Support SHF_GNU_RETAIN ELF section flag

The SHF_GNU_RETAIN section flag is an extension to the GNU ELF OSABI.
It is defined as follows:

=========================================================
Section Attribute Flags
+-------------------------------------+
| Name           | Value              |
+-------------------------------------+
| SHF_GNU_RETAIN | 0x200000 (1 << 21) |
+-------------------------------------+

SHF_GNU_RETAIN
  The link editor should not garbage collect the section.
=========================================================

The .section directive accepts the "R" flag, which indicates
SHF_GNU_RETAIN should be applied to the section.

There is not a direct mapping of SHF_GNU_RETAIN to the BFD
section flag SEC_KEEP. Keeping these flags distinct allows
SHF_GNU_RETAIN sections to be explicitly removed by placing them in
/DISCARD/.

bfd/ChangeLog:

	* elf-bfd.h (enum elf_gnu_osabi): Add elf_gnu_osabi_retain.
	(struct elf_obj_tdata): Increase has_gnu_osabi to 4 bits.
	* elf.c (_bfd_elf_make_section_from_shdr): Set elf_gnu_osabi_retain
	for SHF_GNU_RETAIN.
	(_bfd_elf_final_write_processing): Report if SHF_GNU_RETAIN is
	not supported by the OSABI.
	Adjust error messages.
	* elflink.c (elf_link_input_bfd): Copy enabled has_gnu_osabi bits from
	input BFD to output BFD.
	(bfd_elf_gc_sections): gc_mark the section if SHF_GNU_RETAIN is set.

binutils/ChangeLog:

	* NEWS: Announce SHF_GNU_RETAIN support.
	* readelf.c (get_elf_section_flags): Handle SHF_GNU_RETAIN.
	Recognize SHF_GNU_RETAIN and SHF_GNU_MBIND only for supported OSABIs.
	* testsuite/binutils-all/readelf.exp: Run new tests.
	Don't run run_dump_test when there isn't an assembler available.
	* testsuite/lib/binutils-common.exp (supports_gnu_osabi): Adjust
	comment.
	* testsuite/binutils-all/readelf-maskos-1a.d: New test.
	* testsuite/binutils-all/readelf-maskos-1b.d: New test.
	* testsuite/binutils-all/readelf-maskos.s: New test.
	* testsuite/binutils-all/retain1.s: New test.
	* testsuite/binutils-all/retain1a.d: New test.
	* testsuite/binutils-all/retain1b.d: New test.

gas/ChangeLog:

	* NEWS: Announce SHF_GNU_RETAIN support.
	* config/obj-elf.c (obj_elf_change_section): Merge SHF_GNU_RETAIN bit
	between section declarations.
	(obj_elf_parse_section_letters): Handle 'R' flag.
	Handle numeric flag values within the SHF_MASKOS range.
	(obj_elf_section): Validate SHF_GNU_RETAIN usage.
	* doc/as.texi: Document 'R' flag to .section directive.
	* testsuite/gas/elf/elf.exp: Run new tests.
	* testsuite/gas/elf/section10.d: Unset SHF_GNU_RETAIN bit.
	* testsuite/gas/elf/section10.s: Likewise.
	* testsuite/gas/elf/section22.d: New test.
	* testsuite/gas/elf/section22.s: New test.
	* testsuite/gas/elf/section23.s: New test.
	* testsuite/gas/elf/section23a.d: New test.
	* testsuite/gas/elf/section23b.d: New test.
	* testsuite/gas/elf/section23b.err: New test.
	* testsuite/gas/elf/section24.l: New test.
	* testsuite/gas/elf/section24.s: New test.
	* testsuite/gas/elf/section24a.d: New test.
	* testsuite/gas/elf/section24b.d: New test.

include/ChangeLog:

	* elf/common.h (SHF_GNU_RETAIN): Define.

ld/ChangeLog:

	* NEWS: Announce support for SHF_GNU_RETAIN.
	* ld.texi (garbage collection): Document SHF_GNU_RETAIN.
	(Output Section Discarding): Likewise.
	* testsuite/ld-elf/elf.exp: Run new tests.
	* testsuite/ld-elf/retain1.s: New test.
	* testsuite/ld-elf/retain1a.d: New test.
	* testsuite/ld-elf/retain1b.d: New test.
	* testsuite/ld-elf/retain2.d: New test.
	* testsuite/ld-elf/retain2.ld: New test.
	* testsuite/ld-elf/retain2.map: New test.
	* testsuite/ld-elf/retain3.d: New test.
	* testsuite/ld-elf/retain3.s: New test.
	* testsuite/ld-elf/retain4.d: New test.
	* testsuite/ld-elf/retain4.s: New test.
	* testsuite/ld-elf/retain5.d: New test.
	* testsuite/ld-elf/retain5.map: New test.
	* testsuite/ld-elf/retain5lib.s: New test.
	* testsuite/ld-elf/retain5main.s: New test.
	* testsuite/ld-elf/retain6a.d: New test.
	* testsuite/ld-elf/retain6b.d: New test.
	* testsuite/ld-elf/retain6lib.s: New test.
	* testsuite/ld-elf/retain6main.s: New test.

1 files changed, 15 insertions, 6 deletions

diff --git a/bfd/elf.c b/bfd/elf.c
index 3163d34..288548d 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -1066,9 +1066,12 @@ _bfd_elf_make_section_from_shdr (bfd *abfd,
       /* FIXME: We should not recognize SHF_GNU_MBIND for ELFOSABI_NONE,
 	 but binutils as of 2019-07-23 did not set the EI_OSABI header
 	 byte.  */
-    case ELFOSABI_NONE:
     case ELFOSABI_GNU:
     case ELFOSABI_FREEBSD:
+      if ((hdr->sh_flags & SHF_GNU_RETAIN) != 0)
+	elf_tdata (abfd)->has_gnu_osabi |= elf_gnu_osabi_retain;
+      /* Fall through */
+    case ELFOSABI_NONE:
       if ((hdr->sh_flags & SHF_GNU_MBIND) != 0)
 	elf_tdata (abfd)->has_gnu_osabi |= elf_gnu_osabi_mbind;
       break;
@@ -12456,8 +12459,8 @@ _bfd_elf_final_write_processing (bfd *abfd)
     i_ehdrp->e_ident[EI_OSABI] = get_elf_backend_data (abfd)->elf_osabi;
 
   /* Set the osabi field to ELFOSABI_GNU if the binary contains
-     SHF_GNU_MBIND sections or symbols of STT_GNU_IFUNC type or
-     STB_GNU_UNIQUE binding.  */
+     SHF_GNU_MBIND or SHF_GNU_RETAIN sections or symbols of STT_GNU_IFUNC type
+     or STB_GNU_UNIQUE binding.  */
   if (elf_tdata (abfd)->has_gnu_osabi != 0)
     {
       if (i_ehdrp->e_ident[EI_OSABI] == ELFOSABI_NONE)
@@ -12466,11 +12469,17 @@ _bfd_elf_final_write_processing (bfd *abfd)
 	       && i_ehdrp->e_ident[EI_OSABI] != ELFOSABI_FREEBSD)
 	{
 	  if (elf_tdata (abfd)->has_gnu_osabi & elf_gnu_osabi_mbind)
-	    _bfd_error_handler (_("GNU_MBIND section is unsupported"));
+	    _bfd_error_handler (_("GNU_MBIND section is supported only by GNU "
+				  "and FreeBSD targets"));
 	  if (elf_tdata (abfd)->has_gnu_osabi & elf_gnu_osabi_ifunc)
-	    _bfd_error_handler (_("symbol type STT_GNU_IFUNC is unsupported"));
+	    _bfd_error_handler (_("symbol type STT_GNU_IFUNC is supported "
+				  "only by GNU and FreeBSD targets"));
 	  if (elf_tdata (abfd)->has_gnu_osabi & elf_gnu_osabi_unique)
-	    _bfd_error_handler (_("symbol binding STB_GNU_UNIQUE is unsupported"));
+	    _bfd_error_handler (_("symbol binding STB_GNU_UNIQUE is supported "
+				  "only by GNU and FreeBSD targets"));
+	  if (elf_tdata (abfd)->has_gnu_osabi & elf_gnu_osabi_retain)
+	    _bfd_error_handler (_("GNU_RETAIN section is supported "
+				  "only by GNU and FreeBSD targets"));
 	  bfd_set_error (bfd_error_sorry);
 	  return FALSE;
 	}