x86/vdso: Improve the fake section headers

Fully stripping the vDSO has other unfortunate side effects:

 - binutils is unable to find ELF notes without a SHT_NOTE section.

 - Even elfutils has trouble: it can find ELF notes without a section
   table at all, but if a section table is present, it won't look for
   PT_NOTE.

 - gdb wants section names to match between stripped DSOs and their
   symbols; otherwise it will corrupt symbol addresses.

We're also breaking the rules: section 0 is supposed to be SHT_NULL.

Fix these problems by building a better fake section table.  While
we're at it, we might as well let buggy Go versions keep working well
by giving the SHT_DYNSYM entry the correct size.

This is a bit unfortunate: it adds quite a bit of size to the vdso
image.

If/when binutils improves and the improved versions become widespread,
it would be worth considering dropping most of this.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/0e546a5eeaafdf1840e6ee654a55c1e727c26663.1403129369.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/Makefile

@@ -11,7 +11,6 @@ VDSO32-$(CONFIG_COMPAT)		:= y
 
 # files to link into the vdso
 vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vdso-fakesections.o
-vobjs-nox32 := vdso-fakesections.o
 
 # files to link into kernel
 obj-y				+= vma.o
@@ -134,7 +133,7 @@ override obj-dirs = $(dir $(obj)) $(obj)/vdso32/
 
 targets += vdso32/vdso32.lds
 targets += vdso32/note.o vdso32/vclock_gettime.o $(vdso32.so-y:%=vdso32/%.o)
-targets += vdso32/vclock_gettime.o
+targets += vdso32/vclock_gettime.o vdso32/vdso-fakesections.o
 
 $(obj)/vdso32.o: $(vdso32-images:%=$(obj)/%)
 
@@ -155,6 +154,7 @@ $(vdso32-images:%=$(obj)/%.dbg): KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)
 $(vdso32-images:%=$(obj)/%.dbg): $(obj)/vdso32-%.so.dbg: FORCE \
 				 $(obj)/vdso32/vdso32.lds \
 				 $(obj)/vdso32/vclock_gettime.o \
+				 $(obj)/vdso32/vdso-fakesections.o \
 				 $(obj)/vdso32/note.o \
 				 $(obj)/vdso32/%.o
 	$(call if_changed,vdso)

arch/x86/vdso/vdso-fakesections.c

@@ -2,31 +2,23 @@
  * Copyright 2014 Andy Lutomirski
  * Subject to the GNU Public License, v.2
  *
- * Hack to keep broken Go programs working.
- *
- * The Go runtime had a couple of bugs: it would read the section table to try
- * to figure out how many dynamic symbols there were (it shouldn't have looked
- * at the section table at all) and, if there were no SHT_SYNDYM section table
- * entry, it would use an uninitialized value for the number of symbols.  As a
- * workaround, we supply a minimal section table.  vdso2c will adjust the
- * in-memory image so that "vdso_fake_sections" becomes the section table.
- *
- * The bug was introduced by:
- * https://code.google.com/p/go/source/detail?r=56ea40aac72b (2012-08-31)
- * and is being addressed in the Go runtime in this issue:
- * https://code.google.com/p/go/issues/detail?id=8197
+ * String table for loadable section headers.  See vdso2c.h for why
+ * this exists.
  */
 
-#ifndef __x86_64__
-#error This hack is specific to the 64-bit vDSO
-#endif
-
-#include <linux/elf.h>
-
-extern const __visible struct elf64_shdr vdso_fake_sections[];
-const __visible struct elf64_shdr vdso_fake_sections[] = {
-	{
-		.sh_type = SHT_DYNSYM,
-		.sh_entsize = sizeof(Elf64_Sym),
-	}
-};
+const char fake_shstrtab[] __attribute__((section(".fake_shstrtab"))) =
+	".hash\0"
+	".dynsym\0"
+	".dynstr\0"
+	".gnu.version\0"
+	".gnu.version_d\0"
+	".dynamic\0"
+	".rodata\0"
+	".fake_shstrtab\0"  /* Yay, self-referential code. */
+	".note\0"
+	".data\0"
+	".altinstructions\0"
+	".altinstr_replacement\0"
+	".eh_frame_hdr\0"
+	".eh_frame\0"
+	".text";

arch/x86/vdso/vdso-layout.lds.S

@@ -6,6 +6,16 @@
  * This script controls its layout.
  */
 
+#if defined(BUILD_VDSO64)
+# define SHDR_SIZE 64
+#elif defined(BUILD_VDSO32) || defined(BUILD_VDSOX32)
+# define SHDR_SIZE 40
+#else
+# error unknown VDSO target
+#endif
+
+#define NUM_FAKE_SHDRS 16
+
 SECTIONS
 {
 	. = SIZEOF_HEADERS;
@@ -25,15 +35,29 @@ SECTIONS
 
 	.dynamic	: { *(.dynamic) }		:text	:dynamic
 
-	.rodata		: { *(.rodata*) }		:text
+	.rodata		: {
+		*(.rodata*)
+
+		/*
+		 * Ideally this would live in a C file, but that won't
+		 * work cleanly for x32 until we start building the x32
+		 * C code using an x32 toolchain.
+		 */
+		VDSO_FAKE_SECTION_TABLE_START = .;
+		. = . + NUM_FAKE_SHDRS * SHDR_SIZE;
+		VDSO_FAKE_SECTION_TABLE_END = .;
+	}						:text
+
+	.fake_shstrtab	: { *(.fake_shstrtab) }		:text
+
 	.data		: {
-	      *(.data*)
-	      *(.sdata*)
-	      *(.got.plt) *(.got)
-	      *(.gnu.linkonce.d.*)
-	      *(.bss*)
-	      *(.dynbss*)
-	      *(.gnu.linkonce.b.*)
+		*(.data*)
+		*(.sdata*)
+		*(.got.plt) *(.got)
+		*(.gnu.linkonce.d.*)
+		*(.bss*)
+		*(.dynbss*)
+		*(.gnu.linkonce.b.*)
 	}
 
 	.altinstructions	: { *(.altinstructions) }

arch/x86/vdso/vdso.lds.S

@@ -6,6 +6,8 @@
  * the DSO.
  */
 
+#define BUILD_VDSO64
+
 #include "vdso-layout.lds.S"
 
 /*

arch/x86/vdso/vdso2c.c

@@ -23,22 +23,35 @@ enum {
 	sym_vvar_page,
 	sym_hpet_page,
 	sym_end_mapping,
+	sym_VDSO_FAKE_SECTION_TABLE_START,
+	sym_VDSO_FAKE_SECTION_TABLE_END,
 };
 
 const int special_pages[] = {
 	sym_vvar_page,
 	sym_hpet_page,
 };
 
-char const * const required_syms[] = {
-	[sym_vvar_page] = "vvar_page",
-	[sym_hpet_page] = "hpet_page",
-	[sym_end_mapping] = "end_mapping",
-	"VDSO32_NOTE_MASK",
-	"VDSO32_SYSENTER_RETURN",
-	"__kernel_vsyscall",
-	"__kernel_sigreturn",
-	"__kernel_rt_sigreturn",
+struct vdso_sym {
+	const char *name;
+	bool export;
+};
+
+struct vdso_sym required_syms[] = {
+	[sym_vvar_page] = {"vvar_page", true},
+	[sym_hpet_page] = {"hpet_page", true},
+	[sym_end_mapping] = {"end_mapping", true},
+	[sym_VDSO_FAKE_SECTION_TABLE_START] = {
+		"VDSO_FAKE_SECTION_TABLE_START", false
+	},
+	[sym_VDSO_FAKE_SECTION_TABLE_END] = {
+		"VDSO_FAKE_SECTION_TABLE_END", false
+	},
+	{"VDSO32_NOTE_MASK", true},
+	{"VDSO32_SYSENTER_RETURN", true},
+	{"__kernel_vsyscall", true},
+	{"__kernel_sigreturn", true},
+	{"__kernel_rt_sigreturn", true},
 };
 
 __attribute__((format(printf, 1, 2))) __attribute__((noreturn))