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>

3 files changed, 60 insertions, 13 deletions

diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile
index 9769df094035..ba6fc2757ee4 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 000000000000..cb8a8d72c24b
--- /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 d9f6f61aef1c..c6eefaf389b9 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);