[libc-dev] [musl] ppc64le and 32-bit LE userland compatibility

Will Springer via libc-dev libc-dev at lists.llvm.org
Sat May 30 15:56:47 PDT 2020

On Friday, May 29, 2020 12:24:27 PM PDT Rich Felker wrote:
> The argument passing for pread/pwrite is historically a mess and
> differs between archs. musl has a dedicated macro that archs can
> define to override it. But it looks like it should match regardless of
> BE vs LE, and musl already defines it for powerpc with the default
> definition, adding a zero arg to start on an even arg-slot index,
> which is an odd register (since ppc32 args start with an odd one, r3).
> > [6]:
> > https://gist.github.com/Skirmisher/02891c1a8cafa0ff18b2460933ef4f3c
> I don't think this is correct, but I'm confused about where it's
> getting messed up because it looks like it should already be right.

Hmm, interesting. Will have to go back to it I guess...

> > This was enough to fix up the `file` bug. I'm no seasoned kernel
> > hacker, though, and there is still concern over the right way to
> > approach this, whether it should live in the kernel or libc, etc.
> > Frankly, I don't know the ABI structure enough to understand why the
> > register padding has to be different in this case, or what
> > lower-level component is responsible for it.. For comparison, I had a
> > look at the mips tree, since it's bi-endian and has a similar 32/64
> > situation. There is a macro conditional upon endianness that is
> > responsible for munging long longs; it uses __MIPSEB__ and __MIPSEL__
> > instead of an if/else on the generic __LITTLE_ENDIAN__. Not sure what
> > to make of that. (It also simply swaps registers for LE, unlike what
> > I did for ppc.)
> Indeed the problem is probably that you need to swap registers for LE,
> not remove the padding slot. Did you check what happens if you pass a
> value larger than 32 bits?
> If so, the right way to fix this on the kernel side would be to
> construct the value as a union rather than by bitwise ops so it's
> endian-agnostic:
> 	(union { u32 parts[2]; u64 val; }){{ arg1, arg2 }}.val
> But the kernel folks might prefer endian ifdefs for some odd reason...

You are right, this does seem odd considering what the other archs do. 
It's quite possible I made a silly mistake, of course...

I haven't tested with values outside the 32-bit range yet; again, this is 
new territory for me, so I haven't exactly done exhaustive tests on 
everything. I'll give it a closer look.

> > Also worth noting is the one other outstanding bug, where the
> > time-related syscalls in the 32-bit vDSO seem to return garbage. It
> > doesn't look like an endian bug to me, and it doesn't affect standard
> > syscalls (which is why if you run `date` on musl it prints the
> > correct time, unlike on glibc). The vDSO time functions are
> > implemented in ppc asm (arch/powerpc/kernel/vdso32/ gettimeofday.S),
> > and I've never touched the stuff, so if anyone has a clue I'm all
> > ears.
> Not sure about this. Worst-case, just leave it disabled until someone
> finds a fix.

Apparently these asm implementations are being replaced by the generic C 
ones [1], so it may be this fixes itself on its own.

Will [she/her]

[1]: https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=173231

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