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

[Adhemerval Zanella]

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.