Re: [PATCH 08/12] Rewrite abi-note.S in C.

[Zack Weinberg <zackw@panix.com>]

On Thu, Apr 30, 2020 at 1:43 PM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> Using C code with __asm() allows the compiler to add target
> specific object file markings based on CFLAGS.
> This is e.g. needed for building glibc with branch-protection
> on AArch64.

Hmm.  If we're going to do this, should we maybe go further and use
actual C?  This snippet produces a matching .note.ABI-tag section for
me:

#include <stdint.h>
#include <config.h>
#include <endian.h>
#include <abi-tag.h>
const int32_t
__attribute__((section(".note.ABI-tag")))
__abi_tag[] = {
  4,   /* name length: 4  */
  16,  /* data length: 4 32-bit numbers */
  1,
#if BYTE_ORDER == BIG_ENDIAN
  0x474E5500, /* "GNU\0" */
#else
  0x00554E47, /* same, little-endian */
#endif
  __ABI_TAG_OS,
  __ABI_TAG_VERSION
};

I don't see a problem with hardwiring a name length of 4, and code
elsewhere appears to assume that the payload will always be 4 32-bit
integers, so maybe it's OK to hardwire the 16 too.  If it's not OK,
perhaps we could figure out a way to count the commas in
__ABI_TAG_VERSION in the preprocessor and do a little math.

The other potential problem I can think of is that there's a symbol
pointing into the .note.ABI-tag section now; I don't know if that
might break anything.

zw