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Subject[tip:x86/vdso] x86/vdso: Hack to keep 64-bit Go programs working
Commit-ID:  e0bf7b86dace87eccdabdd66d2769ccad19cb81c
Gitweb: http://git.kernel.org/tip/e0bf7b86dace87eccdabdd66d2769ccad19cb81c
Author: Andy Lutomirski <luto@amacapital.net>
AuthorDate: Thu, 12 Jun 2014 17:53:12 -0700
Committer: H. Peter Anvin <hpa@zytor.com>
CommitDate: Thu, 12 Jun 2014 19:02:30 -0700

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: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/d10618176c4bd39b457a5e85c497295c90cab1bc.1402620737.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
---
arch/x86/vdso/Makefile | 18 ++++++++++--------
arch/x86/vdso/vdso-fakesections.c | 32 ++++++++++++++++++++++++++++++++
arch/x86/vdso/vdso2c.h | 23 ++++++++++++++++++-----
3 files changed, 60 insertions(+), 13 deletions(-)

diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile
index 9769df0..ba6fc27 100644
--- a/arch/x86/vdso/Makefile
+++ b/arch/x86/vdso/Makefile
@@ -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.
diff --git a/arch/x86/vdso/vdso-fakesections.c b/arch/x86/vdso/vdso-fakesections.c
new file mode 100644
index 0000000..cb8a8d7
--- /dev/null
+++ b/arch/x86/vdso/vdso-fakesections.c
@@ -0,0 +1,32 @@
+/*
+ * 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),
+ }
+};
diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index d9f6f61..c6eefaf 100644
--- a/arch/x86/vdso/vdso2c.h
+++ b/arch/x86/vdso/vdso2c.h
@@ -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);

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