Re: port-amd64/54052: bump STACK_ALIGNBYTES for COMPAT_LINUX

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

On 2019/03/12 7:54, Joerg Sonnenberger wrote:
> On Mon, Mar 11, 2019 at 05:50:00AM +0000, rokuyama.rk%gmail.com@localhost wrote:
> > > Description:
> > Linux binaries with glibc >= 2.23 randomly crashes in dynamic linker.
> > By bisectioning, the cause turns out to be this commit
> >
> > https://github.molgen.mpg.de/git-mirror/glibc/commit/38d22f9f48a84b441c5777aff103f5b980243b5f
>
> So the real problem is that ld.so doesn't do what any normal startup
> code does by aligning the stack explicitly. *sigh*

I don't get what you means...

(1) The bottom of stack (i.e., %rsp = &argc) is required to be aligned
to 16-byte boundary by "System V ABI - AMD64 Architecture Processor
Supplement"

https://www.uclibc.org/docs/psABI-x86_64.pdf

(see pp. 29-30).

(2) However, we align it to only 8-byte boundary; we don't define
STACK_ALIGNBYTES for amd64, and __ALIGNBYTES = (8 - 1) is used instead:

src/sys/kern/kern_exec.c
https://nxr.netbsd.org/xref/src/sys/kern/kern_exec.c#1394

  1394  static size_t
  1395  calcstack(struct execve_data * restrict data, const size_t gaplen)
  1396  {
  ....
  1415          /* make the stack "safely" aligned */
  1416          return STACK_LEN_ALIGN(stacklen, STACK_ALIGNBYTES);
  1417  }

(3) If the bottom of stack is aligned to 16-byte boundary, ld.so for
Linux works fine.

Therefore, I think that ld.so is legal within System V ABI. Isn't it?

Thanks,
rin