Add --spare-program-headers

This is a new experimental flag to make room at the end of PHDR
so that post-processing tools can add more entries as needed.

Fixes #1148

Author: rui314

elf/cmdline.cc

@@ -141,6 +141,8 @@ inline const char helpmsg[] = R"(
   --sort-common               Ignored
   --sort-section              Ignored
   --spare-dynamic-tags NUMBER Reserve give number of tags in .dynamic section
+  --spare-program-headers NUMBER
+                              Reserve give number of slots in the program header
   --start-lib                 Give following object files in-archive-file semantics
     --end-lib                 End the effect of --start-lib
   --stats                     Print input statistics
@@ -606,6 +608,9 @@ std::vector<std::string> parse_nonpositional_args(Context<E> &ctx) {
       ctx.arg.shared = true;
     } else if (read_arg("spare-dynamic-tags")) {
       ctx.arg.spare_dynamic_tags = parse_number(ctx, "spare-dynamic-tags", arg);
+    } else if (read_arg("spare-program-headers")) {
+      ctx.arg.spare_program_headers
+        = parse_number(ctx, "spare-program-headers", arg);
     } else if (read_flag("start-lib")) {
       remaining.push_back("--start-lib");
     } else if (read_flag("start-stop")) {

elf/mold.h

@@ -1700,6 +1700,7 @@ struct Context {
     bool z_text = false;
     i64 filler = -1;
     i64 spare_dynamic_tags = 5;
+    i64 spare_program_headers = 0;
     i64 thread_count = 0;
     i64 z_stack_size = 0;
     u64 shuffle_sections_seed;

elf/output-chunks.cc

@@ -352,6 +352,7 @@ static std::vector<ElfPhdr<E>> create_phdr(Context<E> &ctx) {
     }
   }
 
+  vec.resize(vec.size() + ctx.arg.spare_program_headers);
   return vec;
 }
 

test/elf/spare-program-headers.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+cat <<EOF | $CC -o $t/a.o -c -xc -
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
+EOF
+
+$CC -B. -o $t/exe1 $t/a.o
+$QEMU $t/exe1 | grep -q 'Hello world'
+[ "$(readelf -Wl $t/exe1 | grep NULL | wc -l)" -eq 0 ]
+
+$CC -B. -o $t/exe2 $t/a.o -Wl,--spare-program-headers=0
+$QEMU $t/exe2 | grep -q 'Hello world'
+[ "$(readelf -Wl $t/exe2 | grep NULL | wc -l)" -eq 0 ]
+
+$CC -B. -o $t/exe3 $t/a.o -Wl,--spare-program-headers=1
+$QEMU $t/exe3 | grep -q 'Hello world'
+[ "$(readelf -Wl $t/exe3 | grep NULL | wc -l)" -eq 1 ]
+
+$CC -B. -o $t/exe4 $t/a.o -Wl,--spare-program-headers=5
+$QEMU $t/exe4 | grep -q 'Hello world'
+[ "$(readelf -Wl $t/exe4 | grep NULL | wc -l)" -eq 5 ]