From eb6c213f2a9aa8a101b2b52a791be369d165e6a9 Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@cs.stanford.edu>
Date: Thu, 16 Nov 2023 11:33:10 +0900
Subject: [PATCH] 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 https://github.com/rui314/mold/issues/1148
---
 elf/cmdline.cc                    |  5 +++++
 elf/mold.h                        |  1 +
 elf/output-chunks.cc              |  1 +
 test/elf/spare-program-headers.sh | 25 +++++++++++++++++++++++++
 4 files changed, 32 insertions(+)
 create mode 100755 test/elf/spare-program-headers.sh

diff --git a/elf/cmdline.cc b/elf/cmdline.cc
index 5261835da..cd250bec5 100644
--- a/elf/cmdline.cc
+++ b/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")) {
diff --git a/elf/mold.h b/elf/mold.h
index ecafca93e..8b7ad4c5a 100644
--- a/elf/mold.h
+++ b/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;
diff --git a/elf/output-chunks.cc b/elf/output-chunks.cc
index f05539831..6b667222b 100644
--- a/elf/output-chunks.cc
+++ b/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;
 }
 
diff --git a/test/elf/spare-program-headers.sh b/test/elf/spare-program-headers.sh
new file mode 100755
index 000000000..782925a19
--- /dev/null
+++ b/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 ]