[almost patch] elf(5): Add vDSO note Linux 4 0

[Krzysztof Żelechowski <krzysztof.zelechowski@xxxxxxxxxxxxxx>]

Please insert this almost patch somewhere under .IR n_type, maybe before .B Default/unknown namespace (e_type != ET_CORE):

.TP
      .\" commit 7648b1330c335601b7c09c25f77a03cda128fcab
.B n_name = Linux (soname = linux-vdso or such)
.RS
.TP 12
.PD 0
.B n_type = 0
Linux kernel version in the desc field (ElfN_Word), to be matched against the GNU ABI tag, see vdso(7).
.PD
.RE

Signed-off-my: Christopher Yeleighton <giecrilj@xxxxxxxxxxxx>

. PLEASE NOTE: for nontrivial patches, you can save the maintainers a lot of time by attending to the following:
o Describe how you obtained the information in your patch. For example, was it:
. by reading (or writing) the relevant kernel or (g)libc source code? (please provide a pointer to the relevant code)
At arch/x86/entry/vdso/vdso-note.S
. by writing a test program? (send it with the patch, but please make sure it is as simple as possible, and provide instructions on how to use and/or a demo run)

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <sys/auxv.h>
#include <elf.h>

struct sysnames { char write [06]; } const sysnames = { "write" };
int 
main (int argc, char const *const argv [])
{ Elf64_Ehdr
    const *const ph = (Elf64_Ehdr const *)
    getauxval (AT_SYSINFO_EHDR)
    ;
  if (printf ("Reading vDSO at %p\n", ph)
      < 0)
    { perror (sysnames .write); return EXIT_FAILURE; }
  if (ph)
    {
      uint16_t const shnum = ph ->e_shnum, shstrndx = ph ->e_shstrndx;
      ; Elf64_Off const shoff = ph ->e_shoff;
      Elf64_Shdr const *const sh_B
	= (Elf64_Shdr const *) (
				ph ->e_ident + shoff)
	; Elf64_Shdr const *const
	    sh_E = sh_B + shnum;
      char const *const shstr = 
	ph ->e_ident +
	sh_B [shstrndx] .sh_offset
	;
      if (SHN_UNDEF == shstrndx || SHN_XINDEX
	  == shstrndx) return EXIT_FAILURE; 
      assert (ELFMAG0 == ph ->e_ident [EI_MAG0
				       ]);
      assert (ELFMAG1 == ph ->e_ident [EI_MAG1]);
      assert (ELFMAG2 == ph ->e_ident [EI_MAG2
				       ]);
      assert (ELFMAG3
	      == ph ->e_ident [EI_MAG3
			       ]);
      if (printf ("%#o section(s) at offset %#o\n", shnum, shoff
		  ) < 0)
	{ perror (sysnames .write); return EXIT_FAILURE; }
      for (Elf64_Shdr const *
	     psh
	     = sh_B
	     ; psh
	     < sh_E
	     ; ++ psh
	   ) {
	if (SHT_NOTE == psh ->sh_type
	    )
	  {
	    char const *const n_B = ph ->e_ident + psh ->sh_offset;
	    uint32_t sz = psh ->sh_size;
	    if (printf ("section %s size %#o\n"
			, shstr + psh ->sh_name, sz
			) < 0)
	      { perror (sysnames .write); return EXIT_FAILURE; }
	    for (
		 uint32_t
		   o1 = 0; o1 < sz; ) {
	      Elf64_Nhdr const *const note =
		(Elf64_Nhdr const *) (
				      n_B + o1
				      )
		; Elf64_Word const nsz = note ->n_namesz, dsz = note ->n_descsz
		    ;
	      if (printf ("note %#o %s(%#o) %#o\n"
			  , nsz,
			  note + 01,
			  note ->n_type
			  ,
			  dsz
			  ) < 0)
		{ perror (sysnames .write); return EXIT_FAILURE; }
	      o1 +=
		sizeof *note + ((nsz + 03) & ~ 03);
	      if (sizeof (int)
		  == dsz)
		if (printf ("version %#o\n"
			    , * (int const *) (n_B + o1)) < 0)
		  { perror (sysnames .write); return EXIT_FAILURE; }
	      if (printf ("off %#o %#o %#o %#o\n"
			  , sizeof *note, nsz, dsz,
			  o1 +=
			  + ((dsz
			      + 03) & ~ 03)) < 0)
		{ perror (sysnames
			  .write); return EXIT_FAILURE
				     ; }
	    }
	  }
      }
    }
  return EXIT_SUCCESS; }