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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>
This commit is contained in:
Andy Lutomirski 2014-06-18 15:59:48 -07:00 committed by H. Peter Anvin
parent c1979c3702
commit bfad381c0d
8 changed files with 237 additions and 67 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
arch/x86/vdso/Makefile
View File

@ -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)

44
arch/x86/vdso/vdso-fakesections.c
View File

@ -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";

40
arch/x86/vdso/vdso-layout.lds.S
View File

@ -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) }

2
arch/x86/vdso/vdso.lds.S
View File

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

31
arch/x86/vdso/vdso2c.c
View File

@ -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))

180
arch/x86/vdso/vdso2c.h
View File

@ -4,6 +4,116 @@
* are built for 32-bit userspace.
*/
/*
* We're writing a section table for a few reasons:
*
* 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. An empty DYNSYM
* table would work, but I see no reason not to write a valid one (and
* keep full performance for old Go programs). This hack is only
* needed on x86_64.
*
* The bug was introduced on 2012-08-31 by:
* https://code.google.com/p/go/source/detail?r=56ea40aac72b
* and was fixed on 2014-06-13 by:
* https://code.google.com/p/go/source/detail?r=fc1cd5e12595
*
* Binutils has issues debugging the vDSO: it reads the section table to
* find SHT_NOTE; it won't look at PT_NOTE for the in-memory vDSO, which
* would break build-id if we removed the section table. Binutils
* also requires that shstrndx != 0. See:
* https://sourceware.org/bugzilla/show_bug.cgi?id=17064
*
* elfutils might not look for PT_NOTE if there is a section table at
* all. I don't know whether this matters for any practical purpose.
*
* For simplicity, rather than hacking up a partial section table, we
* just write a mostly complete one. We omit non-dynamic symbols,
* though, since they're rather large.
*
* Once binutils gets fixed, we might be able to drop this for all but
* the 64-bit vdso, since build-id only works in kernel RPMs, and
* systems that update to new enough kernel RPMs will likely update
* binutils in sync. build-id has never worked for home-built kernel
* RPMs without manual symlinking, and I suspect that no one ever does
* that.
*/
struct BITSFUNC(fake_sections)
{
ELF(Shdr) *table;
unsigned long table_offset;
int count, max_count;
int in_shstrndx;
unsigned long shstr_offset;
const char *shstrtab;
size_t shstrtab_len;
int out_shstrndx;
};
static unsigned int BITSFUNC(find_shname)(struct BITSFUNC(fake_sections) *out,
const char *name)
{
const char *outname = out->shstrtab;
while (outname - out->shstrtab < out->shstrtab_len) {
if (!strcmp(name, outname))
return (outname - out->shstrtab) + out->shstr_offset;
outname += strlen(outname) + 1;
}
if (*name)
printf("Warning: could not find output name \"%s\"\n", name);
return out->shstr_offset + out->shstrtab_len - 1; /* Use a null. */
}
static void BITSFUNC(init_sections)(struct BITSFUNC(fake_sections) *out)
{
if (!out->in_shstrndx)
fail("didn't find the fake shstrndx\n");
memset(out->table, 0, out->max_count * sizeof(ELF(Shdr)));
if (out->max_count < 1)
fail("we need at least two fake output sections\n");
PUT_LE(&out->table[0].sh_type, SHT_NULL);
PUT_LE(&out->table[0].sh_name, BITSFUNC(find_shname)(out, ""));
out->count = 1;
}
static void BITSFUNC(copy_section)(struct BITSFUNC(fake_sections) *out,
int in_idx, const ELF(Shdr) *in,
const char *name)
{
uint64_t flags = GET_LE(&in->sh_flags);
bool copy = flags & SHF_ALLOC;
if (!copy)
return;
if (out->count >= out->max_count)
fail("too many copied sections (max = %d)\n", out->max_count);
if (in_idx == out->in_shstrndx)
out->out_shstrndx = out->count;
out->table[out->count] = *in;
PUT_LE(&out->table[out->count].sh_name,
BITSFUNC(find_shname)(out, name));
/* elfutils requires that a strtab have the correct type. */
if (!strcmp(name, ".fake_shstrtab"))
PUT_LE(&out->table[out->count].sh_type, SHT_STRTAB);
out->count++;
}
static void BITSFUNC(go)(void *addr, size_t len,
FILE *outfile, const char *name)
{
@ -19,7 +129,7 @@ static void BITSFUNC(go)(void *addr, size_t len,
const char *secstrings;
uint64_t syms[NSYMS] = {};
uint64_t fake_sections_value = 0, fake_sections_size = 0;
struct BITSFUNC(fake_sections) fake_sections = {};
ELF(Phdr) *pt = (ELF(Phdr) *)(addr + GET_LE(&hdr->e_phoff));
@ -89,23 +199,57 @@ static void BITSFUNC(go)(void *addr, size_t len,
GET_LE(&sym->st_name);
for (k = 0; k < NSYMS; k++) {
if (!strcmp(name, required_syms[k])) {
if (!strcmp(name, required_syms[k].name)) {
if (syms[k]) {
fail("duplicate symbol %s\n",
required_syms[k]);
required_syms[k].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);
if (!strcmp(name, "fake_shstrtab")) {
ELF(Shdr) *sh;
fake_sections.in_shstrndx = GET_LE(&sym->st_shndx);
fake_sections.shstrtab = addr + GET_LE(&sym->st_value);
fake_sections.shstrtab_len = GET_LE(&sym->st_size);
sh = addr + GET_LE(&hdr->e_shoff) +
GET_LE(&hdr->e_shentsize) *
fake_sections.in_shstrndx;
fake_sections.shstr_offset = GET_LE(&sym->st_value) -
GET_LE(&sh->sh_addr);
}
}
/* Build the output section table. */
if (!syms[sym_VDSO_FAKE_SECTION_TABLE_START] ||
!syms[sym_VDSO_FAKE_SECTION_TABLE_END])
fail("couldn't find fake section table\n");
if ((syms[sym_VDSO_FAKE_SECTION_TABLE_END] -
syms[sym_VDSO_FAKE_SECTION_TABLE_START]) % sizeof(ELF(Shdr)))
fail("fake section table size isn't a multiple of sizeof(Shdr)\n");
fake_sections.table = addr + syms[sym_VDSO_FAKE_SECTION_TABLE_START];
fake_sections.table_offset = syms[sym_VDSO_FAKE_SECTION_TABLE_START];
fake_sections.max_count = (syms[sym_VDSO_FAKE_SECTION_TABLE_END] -
syms[sym_VDSO_FAKE_SECTION_TABLE_START]) /
sizeof(ELF(Shdr));
BITSFUNC(init_sections)(&fake_sections);
for (i = 0; i < GET_LE(&hdr->e_shnum); i++) {
ELF(Shdr) *sh = addr + GET_LE(&hdr->e_shoff) +
GET_LE(&hdr->e_shentsize) * i;
BITSFUNC(copy_section)(&fake_sections, i, sh,
secstrings + GET_LE(&sh->sh_name));
}
if (!fake_sections.out_shstrndx)
fail("didn't generate shstrndx?!?\n");
PUT_LE(&hdr->e_shoff, fake_sections.table_offset);
PUT_LE(&hdr->e_shentsize, sizeof(ELF(Shdr)));
PUT_LE(&hdr->e_shnum, fake_sections.count);
PUT_LE(&hdr->e_shstrndx, fake_sections.out_shstrndx);
/* Validate mapping addresses. */
for (i = 0; i < sizeof(special_pages) / sizeof(special_pages[0]); i++) {
if (!syms[i])
@ -113,25 +257,17 @@ static void BITSFUNC(go)(void *addr, size_t len,
if (syms[i] % 4096)
fail("%s must be a multiple of 4096\n",
required_syms[i]);
required_syms[i].name);
if (syms[i] < data_size)
fail("%s must be after the text mapping\n",
required_syms[i]);
required_syms[i].name);
if (syms[sym_end_mapping] < syms[i] + 4096)
fail("%s overruns end_mapping\n", required_syms[i]);
fail("%s overruns end_mapping\n",
required_syms[i].name);
}
if (syms[sym_end_mapping] % 4096)
fail("end_mapping must be a multiple of 4096\n");
/* 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);
return;
@ -169,9 +305,9 @@ static void BITSFUNC(go)(void *addr, size_t len,
(unsigned long)GET_LE(&alt_sec->sh_size));
}
for (i = 0; i < NSYMS; i++) {
if (syms[i])
if (required_syms[i].export && syms[i])
fprintf(outfile, "\t.sym_%s = 0x%" PRIx64 ",\n",
required_syms[i], syms[i]);
required_syms[i].name, syms[i]);
}
fprintf(outfile, "};\n");
}

1
arch/x86/vdso/vdso32/vdso-fakesections.c Normal file
View File

@ -0,0 +1 @@
#include "../vdso-fakesections.c"

2
arch/x86/vdso/vdsox32.lds.S
View File

@ -6,6 +6,8 @@
* the DSO.
*/
#define BUILD_VDSOX32
#include "vdso-layout.lds.S"
/*