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, 32 insertions, 8 deletions

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) }