Date: Thu, 23 Apr 2020 14:15:58 -0300 From: Adhemerval Zanella <adhemerval.zanella@...aro.org> To: Rich Felker <dalias@...c.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 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): -- 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.
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