elf: Extend glibc.rtld.execstack tunable to force executable stack (BZ 32653)

From the bug report [1], multiple programs still require to dlopen
shared libraries with either missing PT_GNU_STACK or with the executable
bit set.  Although, in some cases, it seems to be a hard-craft assembly
source without the required .note.GNU-stack marking (so the static linker
is forced to set the stack executable if the ABI requires it), other
cases seem that the library uses trampolines [2].

Unfortunately, READ_IMPLIES_EXEC is not an option since on some ABIs
(x86_64), the kernel clears the bit, making it unsupported.  To avoid
reinstating the broken code that changes stack permission on dlopen
(0ca8785a28), this patch extends the glibc.rtld.execstack tunable to
allow an option to force an executable stack at the program startup.

The tunable is a security issue because it defeats the PT_GNU_STACK
hardening.  It has the slight advantage of making it explicit by the
caller, and, as for other tunables, this is disabled for setuid binaries.
A tunable also allows us to eventually remove it, but from previous
experiences, it would require some time.

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=32653
[2] https://github.com/conda-forge/ctng-compiler-activation-feedstock/issues/143
Reviewed-by: Sam James <sam@gentoo.org>

NEWS

@@ -23,7 +23,9 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* The glibc.rtld.execstack now supports a compatibility mode to allow
+  programs that require an executable stack through dynamic loaded
+  shared libraries.
 
 Changes to build and runtime requirements:
 

elf/Makefile

@@ -60,6 +60,7 @@ dl-routines = \
   dl-deps \
   dl-exception \
   dl-execstack \
+  dl-execstack-tunable \
   dl-find_object \
   dl-fini \
   dl-init \
@@ -572,9 +573,11 @@ tests-execstack-yes = \
   tst-execstack \
   tst-execstack-needed \
   tst-execstack-prog \
+  tst-execstack-tunable \
   # tests-execstack-yes
 tests-execstack-static-yes = \
-  tst-execstack-prog-static
+  tst-execstack-prog-static \
+  tst-execstack-prog-static-tunable \
   # tests-execstack-static-yes
 ifeq (yes,$(run-built-tests))
 tests-execstack-special-yes = \
@@ -2023,6 +2026,14 @@ LDFLAGS-tst-execstack-prog = -Wl,-z,execstack
 CFLAGS-tst-execstack-prog.c += -Wno-trampolines
 CFLAGS-tst-execstack-mod.c += -Wno-trampolines
 
+# It expects loading a module with executable stack to work.
+CFLAGS-tst-execstack-tunable.c += -DUSE_PTHREADS=0 -DDEFAULT_RWX_STACK=1
+$(objpfx)tst-execstack-tunable.out: $(objpfx)tst-execstack-mod.so
+tst-execstack-tunable-ENV = GLIBC_TUNABLES=glibc.rtld.execstack=2
+
+LDFLAGS-tst-execstack-prog-static-tunable = -Wl,-z,noexecstack
+tst-execstack-prog-static-tunable-ENV = GLIBC_TUNABLES=glibc.rtld.execstack=2
+
 LDFLAGS-tst-execstack-prog-static = -Wl,-z,execstack
 ifeq ($(have-no-error-execstack),yes)
 LDFLAGS-tst-execstack-prog-static += -Wl,--no-error-execstack

elf/dl-execstack-tunable.c

@@ -0,0 +1,39 @@
+/* Stack executability handling for GNU dynamic linker.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+#include <dl-tunables.h>
+
+void
+_dl_handle_execstack_tunable (void)
+{
+  switch (TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL))
+    {
+    case stack_tunable_mode_disable:
+      if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X))
+	_dl_fatal_printf (
+"Fatal glibc error: executable stack is not allowed\n");
+      break;
+
+    case stack_tunable_mode_force:
+      if (_dl_make_stack_executable (&__libc_stack_end) != 0)
+	_dl_fatal_printf (
+"Fatal glibc error: cannot enable executable stack as tunable requires");
+      break;
+    }
+}

elf/dl-support.c

@@ -331,9 +331,7 @@ _dl_non_dynamic_init (void)
 	break;
       }
 
-  if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X)
-      && TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL) == 0)
-    _dl_fatal_printf ("Fatal glibc error: executable stack is not allowed\n");
+  _dl_handle_execstack_tunable ();
 
   call_function_static_weak (_dl_find_object_init);
 

elf/dl-tunables.list

@@ -138,7 +138,7 @@ glibc {
     execstack {
       type: INT_32
       minval: 0
-      maxval: 1
+      maxval: 2
       default: 1
     }
   }

elf/rtld.c

@@ -1622,9 +1622,9 @@ dl_main (const ElfW(Phdr) *phdr,
 
   bool has_interp = rtld_setup_main_map (main_map);
 
-  if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X)
-      && TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL) == 0)
-    _dl_fatal_printf ("Fatal glibc error: executable stack is not allowed\n");
+  /* Handle this after PT_GNU_STACK parse, because it updates dl_stack_flags
+     if required.  */
+  _dl_handle_execstack_tunable ();
 
   /* If the current libname is different from the SONAME, add the
      latter as well.  */

elf/tst-execstack-prog-static-tunable.c

@@ -0,0 +1 @@
+#include <tst-execstack-prog-static.c>

elf/tst-execstack-tunable.c

@@ -0,0 +1 @@
+#include <tst-execstack.c>

elf/tst-rtld-list-tunables.exp

@@ -13,6 +13,6 @@ glibc.malloc.top_pad: 0x20000 (min: 0x0, max: 0x[f]+)
 glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0x[f]+)
 glibc.rtld.dynamic_sort: 2 (min: 1, max: 2)
 glibc.rtld.enable_secure: 0 (min: 0, max: 1)
-glibc.rtld.execstack: 1 (min: 0, max: 1)
+glibc.rtld.execstack: 1 (min: 0, max: 2)
 glibc.rtld.nns: 0x4 (min: 0x1, max: 0x10)
 glibc.rtld.optional_static_tls: 0x200 (min: 0x0, max: 0x[f]+)

manual/tunables.texi

@@ -365,8 +365,11 @@ change the main stack permission if kernel starts with a non-executable stack.
 The @code{glibc.rtld.execstack} can be used to control whether an executable
 stack is allowed from the main program.  Setting the value to @code{0} disables
 the ABI auto-negotiation (meaning no executable stacks even if the ABI or ELF
-header requires it), while @code{1} enables auto-negotiation (although the
-program might not need an executable stack).
+header requires it), @code{1} enables auto-negotiation (although the program
+might not need an executable stack), while @code{2} forces an executable
+stack at process start.  Tthis is provided for compatibility reasons, when
+the program dynamically loads modules with @code{dlopen} which require
+an executable stack.
 
 When executable stacks are not allowed, and if the main program requires it,
 the loader will fail with an error message.
@@ -380,7 +383,8 @@ of hardware capabilities and kernel configuration.
 @strong{NB:} Trying to load a dynamic shared library with @code{dlopen} or
 @code{dlmopen} that requires an executable stack will always fail if the
 main program does not require an executable stack at loading time.  This
-is enforced regardless of the tunable value.
+can be worked around by setting the tunable to @code{2}, where the stack is
+always executable.
 @end deftp
 
 @node Elision Tunables

sysdeps/generic/ldsodefs.h

@@ -717,6 +717,19 @@ extern const ElfW(Phdr) *_dl_phdr;
 extern size_t _dl_phnum;
 #endif
 
+/* Possible values for the glibc.rtld.execstack tunable.  */
+enum stack_tunable_mode
+  {
+    /* Do not allow executable stacks, even if program requires it.  */
+    stack_tunable_mode_disable = 0,
+    /* Follows either ABI requirement, or the PT_GNU_STACK value.  */
+    stack_tunable_mode_enable = 1,
+    /* Always enable an executable stack.  */
+    stack_tunable_mode_force = 2
+  };
+
+void _dl_handle_execstack_tunable (void) attribute_hidden;
+
 /* This function changes the permission of the memory region pointed
    by STACK_ENDP to executable and update the internal memory protection
    flags for future thread stack creation.  */