Re: [gofrontend-dev] Re: libgo patch committed: Use a C function to call mmap

[Cherry Mui <cherryyz@google.com>]

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On Tue, Jun 20, 2023 at 3:37 PM Ian Lance Taylor <iant@golang.org> wrote:

> On Tue, Jun 20, 2023 at 11:35 AM Andreas Schwab <schwab@linux-m68k.org>
> wrote:
> >
> > On Jun 20 2023, Ian Lance Taylor via Gcc-patches wrote:
> >
> > > This libgo patches changes the runtime pacakge to use a C function to
> call mmap.
> > >
> > > The final argument to mmap, of type off_t, varies. In
> > > https://go.dev/cl/445375
> > > (https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604158.html)
> > > we changed it to always use the C off_t type, but that broke 32-bit
> > > big-endian Linux systems.
> >
> > This has nothing to do with big-endian, armv7 isn't big-endian.
>
> OK, but I think that it does have something to do with big-endian.
> The bug was that on some 32-bit systems it was passing a 64-bit value
> to a function that expected a 32-bit value.  The problem didn't show
> up on 32-bit x86 because it is little-endian, and did show up on
> 32-bit PPC because it is big-endian.  I guess the armv7 case was
> failing for a different reason.


I think there is a calling convention issue. On 32-bit ARM, for the case of
mmap, if the last argument is 32-bit, it is passed 4 bytes at sp+4. If it
is 64-bit, the offset is aligned and it is stored as 8 bytes at sp+8. So if
the callee tries to read at sp+4, it gets the wrong value, even for little
endian. On 32-bit x86 it doesn't seem to have that alignment padding.