debugging: define ElfW when the target has no <link.h>

[vasko-110]

Fixes #2153.

Per review: instead of disabling ABSL_HAVE_ELF_MEM_IMAGE on targets without a dynamic loader, define ElfW ourselves when <link.h> is missing. newlib ships <elf.h>, so that is enough to build.

Two details differ from the snippet in the review. __SIZEOF_POINTER__ == 8 replaces the architecture list, which covers ppc64/s390x/mips64/loongarch64 as well. And the FreeBSD branch keeps defined(__FreeBSD__): __ELF_NATIVE_CLASS is a glibc macro, and by the time we reach this block glibc's <link.h> has already defined ElfW, so it would never fire — while FreeBSD would silently lose __ElfN.

The platform deny-list is untouched.

Verified: elf_mem_image.cc and vdso_support.cc compile under arm-none-eabi-g++, and the host is unchanged — ElfW still comes from <link.h> there.

[mkruskal-google]

Instead of just disabling this logic for bare-metal, we could add support like we did for FreeBSD. Something like this seems like it should work, if we also condition the include of link.h:

#ifndef ElfW
#  if defined(__ELF_NATIVE_CLASS)
     /* FreeBSD native infrastructure uses __ElfN */
#    define ElfW(type) __ElfN(type)
#  elif defined(__x86_64__) || defined(__aarch64__) || defined(__riscv) && (__riscv_xlen == 64)
     /* Fallback block for 64-bit bare-metal environments */
#    define ElfW(type) Elf64_##type
#  else
     /* Fallback block for 32-bit bare-metal environments */
#    define ElfW(type) Elf32_##type
#  endif
#endif```

[vasko-110]

You're right, it builds — newlib ships <elf.h> even though it has no <link.h>, so this is enough:

#if __has_include(<link.h>)
#include <link.h>  // for ElfW
#else
#include <elf.h>
#endif

#ifndef ElfW
#if defined(__FreeBSD__)
#define ElfW(type) __ElfN(type)
#elif __SIZEOF_POINTER__ == 8
#define ElfW(type) Elf64_##type
#else
#define ElfW(type) Elf32_##type
#endif
#endif

Two changes from yours. __ELF_NATIVE_CLASS is glibc's, not FreeBSD's — glibc's link.h defines ElfW before we get here, so that branch never fires, and FreeBSD would quietly lose __ElfN. And __SIZEOF_POINTER__ saves listing ppc64/s390x/mips64/loongarch64.

I'd still rather keep it off here, though. VDSOSupport::Init() needs either getauxval or /proc/self/auxv, so IsPresent() is always false on bare metal — that's 5 KB of .text and open/read/close in the link for a lookup that can't succeed.

Either way works for me, which do you prefer?

[mkruskal-google]

I think either making it work for bare metal, or going your original route with some cleanup (instead of lumping on more complexity) would be preferable. I don't have a strong preference between those, but I'd rather not add this catch-all and leave this list of platforms that presumably don't have link.h either

[vasko-110]

Updated.

Went with your approach. I can't test VxWorks/Hexagon/Xtensa/wasm/Haiku here, so pruning their guards would have been guesswork — this way the list doesn't need to change at all.

Kept two things from my last comment: __SIZEOF_POINTER__ == 8 instead of the arch list, and defined(__FreeBSD__) for the __ElfN branch.