Implement --append-null-phdr option

src/patchelf.cc

@@ -322,6 +322,8 @@ ElfFile<ElfFileParamNames>::ElfFile(FileContents fContents)
         if (rdi(phdrs[i].p_type) == PT_INTERP) isExecutable = true;
     }
 
+    trailingNullPhdrs = 0;
+
     for (int i = 0; i < rdi(hdr()->e_shnum); ++i) {
         Elf_Shdr *shdr = (Elf_Shdr *) (fileContents->data() + rdi(hdr()->e_shoff)) + i;
 
@@ -1162,7 +1164,7 @@ void ElfFile<ElfFileParamNames>::rewriteHeaders(Elf_Addr phdrAddress)
         if (rdi(phdr.p_type) == PT_PHDR) {
             phdr.p_offset = hdr()->e_phoff;
             wri(phdr.p_vaddr, wri(phdr.p_paddr, phdrAddress));
-            wri(phdr.p_filesz, wri(phdr.p_memsz, phdrs.size() * sizeof(Elf_Phdr)));
+            wri(phdr.p_filesz, wri(phdr.p_memsz, (phdrs.size() + trailingNullPhdrs) * sizeof(Elf_Phdr)));
             break;
         }
     }
@@ -1171,6 +1173,17 @@ void ElfFile<ElfFileParamNames>::rewriteHeaders(Elf_Addr phdrAddress)
         sortPhdrs();
     }
 
+    if (trailingNullPhdrs) {
+        Elf_Phdr nullPhdr = {};
+        nullPhdr.p_type = PT_NULL;
+
+        for (int i = 0; i < trailingNullPhdrs; ++i) {
+            phdrs.push_back(nullPhdr);
+        }
+
+        wri(hdr()->e_phnum, rdi(hdr()->e_phnum) + trailingNullPhdrs);
+    }
+
     for (unsigned int i = 0; i < phdrs.size(); ++i)
         * ((Elf_Phdr *) (fileContents->data() + rdi(hdr()->e_phoff)) + i) = phdrs.at(i);
 
@@ -1298,7 +1311,13 @@ void ElfFile<ElfFileParamNames>::rewriteHeaders(Elf_Addr phdrAddress)
     }
 }
 
-
+template<ElfFileParams>
+void ElfFile<ElfFileParamNames>::appendNullPhdrs(int count)
+{
+  trailingNullPhdrs += count;
+  rewriteHeaders(rdi(hdr()->e_phoff));
+  changed = true;
+}
 
 static void setSubstr(std::string & s, unsigned int pos, const std::string & t)
 {
@@ -2376,6 +2395,7 @@ static bool addDebugTag = false;
 static bool renameDynamicSymbols = false;
 static bool printRPath = false;
 static std::string newRPath;
+static int nullPhdrsToAppend;
 static std::set<std::string> neededLibsToRemove;
 static std::map<std::string, std::string> neededLibsToReplace;
 static std::set<std::string> neededLibsToAdd;
@@ -2444,6 +2464,9 @@ static void patchElf2(ElfFile && elfFile, const FileContents & fileContents, con
     if (renameDynamicSymbols)
         elfFile.renameDynamicSymbols(symbolsToRename);
 
+    if (nullPhdrsToAppend)
+        elfFile.appendNullPhdrs(nullPhdrsToAppend);
+
     if (elfFile.isChanged()){
         writeFile(fileName, elfFile.fileContents);
     } else if (alwaysWrite) {
@@ -2496,6 +2519,8 @@ static void showHelp(const std::string & progName)
   [--allowed-rpath-prefixes PREFIXES]\t\tWith '--shrink-rpath', reject rpath entries not starting with the allowed prefix\n\
   [--print-rpath]\n\
   [--force-rpath]\n\
+  [--append-null-phdr]\t\tAppends a PT_NULL program header to the end of the table; may be used multiple times\n\
+  [--append-null-phdrs N]\t\tAppends N PT_NULL program headers to the end of the table; may be used multiple times\n\
   [--add-needed LIBRARY]\n\
   [--remove-needed LIBRARY]\n\
   [--replace-needed LIBRARY NEW_LIBRARY]\n\
@@ -2600,6 +2625,13 @@ static int mainWrapped(int argc, char * * argv)
         else if (arg == "--no-sort") {
             noSort = true;
         }
+        else if (arg == "--append-null-phdr") {
+            ++nullPhdrsToAppend;
+        }
+        else if (arg == "--append-null-phdrs") {
+            if (++i == argc) error("missing argument");
+            nullPhdrsToAppend += std::stoi(argv[i]);
+        }
         else if (arg == "--add-needed") {
             if (++i == argc) error("missing argument");
             neededLibsToAdd.insert(resolveArgument(argv[i]));

src/patchelf.h

@@ -45,6 +45,8 @@ class ElfFile
     std::vector<Elf_Phdr> phdrs;
     std::vector<Elf_Shdr> shdrs;
 
+    int trailingNullPhdrs;
+
     bool littleEndian;
 
     bool changed = false;
@@ -167,6 +169,8 @@ class ElfFile
 
     void addDebugTag();
 
+    void appendNullPhdrs(int count);
+
     void renameDynamicSymbols(const std::unordered_map<std::string_view, std::string>&);
 
     void clearSymbolVersions(const std::set<std::string> & syms);

tests/Makefile.am

@@ -40,6 +40,7 @@ src_TESTS = \
   basic-flags.sh \
   set-empty-rpath.sh \
   phdr-corruption.sh \
+  append-null-phdr.sh \
   replace-needed.sh \
   replace-add-needed.sh \
   add-debug-tag.sh \
@@ -123,7 +124,7 @@ check_DATA = libbig-dynstr.debug
 # - with libtool, it is difficult to control options
 # - with libtool, it is not possible to compile convenience *dynamic* libraries :-(
 check_PROGRAMS += libfoo.so libfoo-scoped.so libbar.so libbar-scoped.so libsimple.so libsimple-execstack.so libbuildid.so libtoomanystrtab.so \
-                  phdr-corruption.so many-syms-main libmany-syms.so liboveralign.so libshared-rpath.so
+                  phdr-corruption.so append-null-phdr.so many-syms-main libmany-syms.so liboveralign.so libshared-rpath.so
 
 libbuildid_so_SOURCES = simple.c
 libbuildid_so_LDFLAGS = $(LDFLAGS_sharedlib) -Wl,--build-id
@@ -180,6 +181,10 @@ phdr_corruption_so_SOURCES = void.c phdr-corruption.ld
 phdr_corruption_so_LDFLAGS = -nostdlib -shared -Wl,-T$(srcdir)/phdr-corruption.ld
 phdr_corruption_so_CFLAGS =
 
+append_null_phdr_so_SOURCES = void.c
+append_null_phdr_so_LDFLAGS = -nostdlib -shared
+append_null_phdr_so_CFLAGS =
+
 many-syms.c:
 	i=1; while [ $$i -le 2000 ]; do echo "void f$$i() {};"; i=$$(($$i + 1)); done > $@
 

tests/append-null-phdr.sh

@@ -0,0 +1,22 @@
+#! /bin/sh -e
+
+PATCHELF="../src/patchelf"
+SONAME="phdr-corruption.so"
+SCRATCH="scratch/$(basename "$0" .sh)"
+SCRATCH_SO="${SCRATCH}/${SONAME}"
+READELF=${READELF:-readelf}
+
+rm -rf "${SCRATCH}"
+mkdir -p "${SCRATCH}"
+cp "${SONAME}" "${SCRATCH}"
+
+"${PATCHELF}" --append-null-phdr --append-null-phdrs 2 "${SCRATCH_SO}"
+
+# Check for PT_NULL entries
+readelfData=$(${READELF} -l "${SCRATCH_SO}" 2>&1)
+
+if [ "$(echo "$readelfData" | grep -c "NULL")" != 3 ]; then
+  # Triggered if patchelf doesn't append two PT_NULL entries
+  echo "ERROR: PT_NULL segments were not appended to the program header table!"
+  exit 1
+fi