ELF: Strip section header in ELF objects

Section header isn't mandatory on ELF executable nor shared library.
This patch adds a new linker option, -z nosectionheader, to omit ELF
section header, a new objcopy and strip option, --strip-section-headers,
to remove ELF section headers.

bfd/

2023-06-06  H.J. Lu  <hongjiu.lu@intel.com>
	    Kaylee Blake  <klkblake@gmail.com>

	PR ld/25617
	* bfd.c (BFD_NO_SECTION_HEADER): New.
	(BFD_FLAGS_SAVED): Add BFD_NO_SECTION_HEADER.
	(BFD_FLAGS_FOR_BFD_USE_MASK): Likewise.
	* elfcode.h (elf_swap_ehdr_out): Omit section header with
	BFD_NO_SECTION_HEADER.
	(elf_write_shdrs_and_ehdr): Likewise.
	* elfxx-target.h (TARGET_BIG_SYM): Add BFD_NO_SECTION_HEADER
	to object_flags.
	(TARGET_LITTLE_SYM): Likewise.
	* bfd-in2.h: Regenerated.

binutils/

2023-06-06  H.J. Lu  <hongjiu.lu@intel.com>

	PR ld/25617
	* NEWS: Mention --strip-section-headers for objcopy and strip.
	* objcopy.c (strip_section_headers): New.
	(command_line_switch): Add OPTION_STRIP_SECTION_HEADERS.
	(strip_options): Add --strip-section-headers.
	(copy_options): Likewise.
	(copy_usage): Add --strip-section-headers.
	(strip_usage): Likewise.
	(copy_object): Handle --strip-section-headers for ELF files.
	(strip_main): Handle OPTION_STRIP_SECTION_HEADERS.
	(copy_main): Likewise.
	* doc/binutils.texi: Document --strip-section-headers for objcopy
	and strip.

ld/

2023-06-06  H.J. Lu  <hongjiu.lu@intel.com>
	    Kaylee Blake  <klkblake@gmail.com>

	PR ld/25617
	* NEWS: Mention -z nosectionheader.
	* emultempl/elf.em: Support -z sectionheader and
	-z nosectionheader.
	* ld.h (ld_config_type): Add no_section_header.
	* ld.texi: Document -z sectionheader and -z nosectionheader.
	* ldlang.c (ldlang_open_output): Handle
	config.no_section_header.
	* lexsup.c (parse_args): Enable --strip-all with
	-z nosectionheader.  Disallow -r with -z nosectionheader.
	(elf_static_list_options): Add -z sectionheader and
	-z nosectionheader.

13 files changed, 114 insertions, 16 deletions

diff --git a/bfd/bfd-in2.h b/bfd/bfd-in2.h
index c4fa0c1..b34c8ef 100644
--- a/bfd/bfd-in2.h
+++ b/bfd/bfd-in2.h
@@ -2049,11 +2049,15 @@ struct bfd
   /* Compress sections in this BFD with SHF_COMPRESSED zstd.  */
 #define BFD_COMPRESS_ZSTD      0x400000
 
+  /* Don't generate ELF section header.  */
+#define BFD_NO_SECTION_HEADER  0x800000
+
   /* Flags bits which are for BFD use only.  */
 #define BFD_FLAGS_FOR_BFD_USE_MASK \
   (BFD_IN_MEMORY | BFD_COMPRESS | BFD_DECOMPRESS | BFD_LINKER_CREATED \
    | BFD_PLUGIN | BFD_TRADITIONAL_FORMAT | BFD_DETERMINISTIC_OUTPUT \
-   | BFD_COMPRESS_GABI | BFD_CONVERT_ELF_COMMON | BFD_USE_ELF_STT_COMMON)
+   | BFD_COMPRESS_GABI | BFD_CONVERT_ELF_COMMON | BFD_USE_ELF_STT_COMMON \
+   | BFD_NO_SECTION_HEADER)
 
   /* The format which belongs to the BFD. (object, core, etc.)  */
   ENUM_BITFIELD (bfd_format) format : 3;
diff --git a/bfd/bfd.c b/bfd/bfd.c
index 804acab..e43a388 100644
--- a/bfd/bfd.c
+++ b/bfd/bfd.c
@@ -192,11 +192,15 @@ CODE_FRAGMENT
 .  {* Compress sections in this BFD with SHF_COMPRESSED zstd.  *}
 .#define BFD_COMPRESS_ZSTD      0x400000
 .
+.  {* Don't generate ELF section header.  *}
+.#define BFD_NO_SECTION_HEADER	0x800000
+.
 .  {* Flags bits which are for BFD use only.  *}
 .#define BFD_FLAGS_FOR_BFD_USE_MASK \
 .  (BFD_IN_MEMORY | BFD_COMPRESS | BFD_DECOMPRESS | BFD_LINKER_CREATED \
 .   | BFD_PLUGIN | BFD_TRADITIONAL_FORMAT | BFD_DETERMINISTIC_OUTPUT \
-.   | BFD_COMPRESS_GABI | BFD_CONVERT_ELF_COMMON | BFD_USE_ELF_STT_COMMON)
+.   | BFD_COMPRESS_GABI | BFD_CONVERT_ELF_COMMON | BFD_USE_ELF_STT_COMMON \
+.   | BFD_NO_SECTION_HEADER)
 .
 .  {* The format which belongs to the BFD. (object, core, etc.)  *}
 .  ENUM_BITFIELD (bfd_format) format : 3;
diff --git a/bfd/elfcode.h b/bfd/elfcode.h
index e4e29f0..495e498 100644
--- a/bfd/elfcode.h
+++ b/bfd/elfcode.h
@@ -266,6 +266,7 @@ elf_swap_ehdr_out (bfd *abfd,
 {
   unsigned int tmp;
   int signed_vma = get_elf_backend_data (abfd)->sign_extend_vma;
+  bool no_section_header = (abfd->flags & BFD_NO_SECTION_HEADER) != 0;
   memcpy (dst->e_ident, src->e_ident, EI_NIDENT);
   /* note that all elements of dst are *arrays of unsigned char* already...  */
   H_PUT_16 (abfd, src->e_type, dst->e_type);
@@ -276,7 +277,10 @@ elf_swap_ehdr_out (bfd *abfd,
   else
     H_PUT_WORD (abfd, src->e_entry, dst->e_entry);
   H_PUT_WORD (abfd, src->e_phoff, dst->e_phoff);
-  H_PUT_WORD (abfd, src->e_shoff, dst->e_shoff);
+  if (no_section_header)
+    H_PUT_WORD (abfd, 0, dst->e_shoff);
+  else
+    H_PUT_WORD (abfd, src->e_shoff, dst->e_shoff);
   H_PUT_32 (abfd, src->e_flags, dst->e_flags);
   H_PUT_16 (abfd, src->e_ehsize, dst->e_ehsize);
   H_PUT_16 (abfd, src->e_phentsize, dst->e_phentsize);
@@ -284,15 +288,24 @@ elf_swap_ehdr_out (bfd *abfd,
   if (tmp > PN_XNUM)
     tmp = PN_XNUM;
   H_PUT_16 (abfd, tmp, dst->e_phnum);
-  H_PUT_16 (abfd, src->e_shentsize, dst->e_shentsize);
-  tmp = src->e_shnum;
-  if (tmp >= (SHN_LORESERVE & 0xffff))
-    tmp = SHN_UNDEF;
-  H_PUT_16 (abfd, tmp, dst->e_shnum);
-  tmp = src->e_shstrndx;
-  if (tmp >= (SHN_LORESERVE & 0xffff))
-    tmp = SHN_XINDEX & 0xffff;
-  H_PUT_16 (abfd, tmp, dst->e_shstrndx);
+  if (no_section_header)
+    {
+      H_PUT_16 (abfd, 0, dst->e_shentsize);
+      H_PUT_16 (abfd, 0, dst->e_shnum);
+      H_PUT_16 (abfd, 0, dst->e_shstrndx);
+    }
+  else
+    {
+      H_PUT_16 (abfd, src->e_shentsize, dst->e_shentsize);
+      tmp = src->e_shnum;
+      if (tmp >= (SHN_LORESERVE & 0xffff))
+	tmp = SHN_UNDEF;
+      H_PUT_16 (abfd, tmp, dst->e_shnum);
+      tmp = src->e_shstrndx;
+      if (tmp >= (SHN_LORESERVE & 0xffff))
+	tmp = SHN_XINDEX & 0xffff;
+      H_PUT_16 (abfd, tmp, dst->e_shstrndx);
+    }
 }
 
 /* Translate an ELF section header table entry in external format into an
@@ -1077,6 +1090,9 @@ elf_write_shdrs_and_ehdr (bfd *abfd)
       || bfd_bwrite (&x_ehdr, amt, abfd) != amt)
     return false;
 
+  if ((abfd->flags & BFD_NO_SECTION_HEADER) != 0)
+    return true;
+
   /* Some fields in the first section header handle overflow of ehdr
      fields.  */
   if (i_ehdrp->e_phnum >= PN_XNUM)
diff --git a/bfd/elfxx-target.h b/bfd/elfxx-target.h
index 8eabad4..f855300 100644
--- a/bfd/elfxx-target.h
+++ b/bfd/elfxx-target.h
@@ -994,7 +994,7 @@ const bfd_target TARGET_BIG_SYM =
   (HAS_RELOC | EXEC_P | HAS_LINENO | HAS_DEBUG | HAS_SYMS | HAS_LOCALS
    | DYNAMIC | WP_TEXT | D_PAGED | BFD_COMPRESS | BFD_DECOMPRESS
    | BFD_COMPRESS_GABI | BFD_COMPRESS_ZSTD | BFD_CONVERT_ELF_COMMON
-   | BFD_USE_ELF_STT_COMMON),
+   | BFD_USE_ELF_STT_COMMON | BFD_NO_SECTION_HEADER),
 
   /* section_flags: mask of all section flags */
   (SEC_HAS_CONTENTS | SEC_ALLOC | SEC_LOAD | SEC_RELOC | SEC_READONLY
@@ -1099,7 +1099,7 @@ const bfd_target TARGET_LITTLE_SYM =
   (HAS_RELOC | EXEC_P | HAS_LINENO | HAS_DEBUG | HAS_SYMS | HAS_LOCALS
    | DYNAMIC | WP_TEXT | D_PAGED | BFD_COMPRESS | BFD_DECOMPRESS
    | BFD_COMPRESS_GABI | BFD_COMPRESS_ZSTD | BFD_CONVERT_ELF_COMMON
-   | BFD_USE_ELF_STT_COMMON),
+   | BFD_USE_ELF_STT_COMMON | BFD_NO_SECTION_HEADER),
 
   /* section_flags: mask of all section flags */
   (SEC_HAS_CONTENTS | SEC_ALLOC | SEC_LOAD | SEC_RELOC | SEC_READONLY
diff --git a/binutils/NEWS b/binutils/NEWS
index 00ef7c4..3aea875 100644
--- a/binutils/NEWS
+++ b/binutils/NEWS
@@ -14,6 +14,9 @@
   sframe_get_funcdesc_with_addr API, change in the behavior of
   sframe_fre_get_ra_offset and sframe_fre_get_fp_offset APIs.
   
+* Add command-line option, --strip-section-headers, to objcopy and strip to
+  remove ELF section header from ELF file.
+
 Changes in 2.40:
 
 * Objdump has a new command line option --show-all-symbols which will make it
diff --git a/binutils/doc/binutils.texi b/binutils/doc/binutils.texi
index 8c14d11..8314cb5 100644
--- a/binutils/doc/binutils.texi
+++ b/binutils/doc/binutils.texi
@@ -1278,6 +1278,7 @@ objcopy [@option{-F} @var{bfdname}|@option{--target=}@var{bfdname}]
         [@option{-R} @var{sectionpattern}|@option{--remove-section=}@var{sectionpattern}]
         [@option{--keep-section=}@var{sectionpattern}]
         [@option{--remove-relocations=}@var{sectionpattern}]
+        [@option{--strip-section-headers}]
         [@option{-p}|@option{--preserve-dates}]
         [@option{-D}|@option{--enable-deterministic-archives}]
         [@option{-U}|@option{--disable-deterministic-archives}]
@@ -1494,6 +1495,10 @@ will remove all relocations for sections matching the pattern
 '.text.*', but will not remove relocations for the section
 '.text.foo'.
 
+@item --strip-section-headers
+Strip section header   This option is specific to ELF files.
+Implies @option{--strip-all} and @option{--merge-notes}.
+
 @item -S
 @itemx --strip-all
 Do not copy relocation and symbol information from the source file.
@@ -3484,6 +3489,7 @@ strip [@option{-F} @var{bfdname} |@option{--target=}@var{bfdname}]
       [@option{-R} @var{sectionname} |@option{--remove-section=}@var{sectionname}]
       [@option{--keep-section=}@var{sectionpattern}]
       [@option{--remove-relocations=}@var{sectionpattern}]
+      [@option{--strip-section-headers}]
       [@option{-o} @var{file}] [@option{-p}|@option{--preserve-dates}]
       [@option{-D}|@option{--enable-deterministic-archives}]
       [@option{-U}|@option{--disable-deterministic-archives}]
@@ -3586,6 +3592,10 @@ will remove all relocations for sections matching the pattern
 '.text.*', but will not remove relocations for the section
 '.text.foo'.
 
+@item --strip-section-headers
+Strip section headers.  This option is specific to ELF files.  Implies
+@option{--strip-all} and @option{--merge-notes}.
+
 @item -s
 @itemx --strip-all
 Remove all symbols.
diff --git a/binutils/objcopy.c b/binutils/objcopy.c
index 4140077..206d80b 100644
--- a/binutils/objcopy.c
+++ b/binutils/objcopy.c
@@ -97,6 +97,7 @@ static int deterministic = -1;		/* Enable deterministic archives.  */
 static int status = 0;			/* Exit status.  */
 
 static bool    merge_notes = false;	/* Merge note sections.  */
+static bool strip_section_headers = false;/* Strip section headers.  */
 
 typedef struct merged_note_section
 {
@@ -365,6 +366,7 @@ enum command_line_switch
   OPTION_SREC_LEN,
   OPTION_STACK,
   OPTION_STRIP_DWO,
+  OPTION_STRIP_SECTION_HEADERS,
   OPTION_STRIP_SYMBOLS,
   OPTION_STRIP_UNNEEDED,
   OPTION_STRIP_UNNEEDED_SYMBOL,
@@ -403,6 +405,7 @@ static struct option strip_options[] =
   {"preserve-dates", no_argument, 0, 'p'},
   {"remove-section", required_argument, 0, 'R'},
   {"remove-relocations", required_argument, 0, OPTION_REMOVE_RELOCS},
+  {"strip-section-headers", no_argument, 0, OPTION_STRIP_SECTION_HEADERS},
   {"strip-all", no_argument, 0, 's'},
   {"strip-debug", no_argument, 0, 'S'},
   {"strip-dwo", no_argument, 0, OPTION_STRIP_DWO},
@@ -492,6 +495,7 @@ static struct option copy_options[] =
   {"remove-leading-char", no_argument, 0, OPTION_REMOVE_LEADING_CHAR},
   {"remove-section", required_argument, 0, 'R'},
   {"remove-relocations", required_argument, 0, OPTION_REMOVE_RELOCS},
+  {"strip-section-headers", no_argument, 0, OPTION_STRIP_SECTION_HEADERS},
   {"rename-section", required_argument, 0, OPTION_RENAME_SECTION},
   {"reverse-bytes", required_argument, 0, OPTION_REVERSE_BYTES},
   {"section-alignment", required_argument, 0, OPTION_PE_SECTION_ALIGNMENT},
@@ -592,6 +596,7 @@ copy_usage (FILE *stream, int exit_status)
      --add-gnu-debuglink=<file>    Add section .gnu_debuglink linking to <file>\n\
   -R --remove-section <name>       Remove section <name> from the output\n\
      --remove-relocations <name>   Remove relocations from section <name>\n\
+     --strip-section-headers              Strip section header from the output\n\
   -S --strip-all                   Remove all symbol and relocation information\n\
   -g --strip-debug                 Remove all debugging symbols & sections\n\
      --strip-dwo                   Remove all DWO sections\n\
@@ -730,6 +735,7 @@ strip_usage (FILE *stream, int exit_status)
   fprintf (stream, _("\
   -R --remove-section=<name>       Also remove section <name> from the output\n\
      --remove-relocations <name>   Remove relocations from section <name>\n\
+     --strip-section-headers       Strip section headers from the output\n\
   -s --strip-all                   Remove all symbol and relocation information\n\
   -g -S -d --strip-debug           Remove all debugging symbols & sections\n\
      --strip-dwo                   Remove all DWO sections\n\
@@ -2695,7 +2701,16 @@ copy_object (bfd *ibfd, bfd *obfd, const bfd_arch_info_type *input_arch)
      necessary.  */
   VerilogDataEndianness = ibfd->xvec->byteorder;
 
-  if (bfd_get_flavour (ibfd) != bfd_target_elf_flavour)
+  if (bfd_get_flavour (ibfd) == bfd_target_elf_flavour)
+    {
+      if (strip_section_headers)
+	{
+	  ibfd->flags |= BFD_NO_SECTION_HEADER;
+	  strip_symbols = STRIP_ALL;
+	  merge_notes = true;
+	}
+    }
+  else
     {
       if ((do_debug_sections & compress) != 0
 	  && do_debug_sections != compress)
@@ -2712,6 +2727,13 @@ copy_object (bfd *ibfd, bfd *obfd, const bfd_arch_info_type *input_arch)
 		     bfd_get_archive_filename (ibfd));
 	  return false;
 	}
+
+      if (strip_section_headers)
+	{
+	  non_fatal (_("--strip_section_headers is unsupported on `%s'"),
+		     bfd_get_archive_filename (ibfd));
+	  return false;
+	}
     }
 
   if (verbose)
@@ -3456,7 +3478,7 @@ copy_object (bfd *ibfd, bfd *obfd, const bfd_arch_info_type *input_arch)
 	  free (merged);
 	}
     }
-  else if (merge_notes && ! is_strip)
+  else if (merge_notes && ! is_strip && ! strip_section_headers)
     non_fatal (_("%s: Could not find any mergeable note sections"),
 	       bfd_get_filename (ibfd));
 
@@ -4759,6 +4781,9 @@ strip_main (int argc, char *argv[])
 	case OPTION_REMOVE_RELOCS:
 	  handle_remove_relocations_option (optarg);
 	  break;
+	case OPTION_STRIP_SECTION_HEADERS:
+	  strip_section_headers = true;
+	  break;
 	case 's':
 	  strip_symbols = STRIP_ALL;
 	  break;
@@ -5228,6 +5253,10 @@ copy_main (int argc, char *argv[])
 	  handle_remove_relocations_option (optarg);
 	  break;
 
+	case OPTION_STRIP_SECTION_HEADERS:
+	  strip_section_headers = true;
+	  break;
+
 	case 'S':
 	  strip_symbols = STRIP_ALL;
 	  break;
diff --git a/ld/NEWS b/ld/NEWS
index 4dee230..dbeea12 100644
--- a/ld/NEWS
+++ b/ld/NEWS
@@ -15,6 +15,9 @@
 * The linker script syntax has a new command for output sections: ASCIZ "string"
   This will insert a zero-terminated string at the current location.
 
+* Add command-line option, -z nosectionheader, to omit ELF section
+  header.
+
 Changes in 2.40:
 
 * The linker has a new command line option to suppress the generation of any
diff --git a/ld/emultempl/elf.em b/ld/emultempl/elf.em
index e367299..3f67f8b 100644
--- a/ld/emultempl/elf.em
+++ b/ld/emultempl/elf.em
@@ -807,6 +807,10 @@ fragment <<EOF
 		     "must be default, internal, hidden, or protected"),
 		   optarg);
 	}
+      else if (strcmp (optarg, "sectionheader") == 0)
+	config.no_section_header = false;
+      else if (strcmp (optarg, "nosectionheader") == 0)
+	config.no_section_header = true;
 EOF
 
 if test x"$GENERATE_SHLIB_SCRIPT" = xyes; then
diff --git a/ld/ld.h b/ld/ld.h
index 1882a28..a0f8a15 100644
--- a/ld/ld.h
+++ b/ld/ld.h
@@ -280,6 +280,9 @@ typedef struct
   /* If set, code and non-code sections should never be in one segment.  */
   bool separate_code;
 
+  /* If set, generation of ELF section header should be suppressed.  */
+  bool no_section_header;
+
   /* The rpath separation character.  Usually ':'.  */
   char rpath_separator;
 
diff --git a/ld/ld.texi b/ld/ld.texi
index a007ae7..aa8b1aa 100644
--- a/ld/ld.texi
+++ b/ld/ld.texi
@@ -1544,6 +1544,11 @@ Don't create an ELF @code{PT_GNU_RELRO} segment if @samp{norelro}.
 Report dynamic relative relocations generated by linker.  Supported for
 Linux/i386 and Linux/x86_64.
 
+@item sectionheader
+@itemx nosectionheader
+Generate section header.  Don't generate section header if
+@samp{nosectionheader} is used.  @option{sectionheader} is the default.
+
 @item separate-code
 @itemx noseparate-code
 Create separate code @code{PT_LOAD} segment header in the object.  This
diff --git a/ld/ldlang.c b/ld/ldlang.c
index e359a89..f4a8b72 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -3434,6 +3434,10 @@ ldlang_open_output (lang_statement_union_type *statement)
 	link_info.output_bfd->flags |= BFD_TRADITIONAL_FORMAT;
       else
 	link_info.output_bfd->flags &= ~BFD_TRADITIONAL_FORMAT;
+      if (config.no_section_header)
+	link_info.output_bfd->flags |= BFD_NO_SECTION_HEADER;
+      else
+	link_info.output_bfd->flags &= ~BFD_NO_SECTION_HEADER;
       break;
 
     case lang_target_statement_enum:
diff --git a/ld/lexsup.c b/ld/lexsup.c
index 4e20e93..fe87223 100644
--- a/ld/lexsup.c
+++ b/ld/lexsup.c
@@ -1998,6 +1998,15 @@ parse_args (unsigned argc, char **argv)
 	break;
       }
 
+  /* -z nosectionheader implies --strip-all.  */
+  if (config.no_section_header)
+    {
+      if (bfd_link_relocatable (&link_info))
+	einfo (_("%F%P: -r and -z nosectionheader may not be used together\n"));
+
+      link_info.strip = strip_all;
+    }
+
   if (!bfd_link_dll (&link_info))
     {
       if (command_line.filter_shlib)
@@ -2272,6 +2281,10 @@ elf_static_list_options (FILE *file)
   fprintf (file, _("\
   -z start-stop-visibility=V  Set visibility of built-in __start/__stop symbols\n\
                                 to DEFAULT, PROTECTED, HIDDEN or INTERNAL\n"));
+  fprintf (file, _("\
+  -z sectionheader            Generate section header (default)\n"));
+  fprintf (file, _("\
+  -z nosectionheader          Do not generate section header\n"));
 }
 
 static void