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Tue, 28 Apr 2020 19:44:22 -0700 (PDT) Date: Wed, 29 Apr 2020 12:39:22 +1000 From: Nicholas Piggin To: Adhemerval Zanella , Rich Felker Cc: binutils@sourceware.org, Christophe Leroy , libc-alpha@sourceware.org, libc-dev@lists.llvm.org, linuxppc-dev@lists.ozlabs.org, Andy Lutomirski , musl@lists.openwall.com, Thomas Gleixner , Vincenzo Frascino References: <1587790194.w180xsw5be.astroid@bobo.none> <9371cac5-20bb-0552-2609-0d537f41fecd@c-s.fr> <1587810370.tg8ym9yjpc.astroid@bobo.none> <976551e8-229e-54c1-8fb2-c5df94b979c3@c-s.fr> <1587855423.jug0f1n0b8.astroid@bobo.none> <20200425231119.GM11469@brightrain.aerifal.cx> <1587872025.rtx2ygrmn0.astroid@bobo.none> In-Reply-To: MIME-Version: 1.0 Message-Id: <1588126678.zjwj4d1d90.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: [musl] Re: New powerpc vdso calling convention Excerpts from Adhemerval Zanella's message of April 27, 2020 11:09 pm: >=20 >=20 > On 26/04/2020 00:41, Nicholas Piggin wrote: >> Excerpts from Rich Felker's message of April 26, 2020 9:11 am: >>> On Sun, Apr 26, 2020 at 08:58:19AM +1000, Nicholas Piggin wrote: >>>> Excerpts from Christophe Leroy's message of April 25, 2020 10:20 pm: >>>>> >>>>> >>>>> Le 25/04/2020 =C3=A0 12:56, Nicholas Piggin a =C3=A9crit=C2=A0: >>>>>> Excerpts from Christophe Leroy's message of April 25, 2020 5:47 pm: >>>>>>> >>>>>>> >>>>>>> Le 25/04/2020 =C3=A0 07:22, Nicholas Piggin a =C3=A9crit=C2=A0: >>>>>>>> As noted in the 'scv' thread, powerpc's vdso calling convention do= es not >>>>>>>> match the C ELF ABI calling convention (or the proposed scv conven= tion). >>>>>>>> I think we could implement a new ABI by basically duplicating func= tion >>>>>>>> entry points with different names. >>>>>>> >>>>>>> I think doing this is a real good idea. >>>>>>> >>>>>>> I've been working at porting powerpc VDSO to the GENERIC C VDSO, an= d the >>>>>>> main pitfall has been that our vdso calling convention is not compa= tible >>>>>>> with C calling convention, so we have go through an ASM entry/exit. >>>>>>> >>>>>>> See https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series= =3D171469 >>>>>>> >>>>>>> We should kill this error flag return through CR[SO] and get it the >>>>>>> "modern" way like other architectectures implementing the C VDSO: r= eturn >>>>>>> 0 when successfull, return -err when failed. >>>>>> >>>>>> Agreed. >>>>>> >>>>>>>> The ELF v2 ABI convention would suit it well, because the caller a= lready >>>>>>>> requires the function address for ctr, so having it in r12 will >>>>>>>> eliminate the need for address calculation, which suits the vdso d= ata >>>>>>>> page access. >>>>>>>> >>>>>>>> Is there a need for ELF v1 specific calls as well, or could those = just be >>>>>>>> deprecated and remain on existing functions or required to use the= ELF >>>>>>>> v2 calls using asm wrappers? >>>>>>> >>>>>>> What's ELF v1 and ELF v2 ? Is ELF v1 what PPC32 uses ? If so, I'd s= ay >>>>>>> yes, it would be good to have it to avoid going through ASM in the = middle. >>>>>> >>>>>> I'm not sure about PPC32. On PPC64, ELFv2 functions must be called w= ith >>>>>> their address in r12 if called at their global entry point. ELFv1 ha= ve a >>>>>> function descriptor with call address and TOC in it, caller has to l= oad >>>>>> the TOC if it's global. >>>>>> >>>>>> The vdso doesn't have TOC, it has one global address (the vdso data >>>>>> page) which it loads by calculating its own address. >>>>>> >>>>>> The kernel doesn't change the vdso based on whether it's called by a= v1 >>>>>> or v2 userspace (it doesn't really know itself and would have to exp= ort >>>>>> different functions). glibc has a hack to create something: >>>>>> >>>>>> # define VDSO_IFUNC_RET(value) \ >>>>>> ({ \ >>>>>> static Elf64_FuncDesc vdso_opd =3D { .fd_toc =3D ~0x0 }; \ >>>>>> vdso_opd.fd_func =3D (Elf64_Addr)value; \ >>>>>> &vdso_opd; \ >>>>>> }) >>>>>> >>>>>> If we could make something which links more like any other dso with >>>>>> ELFv1, that would be good. Otherwise I think v2 is preferable so it >>>>>> doesn't have to calculate its own address. >>>>> >>>>> I see the following in glibc. So looks like PPC32 is like PPC64 elfv1= .=20 >>>>> By the way, they are talking about something not completely finished = in=20 >>>>> the kernel. Can we finish it ? >>>> >>>> Possibly can. It seems like a good idea to fix all loose ends if we ar= e=20 >>>> going to add new versions. Will have to check with the toolchain peopl= e=20 >>>> to make sure we're doing the right thing. >>> >>> "ELFv1" and "ELFv2" are PPC64-specific names for the old and new >>> version of the ELF psABI for PPC64. They have nothing at all to do >>> with PPC32 which is a completely different ABI from either. >>=20 >> Right, I'm just talking about those comments -- it seems like the kernel= =20 >> vdso should contain an .opd section with function descriptors in it for >> elfv1 calls, rather than the hack it has now of creating one in the=20 >> caller's .data section. >>=20 >> But all that function descriptor code is gated by >>=20 >> #if (defined(__PPC64__) || defined(__powerpc64__)) && _CALL_ELF !=3D 2 >>=20 >> So it seems PPC32 does not use function descriptors but a direct pointer= =20 >> to the entry point like PPC64 with ELFv2. >=20 > Yes, this hack is only for ELFv1. The missing ODP has not been an issue=20 > or glibc because it has been using the inline assembly to emulate the=20 > functions call since initial vDSO support (INTERNAL_VSYSCALL_CALL_TYPE). > It just has become an issue when I added a ifunc optimization to=20 > gettimeofday so it can bypass the libc.so and make plt branch to vDSO=20 > directly. I can't understand if it's actually a problem for you or not. Regardless if you can hack around it, it seems to me that if we're going=20 to add sane calling conventions to the vdso, then we should also just=20 have a .opd section for it as well, whether or not a particular libc=20 requires it. > Recently on some y2038 refactoring it was suggested to get rid of this=20 > and make gettimeofday call clock_gettime regardless. But some felt that=20 > the performance degradation was not worth for a symbol that is still used > extensibility, so we stuck with the hack. >=20 > And I think having this synthetic opd entry is not an issue, since for=20 > full relro the program's will be used and correctly set as read-only. I'm not quite sure what this means, I don't really know how glibc ifunc=20 works. How do you set r2 if you have no opd? > The issue is more for glibc itself, and I wouldn't mind to just remove th= e > gettimeofday and time optimizations and use the default vDSO support > (which might increase in function latency though). >=20 > As Rich has put, it would be simpler to just have powerpc vDSO symbols > to have a default function call semantic so we could issue a function > call directly. But for powerpc64, we glibc will need to continue to=20 > support this non-standard call on older kernels and I am not sure if > adding new symbols with a different semantic will help much. Yeah, we will add entry points with default function call semantics. At which point we make the things look like any other dso unless there=20 is good reason otherwise. > GLibc already hides this powerpc semantic on INTERNAL_VSYSCALL_CALL_TYPE, > so internally all syscalls are assumed to have the new semantic (-errno > on error, 0 on success). Adding another ELFv1 would require to add > more logic to handle multiple symbol version for vDSO setup > (sysdeps/unix/sysv/linux/dl-vdso-setup.h), which would mostly likely to > require an arch-specific implementation to handle it. Is it not a build-time choice? The arch can set its own vdso symbol=20 names AFAIKS. Thanks, Nick