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Commit bfad381c authored by Andy Lutomirski's avatar Andy Lutomirski Committed by H. Peter Anvin
Browse files

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: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/0e546a5eeaafdf1840e6ee654a55c1e727c26663.1403129369.git.luto@amacapital.net


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent c1979c37

arch/x86/vdso/Makefile

+2 −2
Original line number Diff line number Diff line
@@ -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

+18 −26
Original line number Diff line number Diff line
@@ -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

+32 −8
Original line number Diff line number Diff line
@@ -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;
@@ -25,7 +35,21 @@ 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*)

arch/x86/vdso/vdso.lds.S

+2 −0
Original line number Diff line number Diff line
@@ -6,6 +6,8 @@ 
 * the DSO.

 */



#define BUILD_VDSO64



#include "vdso-layout.lds.S"



/*

arch/x86/vdso/vdso2c.c

+22 −9
Original line number Diff line number Diff line
@@ -23,6 +23,8 @@ 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[] = {
@@ -30,15 +32,26 @@ const int special_pages[] = { 
	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))