Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Rich Felker <dalias@libc.org>]

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