> +} > >>> + > >>> +static inline void ffp_reset_target(const struct ffp *m) > >>> +{ > >>> + WRITE_ONCE(*m->fn, m->default_fn); > >>> +} > >>> + > >>> +#endif > >>> + > >>> +#define SET_FFP(_fn, _new) ffp_set_target(&__ffp_ ## _fn, _new) > >>> +#define RESET_FFP(_fn) ffp_reset_target(&__ffp_ ## _fn) > >>> + > >>> +#endif > >> > >> I don't understand this interface. There is no wrapper for the call > >> site, so how are we going to patch all call-sites when you update the > >> target? > > > > I=E2=80=99m also confused. > > > > Anyway, we have patchable functions on x86. They=E2=80=99re called PVOP= s, and they=E2=80=99re way overcomplicated. > > > > I=E2=80=99ve proposed a better way that should generate better code, be= more portable, and be more maintainable. It goes like this. > > > > To call the function, you literally just call the default implementati= on. It *might* be necessary to call a nonexistent wrapper to avoid annoyin= g optimizations. At build time, the kernel is built with relocations, so th= e object files contain relocation entries for the call. We collect these en= tries into a table. If we=E2=80=99re using the =E2=80=9Cnonexistent wrapper= =E2=80=9D approach, we can link in a .S or linker script to alias them to t= he default implementation. > > > > To patch them, we just patch them. It can=E2=80=99t necessarily be done= concurrently because nothing forces the right alignment. But we can do it = at boot time and module load time. (Maybe we can patch at runtime on archit= ectures with appropriate instruction alignment. Or we ask gcc for an exten= sion to align calls to a function.) > > > > Most of the machinery already exists: this is roughly how the module lo= ader resolves calls outside of a module. > > Yeah nothing is ever simple on x86 :-( > > So are you saying the approach i use in patch #2 (which would > translate to emitting a jmpq instruction pointing to the default > implementation, and patching it at runtime to point elsewhere) would > not fly on x86? After getting some more sleep, I'm obviously wrong. The text_poke_bp() mechanism will work. It's just really slow. Let me try to summarize some of the issues. First, when emitting jumps and calls from inline asm on x86, there are a few considerations that are annoying: 1. Following the x86_64 ABI calling conventions is basically impossible. x86_64 requires a 128-byte redzone and 16-byte stack alignment. After much discussion a while back, we decided that it was flat-out impossible on current gcc to get the stack pointer aligned in a known manner in an inline asm statement. Instead, if we actually need alignment, we need to align manually. Fortunately, the kernel is built with an override that forces only 8-byte alignment (on *most* GCC versions). But for crypto in particular, it sucks extra, since the crypto code is basically the only thing in the kernel that actually wants 16-byte alignment. I don't think this is a huge problem in practice, but it's annoying. And the kernel is built without a redzone. 2. On x86_64, depending on config, we either need frame pointers or ORC. ORC is no big deal -- it Just Works (tm). Frame pointers need extra asm hackery. It's doable, but it's still annoying. 3. Actually getting the asm constraints right to do what a C programmer expects is distinctly nontrivial. I just fixed an extremely longstanding bug in the vDSO code in which the asm constraints for the syscall fallback were wrong in such a way that GCC didn't notice that the fallback wrote to its output parameter. Whoops. And having all this asm hackery per architecture is ugly and annoying. So my suggestion is to do it like a regular relocation. Call a function the normal way (make it literally be a C function and call it), and rig up the noinline and noclone attributes and whatever else is needed to make sure that it's a *relocatable* call. Then the toolchain emits ELF relocations saying exactly what part of the text needs patching, and we can patch it at runtime. On x86, this is a bit extra annoying because we can't fully reliably parse backwards to find the beginning of the instruction, but objtool could doit. And then we get something that is mostly arch-neutral! Because surely ARM can also use a relocation-based mechanism. I will generally object to x86 containing more than one inline-asm-hackery-based patchable call mechanism, which your series will add. I would *love* to see a non-inline-asm one, and then we could move most of the x86 paravirt crap over to use it for a big win in readability and maintainability. --Andy