[tip:x86/urgent] x86/vdso: Improve the fake section headers
From: tip-bot for Andy Lutomirski
Date: Fri Jun 20 2014 - 22:08:16 EST
Commit-ID: bfad381c0d1e19cae8461e105d8d4387dd2a14fe
Gitweb: http://git.kernel.org/tip/bfad381c0d1e19cae8461e105d8d4387dd2a14fe
Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
AuthorDate: Wed, 18 Jun 2014 15:59:48 -0700
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Thu, 19 Jun 2014 15:45:12 -0700
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@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/0e546a5eeaafdf1840e6ee654a55c1e727c26663.1403129369.git.luto@xxxxxxxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
---
arch/x86/vdso/Makefile | 4 +-
arch/x86/vdso/vdso-fakesections.c | 44 ++++----
arch/x86/vdso/vdso-layout.lds.S | 40 +++++--
arch/x86/vdso/vdso.lds.S | 2 +
arch/x86/vdso/vdso2c.c | 31 ++++--
arch/x86/vdso/vdso2c.h | 180 +++++++++++++++++++++++++++----
arch/x86/vdso/vdso32/vdso-fakesections.c | 1 +
arch/x86/vdso/vdsox32.lds.S | 2 +
8 files changed, 237 insertions(+), 67 deletions(-)
diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile
index 3c0809a..2c1ca98 100644
--- a/arch/x86/vdso/Makefile
+++ b/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)
diff --git a/arch/x86/vdso/vdso-fakesections.c b/arch/x86/vdso/vdso-fakesections.c
index cb8a8d7..56927a7 100644
--- a/arch/x86/vdso/vdso-fakesections.c
+++ b/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";
diff --git a/arch/x86/vdso/vdso-layout.lds.S b/arch/x86/vdso/vdso-layout.lds.S
index c84166c..e4cbc21 100644
--- a/arch/x86/vdso/vdso-layout.lds.S
+++ b/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) }
diff --git a/arch/x86/vdso/vdso.lds.S b/arch/x86/vdso/vdso.lds.S
index 75e3404..6807932 100644
--- a/arch/x86/vdso/vdso.lds.S
+++ b/arch/x86/vdso/vdso.lds.S
@@ -6,6 +6,8 @@
* the DSO.
*/
+#define BUILD_VDSO64
+
#include "vdso-layout.lds.S"
/*
diff --git a/arch/x86/vdso/vdso2c.c b/arch/x86/vdso/vdso2c.c
index 7343899..238dbe82 100644
--- a/arch/x86/vdso/vdso2c.c
+++ b/arch/x86/vdso/vdso2c.c
@@ -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))
diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index 8e185ce..f01ed4b 100644
--- a/arch/x86/vdso/vdso2c.h
+++ b/arch/x86/vdso/vdso2c.h
@@ -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");
}
diff --git a/arch/x86/vdso/vdso32/vdso-fakesections.c b/arch/x86/vdso/vdso32/vdso-fakesections.c
new file mode 100644
index 0000000..541468e
--- /dev/null
+++ b/arch/x86/vdso/vdso32/vdso-fakesections.c
@@ -0,0 +1 @@
+#include "../vdso-fakesections.c"
diff --git a/arch/x86/vdso/vdsox32.lds.S b/arch/x86/vdso/vdsox32.lds.S
index 46b991b..697c11e 100644
--- a/arch/x86/vdso/vdsox32.lds.S
+++ b/arch/x86/vdso/vdsox32.lds.S
@@ -6,6 +6,8 @@
* the DSO.
*/
+#define BUILD_VDSOX32
+
#include "vdso-layout.lds.S"
/*
--
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