Add options to print, clear and set executable stack state

Add options the modify the state of the executable flag of the GNU_STACK
program header. That header indicates whether the object is requiring an
executable stack.

Author: cgzones

.gitignore

@@ -32,6 +32,7 @@ Makefile
 /tests/libbig-dynstr.debug
 /tests/contiguous-note-sections
 /tests/simple-pie
+/tests/simple-execstack
 
 .direnv/
 .vscode/

patchelf.1

@@ -114,6 +114,15 @@ This means that when a shared library has an entry point (so that it
 can be run as an executable), the debugger does not connect to it correctly and
 symbols are not resolved.
 
+.IP "--print-execstack"
+Prints the state of the executable flag of the GNU_STACK program header, if present.
+
+.IP "--clear-execstack"
+Clears the executable flag of the GNU_STACK program header, or adds a new header.
+
+.IP "--set-execstack"
+Sets the executable flag of the GNU_STACK program header, or adds a new header.
+
 .IP "--output FILE"
 Set the output file name.  If not specified, the input will be modified in place.
 

src/patchelf.cc

@@ -1010,10 +1010,10 @@ void ElfFile<ElfFileParamNames>::normalizeNoteSegments()
 
 
 template<ElfFileParams>
-void ElfFile<ElfFileParamNames>::rewriteSections()
+void ElfFile<ElfFileParamNames>::rewriteSections(bool force)
 {
 
-    if (replacedSections.empty()) return;
+    if (!force && replacedSections.empty()) return;
 
     for (auto & i : replacedSections)
         debug("replacing section '%s' with size %d\n",
@@ -1890,6 +1890,85 @@ void ElfFile<ElfFileParamNames>::clearSymbolVersions(const std::set<std::string>
     this->rewriteSections();
 }
 
+template<ElfFileParams>
+void ElfFile<ElfFileParamNames>::modifyExecstack(ExecstackMode op)
+{
+    if (op == ExecstackMode::clear || op == ExecstackMode::set) {
+        size_t nullhdr = (size_t)-1;
+
+        for (size_t i = 0; i < phdrs.size(); i++) {
+            auto & header = phdrs[i];
+            const auto type = rdi(header.p_type);
+            if (type != PT_GNU_STACK) {
+                if (!nullhdr && type == PT_NULL)
+                    nullhdr = i;
+                continue;
+            }
+
+            if (op == ExecstackMode::clear && (rdi(header.p_flags) & PF_X) == PF_X) {
+                debug("simple execstack clear of header %zu\n", i);
+
+                wri(header.p_flags, rdi(header.p_flags) & ~PF_X);
+                * ((Elf_Phdr *) (fileContents->data() + rdi(hdr()->e_phoff)) + i) = header;
+                changed = true;
+            } else if (op == ExecstackMode::set && (rdi(header.p_flags) & PF_X) != PF_X) {
+                debug("simple execstack set of header %zu\n", i);
+
+                wri(header.p_flags, rdi(header.p_flags) | PF_X);
+                * ((Elf_Phdr *) (fileContents->data() + rdi(hdr()->e_phoff)) + i) = header;
+                changed = true;
+            } else {
+                debug("execstack already in requested state\n");
+            }
+
+            return;
+        }
+
+        if (nullhdr != (size_t)-1) {
+            debug("replacement execstack of header %zu\n", nullhdr);
+
+            auto & header = phdrs[nullhdr];
+            header = {};
+            wri(header.p_type,  PT_GNU_STACK);
+            wri(header.p_flags, PF_R | PF_W | (op == ExecstackMode::set ? PF_X : 0));
+            wri(header.p_align, 0x1);
+
+            * ((Elf_Phdr *) (fileContents->data() + rdi(hdr()->e_phoff)) + nullhdr) = header;
+            changed = true;
+            return;
+        }
+
+        debug("header addition for execstack\n");
+
+        Elf_Phdr new_phdr = {};
+        wri(new_phdr.p_type,  PT_GNU_STACK);
+        wri(new_phdr.p_flags, PF_R | PF_W | (op == ExecstackMode::set ? PF_X : 0));
+        wri(new_phdr.p_align, 0x1);
+        phdrs.push_back(new_phdr);
+
+        wri(hdr()->e_phnum, rdi(hdr()->e_phnum) + 1);
+
+        changed = true;
+        rewriteSections(true);
+        return;
+    }
+
+    char result = '?';
+
+    for (const auto & header : phdrs) {
+        if (rdi(header.p_type) != PT_GNU_STACK)
+            continue;
+
+        if ((rdi(header.p_flags) & PF_X) == PF_X)
+            result = 'X';
+        else
+            result = '-';
+        break;
+    }
+
+    printf("execstack: %c\n", result);
+}
+
 static bool printInterpreter = false;
 static bool printOsAbi = false;
 static bool setOsAbi = false;
@@ -1912,6 +1991,9 @@ static std::set<std::string> neededLibsToAdd;
 static std::set<std::string> symbolsToClearVersion;
 static bool printNeeded = false;
 static bool noDefaultLib = false;
+static bool printExecstack = false;
+static bool clearExecstack = false;
+static bool setExecstack = false;
 
 template<class ElfFile>
 static void patchElf2(ElfFile && elfFile, const FileContents & fileContents, const std::string & fileName)
@@ -1937,6 +2019,13 @@ static void patchElf2(ElfFile && elfFile, const FileContents & fileContents, con
     if (printRPath)
         elfFile.modifyRPath(elfFile.rpPrint, {}, "");
 
+    if (printExecstack)
+        elfFile.modifyExecstack(ElfFile::ExecstackMode::print);
+    else if (clearExecstack)
+        elfFile.modifyExecstack(ElfFile::ExecstackMode::clear);
+    else if (setExecstack)
+        elfFile.modifyExecstack(ElfFile::ExecstackMode::set);
+
     if (shrinkRPath)
         elfFile.modifyRPath(elfFile.rpShrink, allowedRpathPrefixes, "");
     else if (removeRPath)
@@ -2019,6 +2108,9 @@ void showHelp(const std::string & progName)
   [--no-sort]\t\tDo not sort program+section headers; useful for debugging patchelf.\n\
   [--clear-symbol-version SYMBOL]\n\
   [--add-debug-tag]\n\
+  [--print-execstack]\t\tPrints whether the object requests an executable stack\n\
+  [--clear-execstack]\n\
+  [--set-execstack]\n\
   [--output FILE]\n\
   [--debug]\n\
   [--version]\n\
@@ -2127,6 +2219,15 @@ int mainWrapped(int argc, char * * argv)
             if (++i == argc) error("missing argument");
             symbolsToClearVersion.insert(resolveArgument(argv[i]));
         }
+        else if (arg == "--print-execstack") {
+            printExecstack = true;
+        }
+        else if (arg == "--clear-execstack") {
+            clearExecstack = true;
+        }
+        else if (arg == "--set-execstack") {
+            setExecstack = true;
+        }
         else if (arg == "--output") {
             if (++i == argc) error("missing argument");
             outputFileName = resolveArgument(argv[i]);

src/patchelf.h

@@ -105,7 +105,7 @@ class ElfFile
 
 public:
 
-    void rewriteSections();
+    void rewriteSections(bool force = false);
 
     std::string getInterpreter();
 
@@ -139,6 +139,10 @@ class ElfFile
 
     void clearSymbolVersions(const std::set<std::string> & syms);
 
+    enum class ExecstackMode { print, set, clear };
+
+    void modifyExecstack(ExecstackMode op);
+
 private:
 
     /* Convert an integer in big or little endian representation (as

tests/Makefile.am

@@ -1,6 +1,6 @@
 LIBS =
 
-check_PROGRAMS = simple-pie simple main too-many-strtab main-scoped big-dynstr no-rpath contiguous-note-sections
+check_PROGRAMS = simple-pie simple simple-execstack main too-many-strtab main-scoped big-dynstr no-rpath contiguous-note-sections
 
 no_rpath_arch_TESTS = \
   no-rpath-amd64.sh \
@@ -43,7 +43,9 @@ src_TESTS = \
   replace-needed.sh \
   replace-add-needed.sh \
   add-debug-tag.sh \
-  empty-note.sh
+  empty-note.sh \
+  print-execstack.sh \
+  modify-execstack.sh
 
 build_TESTS = \
   $(no_rpath_arch_TESTS)
@@ -71,10 +73,15 @@ export NIX_LDFLAGS=
 simple_SOURCES = simple.c
 # no -fpic for simple.o
 simple_CFLAGS =
+simple_LDFLAGS = -Wl,-z,noexecstack
 
 simple_pie_SOURCES = simple.c
 simple_pie_CFLAGS = -fPIC -pie
 
+simple_execstack_SOURCES = simple.c
+simple_execstack_CFLAGS =
+simple_execstack_LDFLAGS = -Wl,-z,execstack
+
 main_SOURCES = main.c
 main_LDADD = -lfoo $(AM_LDADD)
 main_DEPENDENCIES = libfoo.so
@@ -108,7 +115,7 @@ check_DATA = libbig-dynstr.debug
 # - without libtool, only archives (static libraries) can be built by automake
 # - 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 libbuildid.so libtoomanystrtab.so \
+check_PROGRAMS += libfoo.so libfoo-scoped.so libbar.so libbar-scoped.so libsimple.so libsimple-execstack.so libbuildid.so libtoomanystrtab.so \
                   phdr-corruption.so
 
 libbuildid_so_SOURCES = simple.c
@@ -131,7 +138,10 @@ libbar_scoped_so_SOURCES = bar.c
 libbar_scoped_so_LDFLAGS = $(LDFLAGS_sharedlib)
 
 libsimple_so_SOURCES = simple.c
-libsimple_so_LDFLAGS = $(LDFLAGS_sharedlib)
+libsimple_so_LDFLAGS = $(LDFLAGS_sharedlib) -Wl,-z,noexecstack
+
+libsimple_execstack_so_SOURCES = simple.c
+libsimple_execstack_so_LDFLAGS = $(LDFLAGS_sharedlib) -Wl,-z,execstack
 
 too_many_strtab_SOURCES = too-many-strtab.c too-many-strtab2.s
 libtoomanystrtab_so_SOURCES = too-many-strtab.c too-many-strtab2.s