elf: Add ELF_DYNAMIC_AFTER_RELOC to rewrite PLT

Add ELF_DYNAMIC_AFTER_RELOC to allow target specific processing after
relocation.

For x86-64, add

 #define DT_X86_64_PLT     (DT_LOPROC + 0)
 #define DT_X86_64_PLTSZ   (DT_LOPROC + 1)
 #define DT_X86_64_PLTENT  (DT_LOPROC + 3)

1. DT_X86_64_PLT: The address of the procedure linkage table.
2. DT_X86_64_PLTSZ: The total size, in bytes, of the procedure linkage
table.
3. DT_X86_64_PLTENT: The size, in bytes, of a procedure linkage table
entry.

With the r_addend field of the R_X86_64_JUMP_SLOT relocation set to the
memory offset of the indirect branch instruction.

Define ELF_DYNAMIC_AFTER_RELOC for x86-64 to rewrite the PLT section
with direct branch after relocation when the lazy binding is disabled.

PLT rewrite is disabled by default since SELinux may disallow modifying
code pages and ld.so can't detect it in all cases.  Use

$ export GLIBC_TUNABLES=glibc.cpu.plt_rewrite=1

to enable PLT rewrite with 32-bit direct jump at run-time or

$ export GLIBC_TUNABLES=glibc.cpu.plt_rewrite=2

to enable PLT rewrite with 32-bit direct jump and on APX processors with
64-bit absolute jump at run-time.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

8 files changed, 387 insertions, 1 deletions

diff --git a/sysdeps/x86_64/Makefile b/sysdeps/x86_64/Makefile
index 00120ca9ca..374bca80d0 100644
--- a/sysdeps/x86_64/Makefile
+++ b/sysdeps/x86_64/Makefile
@@ -1,6 +1,14 @@
 # The i387 `long double' is a distinct type we support.
 long-double-fcts = yes
 
+ifeq (yes,$(have-z-mark-plt))
+# Always generate DT_X86_64_PLT* tags.
+sysdep-LDFLAGS += -Wl,-z,mark-plt
+# Never generate DT_X86_64_PLT* tags on ld.so to avoid changing its own
+# PLT.
+LDFLAGS-rtld += -Wl,-z,nomark-plt
+endif
+
 ifeq ($(subdir),csu)
 gen-as-const-headers += link-defines.sym
 endif
@@ -175,6 +183,25 @@ ifeq (no,$(build-hardcoded-path-in-tests))
 tests-container += tst-glibc-hwcaps-cache
 endif
 
+ifeq (yes,$(have-z-mark-plt))
+tests += \
+  tst-plt-rewrite1 \
+# tests
+modules-names += \
+  tst-plt-rewritemod1 \
+# modules-names
+
+tst-plt-rewrite1-no-pie = yes
+LDFLAGS-tst-plt-rewrite1 = -Wl,-z,now
+LDFLAGS-tst-plt-rewritemod1.so = -Wl,-z,now
+tst-plt-rewrite1-ENV = GLIBC_TUNABLES=glibc.cpu.plt_rewrite=1 LD_DEBUG=files:bindings
+$(objpfx)tst-plt-rewrite1: $(objpfx)tst-plt-rewritemod1.so
+$(objpfx)tst-plt-rewrite1.out: /dev/null $(objpfx)tst-plt-rewrite1
+	$(tst-plt-rewrite1-ENV) $(make-test-out) > $@ 2>&1; \
+	grep -q -E "changing 'bar' PLT entry in .*/elf/tst-plt-rewritemod1.so' to direct branch" $@; \
+	$(evaluate-test)
+endif
+
 endif # $(subdir) == elf
 
 ifeq ($(subdir),csu)
diff --git a/sysdeps/x86_64/configure b/sysdeps/x86_64/configure
index b4a80b8035..418cc4a9b8 100755
--- a/sysdeps/x86_64/configure
+++ b/sysdeps/x86_64/configure
@@ -25,6 +25,41 @@ printf "%s\n" "$libc_cv_cc_mprefer_vector_width" >&6; }
 config_vars="$config_vars
 config-cflags-mprefer-vector-width = $libc_cv_cc_mprefer_vector_width"
 
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for linker that supports -z mark-plt" >&5
+printf %s "checking for linker that supports -z mark-plt... " >&6; }
+libc_linker_feature=no
+cat > conftest.c <<EOF
+int _start (void) { return 42; }
+EOF
+if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp
+		  -Wl,-z,mark-plt -nostdlib -nostartfiles
+		  -fPIC -shared -o conftest.so conftest.c
+		  1>&5'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }
+then
+  if ${CC-cc} $CFLAGS $CPPFLAGS $LDFLAGS $no_ssp -Wl,-z,mark-plt -nostdlib \
+      -nostartfiles -fPIC -shared -o conftest.so conftest.c 2>&1 \
+      | grep "warning: -z mark-plt ignored" > /dev/null 2>&1; then
+    true
+  else
+    libc_linker_feature=yes
+  fi
+fi
+rm -f conftest*
+if test $libc_linker_feature = yes; then
+  libc_cv_z_mark_plt=yes
+else
+  libc_cv_z_mark_plt=no
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $libc_linker_feature" >&5
+printf "%s\n" "$libc_linker_feature" >&6; }
+config_vars="$config_vars
+have-z-mark-plt = $libc_cv_z_mark_plt"
+
 if test x"$build_mathvec" = xnotset; then
   build_mathvec=yes
 fi
diff --git a/sysdeps/x86_64/configure.ac b/sysdeps/x86_64/configure.ac
index 937d1aff7e..d1f803c02e 100644
--- a/sysdeps/x86_64/configure.ac
+++ b/sysdeps/x86_64/configure.ac
@@ -10,6 +10,10 @@ LIBC_TRY_CC_OPTION([-mprefer-vector-width=128],
 LIBC_CONFIG_VAR([config-cflags-mprefer-vector-width],
 		[$libc_cv_cc_mprefer_vector_width])
 
+LIBC_LINKER_FEATURE([-z mark-plt], [-Wl,-z,mark-plt],
+                    [libc_cv_z_mark_plt=yes], [libc_cv_z_mark_plt=no])
+LIBC_CONFIG_VAR([have-z-mark-plt], [$libc_cv_z_mark_plt])
+
 if test x"$build_mathvec" = xnotset; then
   build_mathvec=yes
 fi
diff --git a/sysdeps/x86_64/dl-dtprocnum.h b/sysdeps/x86_64/dl-dtprocnum.h
new file mode 100644
index 0000000000..01573aaf4a
--- /dev/null
+++ b/sysdeps/x86_64/dl-dtprocnum.h
@@ -0,0 +1,21 @@
+/* Configuration of lookup functions.  x64-64 version.
+   Copyright (C) 2024 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/>.  */
+
+/* Number of extra dynamic section entries for this architecture.  By
+   default there are none.  */
+#define DT_THISPROCNUM DT_X86_64_NUM
diff --git a/sysdeps/x86_64/dl-machine.h b/sysdeps/x86_64/dl-machine.h
index e0a9b14469..ca290ef082 100644
--- a/sysdeps/x86_64/dl-machine.h
+++ b/sysdeps/x86_64/dl-machine.h
@@ -22,6 +22,7 @@
 #define ELF_MACHINE_NAME "x86_64"
 
 #include <assert.h>
+#include <stdint.h>
 #include <sys/param.h>
 #include <sysdep.h>
 #include <tls.h>
@@ -35,6 +36,9 @@
 # define RTLD_START_ENABLE_X86_FEATURES
 #endif
 
+/* Translate a processor specific dynamic tag to the index in l_info array.  */
+#define DT_X86_64(x) (DT_X86_64_##x - DT_LOPROC + DT_NUM)
+
 /* Return nonzero iff ELF header is compatible with the running host.  */
 static inline int __attribute__ ((unused))
 elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
@@ -312,8 +316,10 @@ and creates an unsatisfiable circular dependency.\n",
 
       switch (r_type)
 	{
-	case R_X86_64_GLOB_DAT:
 	case R_X86_64_JUMP_SLOT:
+	  map->l_has_jump_slot_reloc = true;
+	  /* fallthrough */
+	case R_X86_64_GLOB_DAT:
 	  *reloc_addr = value;
 	  break;
 
@@ -549,3 +555,211 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
 }
 
 #endif /* RESOLVE_MAP */
+
+#if !defined ELF_DYNAMIC_AFTER_RELOC && !defined RTLD_BOOTSTRAP \
+    && defined SHARED
+# define ELF_DYNAMIC_AFTER_RELOC(map, lazy) \
+  x86_64_dynamic_after_reloc (map, (lazy))
+
+# define JMP32_INSN_OPCODE	0xe9
+# define JMP32_INSN_SIZE	5
+# define JMPABS_INSN_OPCODE	0xa100d5
+# define JMPABS_INSN_SIZE	11
+# define INT3_INSN_OPCODE	0xcc
+
+static const char *
+x86_64_reloc_symbol_name (struct link_map *map, const ElfW(Rela) *reloc)
+{
+  const ElfW(Sym) *const symtab
+    = (const void *) map->l_info[DT_SYMTAB]->d_un.d_ptr;
+  const ElfW(Sym) *const refsym = &symtab[ELFW (R_SYM) (reloc->r_info)];
+  const char *strtab = (const char *) map->l_info[DT_STRTAB]->d_un.d_ptr;
+  return strtab + refsym->st_name;
+}
+
+static void
+x86_64_rewrite_plt (struct link_map *map, ElfW(Addr) plt_rewrite)
+{
+  ElfW(Addr) l_addr = map->l_addr;
+  ElfW(Addr) pltent = map->l_info[DT_X86_64 (PLTENT)]->d_un.d_val;
+  ElfW(Addr) start = map->l_info[DT_JMPREL]->d_un.d_ptr;
+  ElfW(Addr) size = map->l_info[DT_PLTRELSZ]->d_un.d_val;
+  const ElfW(Rela) *reloc = (const void *) start;
+  const ElfW(Rela) *reloc_end = (const void *) (start + size);
+
+  unsigned int feature_1 = THREAD_GETMEM (THREAD_SELF,
+					  header.feature_1);
+  bool ibt_enabled_p
+    = (feature_1 & GNU_PROPERTY_X86_FEATURE_1_IBT) != 0;
+
+  if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+    _dl_debug_printf ("\nchanging PLT in '%s' to direct branch\n",
+		      DSO_FILENAME (map->l_name));
+
+  for (; reloc < reloc_end; reloc++)
+    if (ELFW(R_TYPE) (reloc->r_info) == R_X86_64_JUMP_SLOT)
+      {
+	/* Get the value from the GOT entry.  */
+	ElfW(Addr) value = *(ElfW(Addr) *) (l_addr + reloc->r_offset);
+
+	/* Get the corresponding PLT entry from r_addend.  */
+	ElfW(Addr) branch_start = l_addr + reloc->r_addend;
+	/* Skip ENDBR64 if IBT isn't enabled.  */
+	if (!ibt_enabled_p)
+	  branch_start = ALIGN_DOWN (branch_start, pltent);
+	/* Get the displacement from the branch target.  */
+	ElfW(Addr) disp = value - branch_start - JMP32_INSN_SIZE;
+	ElfW(Addr) plt_end;
+	ElfW(Addr) pad;
+
+	plt_end = (branch_start | (pltent - 1)) + 1;
+
+	/* Update the PLT entry.  */
+	if (((uint64_t) disp + (uint64_t) ((uint32_t) INT32_MIN))
+	    <= (uint64_t) UINT32_MAX)
+	  {
+	    pad = branch_start + JMP32_INSN_SIZE;
+
+	    if (__glibc_unlikely (pad > plt_end))
+	      continue;
+
+	    /* If the target branch can be reached with a direct branch,
+	       rewrite the PLT entry with a direct branch.  */
+	    if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_BINDINGS))
+	      {
+		const char *sym_name = x86_64_reloc_symbol_name (map,
+								 reloc);
+		_dl_debug_printf ("changing '%s' PLT entry in '%s' to "
+				  "direct branch\n", sym_name,
+				  DSO_FILENAME (map->l_name));
+	      }
+
+	    /* Write out direct branch.  */
+	    *(uint8_t *) branch_start = JMP32_INSN_OPCODE;
+	    *(uint32_t *) (branch_start + 1) = disp;
+	  }
+	else
+	  {
+	    if (GL(dl_x86_feature_control).plt_rewrite
+		!= plt_rewrite_jmpabs)
+	      {
+		if (__glibc_unlikely (GLRO(dl_debug_mask)
+				      & DL_DEBUG_BINDINGS))
+		  {
+		    const char *sym_name
+		      = x86_64_reloc_symbol_name (map, reloc);
+		    _dl_debug_printf ("skipping '%s' PLT entry in '%s'\n",
+				      sym_name,
+				      DSO_FILENAME (map->l_name));
+		  }
+		continue;
+	      }
+
+	    pad = branch_start + JMPABS_INSN_SIZE;
+
+	    if (__glibc_unlikely (pad > plt_end))
+	      continue;
+
+	    /* Rewrite the PLT entry with JMPABS.  */
+	    if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_BINDINGS))
+	      {
+		const char *sym_name = x86_64_reloc_symbol_name (map,
+								 reloc);
+		_dl_debug_printf ("changing '%s' PLT entry in '%s' to "
+				  "JMPABS\n", sym_name,
+				  DSO_FILENAME (map->l_name));
+	      }
+
+	    /* "jmpabs $target" for 64-bit displacement.  NB: JMPABS has
+	       a 3-byte opcode + 64bit address.  There is a 1-byte overlap
+	       between 4-byte write and 8-byte write.  */
+	    *(uint32_t *) (branch_start) = JMPABS_INSN_OPCODE;
+	    *(uint64_t *) (branch_start + 3) = value;
+	  }
+
+	/* Fill the unused part of the PLT entry with INT3.  */
+	for (; pad < plt_end; pad++)
+	  *(uint8_t *) pad = INT3_INSN_OPCODE;
+      }
+}
+
+static inline void
+x86_64_rewrite_plt_in_place (struct link_map *map)
+{
+  /* Adjust DT_X86_64_PLT address and DT_X86_64_PLTSZ values.  */
+  ElfW(Addr) plt = (map->l_info[DT_X86_64 (PLT)]->d_un.d_ptr
+		    + map->l_addr);
+  size_t pagesize = GLRO(dl_pagesize);
+  ElfW(Addr) plt_aligned = ALIGN_DOWN (plt, pagesize);
+  size_t pltsz = (map->l_info[DT_X86_64 (PLTSZ)]->d_un.d_val
+		  + plt - plt_aligned);
+
+  if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+    _dl_debug_printf ("\nchanging PLT in '%s' to writable\n",
+		      DSO_FILENAME (map->l_name));
+
+  if (__glibc_unlikely (__mprotect ((void *) plt_aligned, pltsz,
+				    PROT_WRITE | PROT_READ) < 0))
+    {
+      if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+	_dl_debug_printf ("\nfailed to change PLT in '%s' to writable\n",
+			  DSO_FILENAME (map->l_name));
+      return;
+    }
+
+  x86_64_rewrite_plt (map, plt_aligned);
+
+  if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_FILES))
+    _dl_debug_printf ("\nchanging PLT in '%s' back to read-only\n",
+		      DSO_FILENAME (map->l_name));
+
+  if (__glibc_unlikely (__mprotect ((void *) plt_aligned, pltsz,
+				    PROT_EXEC | PROT_READ) < 0))
+    _dl_signal_error (0, DSO_FILENAME (map->l_name), NULL,
+		      "failed to change PLT back to read-only");
+}
+
+/* Rewrite PLT entries to direct branch if possible.  */
+
+static inline void
+x86_64_dynamic_after_reloc (struct link_map *map, int lazy)
+{
+  /* Ignore DT_X86_64_PLT if the lazy binding is enabled.  */
+  if (lazy != 0)
+    return;
+
+  /* Ignore DT_X86_64_PLT if PLT rewrite isn't enabled.  */
+  if (__glibc_likely (GL(dl_x86_feature_control).plt_rewrite
+		      == plt_rewrite_none))
+    return;
+
+  if (__glibc_likely (map->l_info[DT_X86_64 (PLT)] == NULL))
+    return;
+
+  /* Ignore DT_X86_64_PLT if there is no R_X86_64_JUMP_SLOT.  */
+  if (map->l_has_jump_slot_reloc == 0)
+    return;
+
+  /* Ignore DT_X86_64_PLT if
+     1. DT_JMPREL isn't available or its value is 0.
+     2. DT_PLTRELSZ is 0.
+     3. DT_X86_64_PLTENT isn't available or its value is smaller than
+	16 bytes.
+     4. DT_X86_64_PLTSZ isn't available or its value is smaller than
+	DT_X86_64_PLTENT's value or isn't a multiple of DT_X86_64_PLTENT's
+	value.  */
+  if (map->l_info[DT_JMPREL] == NULL
+      || map->l_info[DT_JMPREL]->d_un.d_ptr == 0
+      || map->l_info[DT_PLTRELSZ]->d_un.d_val == 0
+      || map->l_info[DT_X86_64 (PLTSZ)] == NULL
+      || map->l_info[DT_X86_64 (PLTENT)] == NULL
+      || map->l_info[DT_X86_64 (PLTENT)]->d_un.d_val < 16
+      || (map->l_info[DT_X86_64 (PLTSZ)]->d_un.d_val
+	  < map->l_info[DT_X86_64 (PLTENT)]->d_un.d_val)
+      || (map->l_info[DT_X86_64 (PLTSZ)]->d_un.d_val
+	  % map->l_info[DT_X86_64 (PLTENT)]->d_un.d_val) != 0)
+    return;
+
+  x86_64_rewrite_plt_in_place (map);
+}
+#endif
diff --git a/sysdeps/x86_64/link_map.h b/sysdeps/x86_64/link_map.h
new file mode 100644
index 0000000000..537f56ace5
--- /dev/null
+++ b/sysdeps/x86_64/link_map.h
@@ -0,0 +1,22 @@
+/* Additional fields in struct link_map.  x86-64 version.
+   Copyright (C) 2024 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/>.  */
+
+/* Has R_X86_64_JUMP_SLOT relocation.  */
+bool l_has_jump_slot_reloc;
+
+#include <sysdeps/x86/link_map.h>
diff --git a/sysdeps/x86_64/tst-plt-rewrite1.c b/sysdeps/x86_64/tst-plt-rewrite1.c
new file mode 100644
index 0000000000..86785957e2
--- /dev/null
+++ b/sysdeps/x86_64/tst-plt-rewrite1.c
@@ -0,0 +1,31 @@
+/* Test PLT rewrite.
+   Copyright (C) 2024 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 <string.h>
+#include <support/check.h>
+
+extern const char *foo (void);
+
+static int
+do_test (void)
+{
+  TEST_COMPARE (strcmp (foo (), "PLT rewrite works"), 0);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/x86_64/tst-plt-rewritemod1.c b/sysdeps/x86_64/tst-plt-rewritemod1.c
new file mode 100644
index 0000000000..99f21fba5a
--- /dev/null
+++ b/sysdeps/x86_64/tst-plt-rewritemod1.c
@@ -0,0 +1,32 @@
+/* Check PLT rewrite works correctly.
+   Copyright (C) 2024 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/>.  */
+
+/* foo calls bar with indirect branch via PLT.  PLT rewrite should
+   change it to direct branch.  */
+
+const char *
+bar (void)
+{
+  return "PLT rewrite works";
+}
+
+const char *
+foo (void)
+{
+  return bar ();
+}