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Commit e0bf7b86 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by H. Peter Anvin
Browse files

x86/vdso: Hack to keep 64-bit Go programs working 



The Go runtime has a buggy vDSO parser that currently segfaults.
This writes an empty SHT_DYNSYM entry that causes Go's runtime to
malfunction by thinking that the vDSO is empty rather than
malfunctioning by running off the end and segfaulting.

This affects x86-64 only as far as we know, so we do not need this for
the i386 and x32 vdsos.

Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/d10618176c4bd39b457a5e85c497295c90cab1bc.1402620737.git.luto@amacapital.net


Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent b4b31f61

arch/x86/vdso/Makefile

+10 −8
Original line number Diff line number Diff line
@@ -15,12 +15,8 @@ vdso-install-$(VDSO32-y) += $(vdso32-images) 




# files to link into the vdso

vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o



vobjs-$(VDSOX32-y) += $(vobjx32s-compat)



# Filter out x32 objects.

vobj64s := $(filter-out $(vobjx32s-compat),$(vobjs-y))

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
@@ -34,7 +30,7 @@ vdso_img-$(VDSO32-y) += 32-sysenter 


obj-$(VDSO32-y)			+= vdso32-setup.o



vobjs := $(foreach F,$(vobj64s),$(obj)/$F)

vobjs := $(foreach F,$(vobjs-y),$(obj)/$F)



$(obj)/vdso.o: $(obj)/vdso.so
@@ -104,7 +100,13 @@ VDSO_LDFLAGS_vdsox32.lds = -Wl,-m,elf32_x86_64 \ 
			   -Wl,-z,max-page-size=4096 \

			   -Wl,-z,common-page-size=4096



vobjx32s-y := $(vobj64s:.o=-x32.o)

# 64-bit objects to re-brand as x32

vobjs64-for-x32 := $(filter-out $(vobjs-nox32),$(vobjs-y))



# x32-rebranded versions

vobjx32s-y := $(vobjs64-for-x32:.o=-x32.o)



# same thing, but in the output directory

vobjx32s := $(foreach F,$(vobjx32s-y),$(obj)/$F)



# Convert 64bit object file to x32 for x32 vDSO.

arch/x86/vdso/vdso-fakesections.c

0 → 100644
+32 −0
Original line number Diff line number Diff line 
/*

 * 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

 */



#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),

	}

};

arch/x86/vdso/vdso2c.h

+18 −5
Original line number Diff line number Diff line
@@ -18,6 +18,8 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name) 
	const char *secstrings;

	uint64_t syms[NSYMS] = {};



	uint64_t fake_sections_value = 0, fake_sections_size = 0;



	Elf_Phdr *pt = (Elf_Phdr *)(addr + GET_LE(&hdr->e_phoff));



	/* Walk the segment table. */
@@ -84,6 +86,7 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name) 
			GET_LE(&symtab_hdr->sh_entsize) * i;

		const char *name = addr + GET_LE(&strtab_hdr->sh_offset) +

			GET_LE(&sym->st_name);



		for (k = 0; k < NSYMS; k++) {

			if (!strcmp(name, required_syms[k])) {

				if (syms[k]) {
@@ -93,6 +96,13 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name) 
				syms[k] = GET_LE(&sym->st_value);

			}

		}



		if (!strcmp(name, "vdso_fake_sections")) {

			if (fake_sections_value)

				fail("duplicate vdso_fake_sections\n");

			fake_sections_value = GET_LE(&sym->st_value);

			fake_sections_size = GET_LE(&sym->st_size);

		}

	}



	/* Validate mapping addresses. */
@@ -112,11 +122,14 @@ static void GOFUNC(void *addr, size_t len, FILE *outfile, const char *name) 
	if (syms[sym_end_mapping] % 4096)

		fail("end_mapping must be a multiple of 4096\n");



	/* Remove sections. */

	hdr->e_shoff = 0;

	hdr->e_shentsize = 0;

	hdr->e_shnum = 0;

	hdr->e_shstrndx = SHN_UNDEF; /* SHN_UNDEF == 0 */

	/* Remove sections or use fakes */

	if (fake_sections_size % sizeof(Elf_Shdr))

		fail("vdso_fake_sections size is not a multiple of %ld\n",

		     (long)sizeof(Elf_Shdr));

	PUT_LE(&hdr->e_shoff, fake_sections_value);

	PUT_LE(&hdr->e_shentsize, fake_sections_value ? sizeof(Elf_Shdr) : 0);

	PUT_LE(&hdr->e_shnum, fake_sections_size / sizeof(Elf_Shdr));

	PUT_LE(&hdr->e_shstrndx, SHN_UNDEF);



	if (!name) {

		fwrite(addr, load_size, 1, outfile);