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>

1 files changed, 22 insertions, 9 deletions

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))