From bfad381c0d1e19cae8461e105d8d4387dd2a14fe Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Wed, 18 Jun 2014 15:59:48 -0700 Subject: [PATCH] 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 Link: http://lkml.kernel.org/r/0e546a5eeaafdf1840e6ee654a55c1e727c26663.1403129369.git.luto@amacapital.net Signed-off-by: H. Peter Anvin --- 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(-) create mode 100644 arch/x86/vdso/vdso32/vdso-fakesections.c diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile index 3c0809a0631f..2c1ca98eb612 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 cb8a8d72c24b..56927a7e4977 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 - -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 c84166cbcd28..e4cbc2145bab 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 75e3404c83b1..6807932643c2 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 734389976cc0..238dbe82776e 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 8e185ce39e69..f01ed4bde880 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 000000000000..541468e25265 --- /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 46b991b578a8..697c11ece90c 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" /* -- 2.30.2