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.

16 files changed, 250 insertions, 22 deletions

diff --git a/gas/ChangeLog b/gas/ChangeLog
index cdbc1ca..fcfce70 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,26 @@
+2020-11-18  Jozef Lawrynowicz  <jozef.l@mittosystems.com>
+	H.J. Lu  <hongjiu.lu@intel.com>
+
+	* 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.s: New test.
+	* testsuite/gas/elf/section24a.d: New test.
+	* testsuite/gas/elf/section24b.d: New test.
+
 2020-11-13  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>
 
 	* NEWS: Update news.
diff --git a/gas/NEWS b/gas/NEWS
index 6969d1c..f44861c 100644
--- a/gas/NEWS
+++ b/gas/NEWS
@@ -52,6 +52,11 @@
 
 * Configure with --enable-x86-used-note by default for Linux/x86.
 
+* Add support for the SHF_GNU_RETAIN flag, which can be applied to
+  sections using the 'R' flag in the .section directive.
+  SHF_GNU_RETAIN specifies that the section should not be garbage
+  collected by the linker.  It requires the GNU or FreeBSD ELF OSABIs.
+
 Changes in 2.35:
 
 * X86 NaCl target support is removed.
diff --git a/gas/config/obj-elf.c b/gas/config/obj-elf.c
index 7aed812..54d42d9 100644
--- a/gas/config/obj-elf.c
+++ b/gas/config/obj-elf.c
@@ -806,9 +806,17 @@ obj_elf_change_section (const char *name,
 		as_bad (_("changed section attributes for %s"), name);
 	    }
 	  else
-	    /* FIXME: Maybe we should consider removing a previously set
-	       processor or application specific attribute as suspicious ?  */
-	    elf_section_flags (sec) = attr;
+	    {
+	      /* Don't overwrite a previously set SHF_GNU_RETAIN flag for the
+		 section.  The entire section must be marked retained.  */
+	      if ((elf_tdata (stdoutput)->has_gnu_osabi & elf_gnu_osabi_retain)
+		  && ((elf_section_flags (old_sec) & SHF_GNU_RETAIN)))
+		attr |= SHF_GNU_RETAIN;
+
+	      /* FIXME: Maybe we should consider removing a previously set
+		 processor or application specific attribute as suspicious ?  */
+	      elf_section_flags (sec) = attr;
+	    }
 
 	  if ((flags & SEC_MERGE) && old_sec->entsize != (unsigned) entsize)
 	    as_bad (_("changed section entity size for %s"), name);
@@ -861,6 +869,9 @@ obj_elf_parse_section_letters (char *str, size_t len,
 	case 'd':
 	  *gnu_attr |= SHF_GNU_MBIND;
 	  break;
+	case 'R':
+	  *gnu_attr |= SHF_GNU_RETAIN;
+	  break;
 	case '?':
 	  *is_clone = TRUE;
 	  break;
@@ -890,8 +901,32 @@ obj_elf_parse_section_letters (char *str, size_t len,
 	      if (ISDIGIT (*str))
 		{
 		  char * end;
+		  struct elf_backend_data *bed;
+		  bfd_vma numeric_flags = strtoul (str, &end, 0);
+
+		  attr |= numeric_flags;
+
+		  bed = (struct elf_backend_data *)
+		    get_elf_backend_data (stdoutput);
+
+		  if (bed->elf_osabi == ELFOSABI_NONE
+		      || bed->elf_osabi == ELFOSABI_STANDALONE
+		      || bed->elf_osabi == ELFOSABI_GNU
+		      || bed->elf_osabi == ELFOSABI_FREEBSD)
+		    {
+		      /* Add flags in the SHF_MASKOS range to gnu_attr for
+			 OSABIs that support those flags.
+			 Also adding the flags for ELFOSABI_{NONE,STANDALONE}
+			 allows them to be validated later in obj_elf_section.
+			 We can't just always set these bits in gnu_attr for
+			 all OSABIs, since Binutils does not recognize all
+			 SHF_MASKOS bits for non-GNU OSABIs.  It's therefore
+			 possible that numeric flags are being used to set bits
+			 in the SHF_MASKOS range for those targets, and we
+			 don't want assembly to fail in those situations.  */
+		      *gnu_attr |= (numeric_flags & SHF_MASKOS);
+		    }
 
-		  attr |= strtoul (str, & end, 0);
 		  /* Update str and len, allowing for the fact that
 		     we will execute str++ and len-- below.  */
 		  end --;
@@ -1387,26 +1422,37 @@ obj_elf_section (int push)
  done:
   demand_empty_rest_of_line ();
 
-  obj_elf_change_section (name, type, attr, entsize, &match, linkonce,
-			  push);
-
-  if ((gnu_attr & SHF_GNU_MBIND) != 0)
+  if ((gnu_attr & (SHF_GNU_MBIND | SHF_GNU_RETAIN)) != 0)
     {
       struct elf_backend_data *bed;
+      bfd_boolean mbind_p = (gnu_attr & SHF_GNU_MBIND) != 0;
 
-      if ((attr & SHF_ALLOC) == 0)
+      if (mbind_p && (attr & SHF_ALLOC) == 0)
 	as_bad (_("SHF_ALLOC isn't set for GNU_MBIND section: %s"), name);
 
       bed = (struct elf_backend_data *) get_elf_backend_data (stdoutput);
-      if (bed->elf_osabi == ELFOSABI_NONE)
-	bed->elf_osabi = ELFOSABI_GNU;
-      else if (bed->elf_osabi != ELFOSABI_GNU
-	       && bed->elf_osabi != ELFOSABI_FREEBSD)
-	as_bad (_("GNU_MBIND section is supported only by GNU "
-		  "and FreeBSD targets"));
-      elf_tdata (stdoutput)->has_gnu_osabi |= elf_gnu_osabi_mbind;
+
+      if (bed->elf_osabi != ELFOSABI_GNU
+	  && bed->elf_osabi != ELFOSABI_FREEBSD
+	  && bed->elf_osabi != ELFOSABI_NONE)
+	as_bad (_("%s section is supported only by GNU and FreeBSD targets"),
+		mbind_p ? "GNU_MBIND" : "GNU_RETAIN");
+      else
+	{
+	  if (bed->elf_osabi == ELFOSABI_NONE)
+	    bed->elf_osabi = ELFOSABI_GNU;
+
+	  if (mbind_p)
+	    elf_tdata (stdoutput)->has_gnu_osabi |= elf_gnu_osabi_mbind;
+	  if ((gnu_attr & SHF_GNU_RETAIN) != 0)
+	    elf_tdata (stdoutput)->has_gnu_osabi |= elf_gnu_osabi_retain;
+
+	  attr |= gnu_attr;
+	}
     }
-  elf_section_flags (now_seg) |= gnu_attr;
+
+  obj_elf_change_section (name, type, attr, entsize, &match, linkonce,
+			  push);
 
   if (linked_to_section_index != -1UL)
     {
diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index 278de41..84a7e61 100644
--- a/gas/doc/as.texi
+++ b/gas/doc/as.texi
@@ -6659,6 +6659,9 @@ section is a member of a section group
 section is used for thread-local-storage
 @item ?
 section is a member of the previously-current section's group, if any
+@item R
+retained section (apply SHF_GNU_RETAIN to prevent linker garbage
+collection, GNU ELF extension)
 @item @code{<number>}
 a numeric value indicating the bits to be set in the ELF section header's flags
 field.  Note - if one or more of the alphabetic characters described above is
diff --git a/gas/testsuite/gas/elf/elf.exp b/gas/testsuite/gas/elf/elf.exp
index edacf27..0ba32c7 100644
--- a/gas/testsuite/gas/elf/elf.exp
+++ b/gas/testsuite/gas/elf/elf.exp
@@ -261,8 +261,12 @@ if { [is_elf_format] } then {
     run_dump_test "section19"
     run_dump_test "section20"
     run_dump_test "section21"
+    run_dump_test "section22"
+    run_dump_test "section23a"
+    run_dump_test "section23b"
+    run_dump_test "section24a"
+    run_dump_test "section24b"
     run_dump_test "sh-link-zero"
-
     run_dump_test "dwarf2-1" $dump_opts
     run_dump_test "dwarf2-2" $dump_opts
     run_dump_test "dwarf2-3" $dump_opts
diff --git a/gas/testsuite/gas/elf/section10.d b/gas/testsuite/gas/elf/section10.d
index 554a791..6aa7b08 100644
--- a/gas/testsuite/gas/elf/section10.d
+++ b/gas/testsuite/gas/elf/section10.d
@@ -18,7 +18,7 @@
 #...
 [ 	]*\[.*\][ 	]+sec3
 [ 	]*PROGBITS.*
-[ 	]*\[.*fefff030\]: MERGE, STRINGS,.* EXCLUDE, OS \(.*ef00000\), PROC \(.*[3467]0000000\), UNKNOWN \(0+0ff000\)
+[ 	]*\[.*fedff030\]: MERGE, STRINGS,.* EXCLUDE, OS \(.*ed00000\), PROC \(.*[3467]0000000\), UNKNOWN \(0+0ff000\)
 #...
 [ 	]*\[.*\][ 	]+sec4
 [ 	]*LOOS\+0x11[ 	].*
@@ -26,7 +26,7 @@
 #...
 [ 	]*\[.*\][ 	]+sec5
 [ 	]*LOUSER\+0x9[ 	].*
-[ 	]*\[.*feff0000\]:.* EXCLUDE, OS \(.*ef00000\), PROC \(.*[3467]0000000\), UNKNOWN \(.*f0000\)
+[ 	]*\[.*fedf0000\]:.* EXCLUDE, OS \(.*ed00000\), PROC \(.*[3467]0000000\), UNKNOWN \(.*f0000\)
 [ 	]*\[.*\][ 	]+.data.foo
 [ 	]*LOUSER\+0x7f000000[ 	].*
 [ 	]*\[0+003\]: WRITE, ALLOC
diff --git a/gas/testsuite/gas/elf/section10.s b/gas/testsuite/gas/elf/section10.s
index 29f1184..d52b345 100644
--- a/gas/testsuite/gas/elf/section10.s
+++ b/gas/testsuite/gas/elf/section10.s
@@ -7,7 +7,7 @@
 	.word 2
 
 	# Make sure that specifying further arguments to .sections is still supported
-	.section sec3, "0xfefff000MS", %progbits, 32
+	.section sec3, "0xfedff000MS", %progbits, 32
 	.word 3
 
 	# Make sure that extra flags can be set for well known sections as well.
@@ -19,7 +19,7 @@
 	.word 5
 
 	# Test both together, with a quoted type value.
-	.section sec5, "0xfeff0000", "0x80000009"
+	.section sec5, "0xfedf0000", "0x80000009"
 	.word 6
 
 	# Test that declaring an extended version of a known special section works.
diff --git a/gas/testsuite/gas/elf/section22.d b/gas/testsuite/gas/elf/section22.d
new file mode 100644
index 0000000..8aa7fcf
--- /dev/null
+++ b/gas/testsuite/gas/elf/section22.d
@@ -0,0 +1,19 @@
+#readelf: -h -S --wide
+#name: SHF_GNU_RETAIN sections 22
+#notarget: ![supports_gnu_osabi]
+
+#...
+ +OS/ABI: +UNIX - GNU
+#...
+  \[..\] .text.discard0[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00  AX.*
+#...
+  \[..\] .data.discard1[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00  WA.*
+#...
+  \[..\] .bss.discard2[ 	]+NOBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00  WA.*
+#...
+  \[..\] .bss.retain0[ 	]+NOBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 WAR.*
+#...
+  \[..\] .data.retain1[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 WAR.*
+#...
+  \[..\] .text.retain2[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 AXR.*
+#pass
diff --git a/gas/testsuite/gas/elf/section22.s b/gas/testsuite/gas/elf/section22.s
new file mode 100644
index 0000000..66ed990
--- /dev/null
+++ b/gas/testsuite/gas/elf/section22.s
@@ -0,0 +1,34 @@
+	.section	.text.discard0,"ax",%progbits
+	.global	discard0
+	.type	discard0, %function
+discard0:
+	.word	0
+
+	.section	.data.discard1,"aw"
+	.global	discard1
+	.type	discard1, %object
+discard1:
+	.word	1
+
+	.section	.bss.discard2,"aw"
+	.global	discard2
+	.type	discard2, %object
+discard2:
+	.zero	2
+
+	.section	.bss.retain0,"awR",%nobits
+	.global	retain0
+	.type	retain0, %object
+retain0:
+	.zero	2
+
+	.section	.data.retain1,"awR",%progbits
+	.type	retain1, %object
+retain1:
+	.word	1
+
+	.section	.text.retain2,"axR",%progbits
+	.global	retain2
+	.type	retain2, %function
+retain2:
+	.word	0
diff --git a/gas/testsuite/gas/elf/section23.s b/gas/testsuite/gas/elf/section23.s
new file mode 100644
index 0000000..d671119
--- /dev/null
+++ b/gas/testsuite/gas/elf/section23.s
@@ -0,0 +1,11 @@
+  .section	.data.retain_var,"0x200003"
+	.global	retain_var
+	.type	retain_var, %object
+retain_var:
+	.long	2
+
+	.section	.text._start,"ax"
+	.global	_start
+	.type	_start, %function
+_start:
+	.word 0
diff --git a/gas/testsuite/gas/elf/section23a.d b/gas/testsuite/gas/elf/section23a.d
new file mode 100644
index 0000000..2e413e1
--- /dev/null
+++ b/gas/testsuite/gas/elf/section23a.d
@@ -0,0 +1,10 @@
+#name: SHF_GNU_RETAIN set with numeric flag value in .section
+#source: section23.s
+#target: [supports_gnu_osabi]
+#readelf: -h -S --wide
+
+#...
+ +OS/ABI: +UNIX - GNU
+#...
+  \[..\] .data.retain_var[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 WAR.*
+#pass
diff --git a/gas/testsuite/gas/elf/section23b.d b/gas/testsuite/gas/elf/section23b.d
new file mode 100644
index 0000000..c85200e
--- /dev/null
+++ b/gas/testsuite/gas/elf/section23b.d
@@ -0,0 +1,6 @@
+#name: SHF_GNU_RETAIN set with numeric flag value in .section for non-GNU OSABI target
+#source: section23.s
+#error_output: section23b.err
+#target: msp430-*-elf visium-*-elf
+
+# This test only runs for targets which set ELFOSABI_STANDALONE.
diff --git a/gas/testsuite/gas/elf/section23b.err b/gas/testsuite/gas/elf/section23b.err
new file mode 100644
index 0000000..83de60c
--- /dev/null
+++ b/gas/testsuite/gas/elf/section23b.err
@@ -0,0 +1,2 @@
+.*: Assembler messages:
+.*:1: Error: GNU_RETAIN section is supported only by GNU and FreeBSD targets
diff --git a/gas/testsuite/gas/elf/section24.s b/gas/testsuite/gas/elf/section24.s
new file mode 100644
index 0000000..adcff4a
--- /dev/null
+++ b/gas/testsuite/gas/elf/section24.s
@@ -0,0 +1,38 @@
+	.section	.text,"ax",%progbits
+  .word 0
+	.section	.data,"aw"
+  .word 0
+	.section	.bss,"aw",%nobits
+  .word 0
+	.section	.rodata,"a"
+  .word 0
+
+/* Test that we can set the 'R' flag on an existing section.  */
+	.section	.text,"axR",%progbits
+  .word 0
+	.section	.data,"awR"
+  .word 0
+	.section	.bss,"awR",%nobits
+  .word 0
+	.section	.rodata,"aR"
+  .word 0
+
+/* Test that the 'R' flag does not get clobbered when the section is switched
+   back to.  */
+	.section	.text,"ax",%progbits
+  .word 0
+	.section	.data,"aw"
+  .word 0
+	.section	.bss,"aw",%nobits
+  .word 0
+	.section	.rodata,"a"
+  .word 0
+
+	.section	.text
+  .word 0
+	.section	.data
+  .word 0
+	.section	.bss
+  .word 0
+	.section	.rodata
+  .word 0
diff --git a/gas/testsuite/gas/elf/section24a.d b/gas/testsuite/gas/elf/section24a.d
new file mode 100644
index 0000000..8f316d3
--- /dev/null
+++ b/gas/testsuite/gas/elf/section24a.d
@@ -0,0 +1,17 @@
+#name: Merge SHF_GNU_RETAIN for non-unique sections
+#notarget: ![supports_gnu_osabi]
+#source: section24.s
+#readelf: -h -S --wide
+
+#...
+ +OS/ABI: +UNIX - GNU
+#...
+  \[..\] .text[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 AXR .*
+#...
+  \[..\] .data[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 WAR .*
+#...
+  \[..\] .bss[ 	]+NOBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 WAR .*
+#...
+  \[..\] .rodata[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00  AR .*
+#pass
+
diff --git a/gas/testsuite/gas/elf/section24b.d b/gas/testsuite/gas/elf/section24b.d
new file mode 100644
index 0000000..451ec21
--- /dev/null
+++ b/gas/testsuite/gas/elf/section24b.d
@@ -0,0 +1,10 @@
+#name: Merge SHF_GNU_RETAIN for non-unique sections (check no unmerged)
+#notarget: ![supports_gnu_osabi]
+#source: section24.s
+#readelf: -S --wide
+
+#failif
+#...
+  \[..\] .(text|data|bss|rodata)[ 	]+PROGBITS[ 	]+[0-9a-f]+ [0-9a-f]+ [0-9a-f]+ 00 [^R] .*
+#pass
+