Date: Thu, 23 Apr 2020 13:42:14 -0400 From: Rich Felker <dalias@...c.org> To: Adhemerval Zanella <adhemerval.zanella@...aro.org> Cc: Nicholas Piggin <npiggin@...il.com>, libc-alpha@...rceware.org, libc-dev@...ts.llvm.org, linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com Subject: Re: Powerpc Linux 'scv' system call ABI proposal take 2 On Thu, Apr 23, 2020 at 02:15:58PM -0300, Adhemerval Zanella wrote: > > > On 23/04/2020 13:43, Rich Felker wrote: > > On Thu, Apr 23, 2020 at 01:35:01PM -0300, Adhemerval Zanella wrote: > >> > >> > >> On 23/04/2020 13:18, Rich Felker wrote: > >>> On Thu, Apr 23, 2020 at 09:13:57AM -0300, Adhemerval Zanella wrote: > >>>> > >>>> > >>>> On 22/04/2020 23:36, Rich Felker wrote: > >>>>> On Wed, Apr 22, 2020 at 04:18:36PM +1000, Nicholas Piggin wrote: > >>>>>> Yeah I had a bit of a play around with musl (which is very nice code I > >>>>>> must say). The powerpc64 syscall asm is missing ctr clobber by the way. > >>>>>> Fortunately adding it doesn't change code generation for me, but it > >>>>>> should be fixed. glibc had the same bug at one point I think (probably > >>>>>> due to syscall ABI documentation not existing -- something now lives in > >>>>>> linux/Documentation/powerpc/syscall64-abi.rst). > >>>>> > >>>>> Do you know anywhere I can read about the ctr issue, possibly the > >>>>> relevant glibc bug report? I'm not particularly familiar with ppc > >>>>> register file (at least I have to refamiliarize myself every time I > >>>>> work on this stuff) so it'd be nice to understand what's > >>>>> potentially-wrong now. > >>>> > >>>> My understanding is the ctr issue only happens for vDSO calls where it > >>>> fallback to a syscall in case an error (invalid argument, etc. and > >>>> assuming if vDSO does not fallback to a syscall it always succeed). > >>>> This makes the vDSO call on powerpc to have same same ABI constraint > >>>> as a syscall, where it clobbers CR0. > >>> > >>> I think you mean "vsyscall", the old thing glibc used where there are > >>> in-userspace implementations of some syscalls with call interfaces > >>> roughly equivalent to a syscall. musl has never used this. It only > >>> uses the actual exported functions from the vdso which have normal > >>> external function call ABI. > >> > >> I wasn't thinking in vsyscall in fact, which afaik it is a x86 thing. > >> The issue is indeed when calling the powerpc provided functions in > >> vDSO, which musl might want to do eventually. > > > > AIUI (at least this is true for all other archs) the functions have > > normal external function call ABI and calling them has nothing to do > > with syscall mechanisms. > > My point is powerpc specifically does not follow it, since it issues a > syscall in fallback and its semantic follow kernel syscalls (error > signalled in cr0, r3 being always a positive value): Oh, then I think we'll just ignore these unless the kernel can make ones with a reasonable ABI. It's not worth having ppc-specific code for this... It would be really nice if ones that actually behave like functions could be added though. > -- > V_FUNCTION_BEGIN(__kernel_clock_gettime) > .cfi_startproc > [...] > /* > * syscall fallback > */ > 99: > li r0,__NR_clock_gettime > .cfi_restore lr > sc > blr > .cfi_endproc > V_FUNCTION_END(__kernel_clock_gettime) > > > > > > It looks like we're not using them right now and I'm not sure why. It > > could be that there are ABI mismatch issues (are 32-bit ones > > compatible with secure-plt? are 64-bit ones compatible with ELFv2?) or > > just that nobody proposed adding them. Also as of 5.4 32-bit ppc > > lacked time64 versions of them; not sure if this is fixed yet. > > For 64-bit it also have an issue where vDSO does not provide an OPD > for ELFv1, which has bitten glibc while trying to implement an ifunc > optimization. I don't recall any issue for ELFv2. > > For 32-bit I am not sure secure-plt will change anything, at least not > on powerpc where we use the same strategy for 64-bit and use a > mtctr/bctr directly. Indeed, I don't think there's a secure-plt distinction unless you're making outgoing calls to possibly-cross-DSO functions. Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.