Support -z nosectionheader

docs/mold.1

@@ -1,6 +1,6 @@
 .\" generated with Ronn-NG/v0.9.1
 .\" http://github.com/apjanke/ronn-ng/tree/0.9.1
-.TH "MOLD" "1" "March 2023" ""
+.TH "MOLD" "1" "July 2023" ""
 .SH "NAME"
 \fBmold\fR \- a modern linker
 .SH "SYNOPSIS"
@@ -425,6 +425,9 @@ Some sections such as \fB\.dynamic\fR have to be writable only during an executa
 .IP
 By default, \fBmold\fR generates a relro segment\. \fB\-z norelro\fR disables the feature\.
 .TP
+\fB\-z sectionheader\fR, \fB\-z nosectionheader\fR
+\fB\-z nosectionheader\fR tell the linker to omit the section header\. By default, the linker does not omit the section header\.
+.TP
 \fB\-z separate\-loadable\-segments\fR, \fB\-z separate\-code\fR, \fB\-z noseparate\-code\fR
 If one memory page contains multiple segments, the page protection bits are set in such a way that the needed attributes (writable or executable) are satisfied for all segments\. This usually happens at a boundary of two segments with two different attributes\.
 .IP

docs/mold.md

@@ -697,6 +697,10 @@ arguments.
   By default, `mold` generates a relro segment. `-z norelro` disables the
   feature.
 
+* `-z sectionheader`, `-z nosectionheader`:
+  `-z nosectionheader` tell the linker to omit the section header.
+  By default, the linker does not omit the section header.
+
 * `-z separate-loadable-segments`, `-z separate-code`, `-z noseparate-code`:
   If one memory page contains multiple segments, the page protection bits are
   set in such a way that the needed attributes (writable or executable) are

elf/cmdline.cc

@@ -188,6 +188,8 @@ Options:
   -z origin                   Mark object requiring immediate $ORIGIN processing at runtime
   -z pack-relative-relocs     Alias for --pack-dyn-relocs=relr
     -z nopack-relative-relocs
+  -z sectionheader            Do not omit section header (default)
+    -z nosectionheader        Omit section header
   -z separate-loadable-segments
                               Separate all loadable segments to different pages
     -z separate-code          Separate code and data into different pages
@@ -886,6 +888,10 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.z_dynamic_undefined_weak = true;
     } else if (read_z_flag("nodynamic-undefined-weak")) {
       ctx.arg.z_dynamic_undefined_weak = false;
+    } else if (read_z_flag("sectionheader")) {
+      ctx.arg.z_sectionheader = true;
+    } else if (read_z_flag("nosectionheader")) {
+      ctx.arg.z_sectionheader = false;
     } else if (read_flag("no-undefined")) {
       ctx.arg.z_defs = true;
     } else if (read_flag("fatal-warnings")) {

elf/mold.h

@@ -1794,6 +1794,7 @@ struct Context {
     bool z_now = false;
     bool z_origin = false;
     bool z_relro = true;
+    bool z_sectionheader = true;
     bool z_shstk = false;
     bool z_text = false;
     i64 filler = -1;

elf/output-chunks.cc

@@ -101,10 +101,12 @@ void OutputEhdr<E>::copy_buf(Context<E> &ctx) {
 
   // If e_shstrndx is too large, a dummy value is set to e_shstrndx.
   // The real value is stored to the zero'th section's sh_link field.
-  if (ctx.shstrtab->shndx < SHN_LORESERVE)
-    hdr.e_shstrndx = ctx.shstrtab->shndx;
-  else
-    hdr.e_shstrndx = SHN_XINDEX;
+  if (ctx.shstrtab) {
+    if (ctx.shstrtab->shndx < SHN_LORESERVE)
+      hdr.e_shstrndx = ctx.shstrtab->shndx;
+    else
+      hdr.e_shstrndx = SHN_XINDEX;
+  }
 
   if (ctx.arg.relocatable)
     hdr.e_type = ET_REL;
@@ -140,7 +142,7 @@ void OutputShdr<E>::copy_buf(Context<E> &ctx) {
   if (UINT16_MAX < shnum)
     hdr->sh_size = shnum;
 
-  if (SHN_LORESERVE <= ctx.shstrtab->shndx)
+  if (ctx.shstrtab && SHN_LORESERVE <= ctx.shstrtab->shndx)
     hdr->sh_link = ctx.shstrtab->shndx;
 
   for (Chunk<E> *chunk : ctx.chunks)

elf/passes.cc

@@ -57,7 +57,8 @@ void create_synthetic_sections(Context<E> &ctx) {
     else
       ctx.phdr = push(new OutputPhdr<E>(0));
 
-    ctx.shdr = push(new OutputShdr<E>);
+    if (ctx.arg.z_sectionheader)
+      ctx.shdr = push(new OutputShdr<E>);
   }
 
   ctx.got = push(new GotSection<E>);
@@ -81,7 +82,7 @@ void create_synthetic_sections(Context<E> &ctx) {
   ctx.copyrel = push(new CopyrelSection<E>(false));
   ctx.copyrel_relro = push(new CopyrelSection<E>(true));
 
-  if (!ctx.arg.oformat_binary)
+  if (ctx.shdr)
     ctx.shstrtab = push(new ShstrtabSection<E>);
 
   if (!ctx.arg.dynamic_linker.empty())
@@ -2522,7 +2523,8 @@ i64 compress_debug_sections(Context<E> &ctx) {
     ctx.chunks[i] = comp;
   });
 
-  ctx.shstrtab->update_shdr(ctx);
+  if (ctx.shstrtab)
+    ctx.shstrtab->update_shdr(ctx);
 
   if (ctx.ehdr)
     ctx.ehdr->update_shdr(ctx);

test/elf/z-sectionheader.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+#include <stdio.h>
+int main() {
+  printf("Hello ");
+  puts("world");
+}
+EOF
+
+$CC -B. -o $t/exe $t/a.o -Wl,-z,nosectionheader
+$QEMU $t/exe | grep -q 'Hello world'
+
+readelf -h $t/exe 2>&1 | grep -Eq 'Size of section headers:\s+0 '