Use non-absolute addresses in librrpage

This matches the kernel's VDSO, so applications that inspect the VDSO library
won't get surprised.

Resolves #3791

CMakeLists.txt

@@ -1413,6 +1413,7 @@ set(BASIC_TESTS
   userfaultfd
   utimes
   vdso_parts
+  vdso_symbols
   vfork_done
   vfork_flush
   vfork_setopts

src/preload/rr_page.ld.in

@@ -8,7 +8,8 @@ PHDRS
 }
 SECTIONS
 {
-  . = 0x70000000 - 3 * @PRELOAD_LIBRARY_PAGE_SIZE@ + SIZEOF_HEADERS;
+  rr_page_start = 3 * @PRELOAD_LIBRARY_PAGE_SIZE@;
+  . = SIZEOF_HEADERS;
   .eh_frame_hdr   : { *(.eh_frame_hdr) *(.eh_frame_entry .eh_frame_entry.*) } :text :eh_frame
   .eh_frame       : { KEEP (*(.eh_frame)) *(.eh_frame.*) } :text :eh_frame
   .note.gnu.build-id  : { *(.note.gnu.build-id) } :text :note
@@ -23,19 +24,19 @@ SECTIONS
   .gnu.version_r  : { *(.gnu.version_r) } :text
   .got            : { *(.got) } :text
   .got.plt        : { *(.got.plt) } :text
-  . = 0x70000000 - 2 * @PRELOAD_LIBRARY_PAGE_SIZE@;
+  . = rr_page_start - 2 * @PRELOAD_LIBRARY_PAGE_SIZE@;
   /* This space in .sh_placeholder is reserved for the section table
      symtab/strtab, which ordinarily go after the text sections,
      but we need to have before the rr page.
      We move it there in a post-processing step, since linker
      scripts can't specify these locations for legacy reasons */
   .sh_placeholder : { *(.sh_placeholder) } :text
-  . = 0x70000000 - @PRELOAD_LIBRARY_PAGE_SIZE@;
+  . = rr_page_start - @PRELOAD_LIBRARY_PAGE_SIZE@;
   .plt       : { *(.plt) } :text
   .vdso.text : { *(.vdso.text) } :text
-  . = 0x70000000;
+  . = rr_page_start;
   .record.text : { *(.record.text) } :text
-  . = 0x70000000 + @PRELOAD_LIBRARY_PAGE_SIZE@;
+  . = rr_page_start + @PRELOAD_LIBRARY_PAGE_SIZE@;
   .replay.text : { *(.replay.text) } :replay
   /DISCARD/ : { *(.debug_* ) }
 }

src/test/vdso_symbols.c

@@ -0,0 +1,48 @@
+/* -*- Mode: C; tab-width: 8; c-basic-offset: 2; indent-tabs-mode: nil; -*- */
+
+#include "util.h"
+
+#ifdef __x86_64__
+typedef Elf64_Ehdr Ehdr;
+typedef Elf64_Shdr Shdr;
+typedef Elf64_Sym Sym;
+#else
+typedef Elf32_Ehdr Ehdr;
+typedef Elf32_Shdr Shdr;
+typedef Elf32_Sym Sym;
+#endif
+
+static const char* find_string(Ehdr* ehdr, uintptr_t offset) {
+  Shdr* strings = (Shdr*)((char*)ehdr + ehdr->e_shoff + ehdr->e_shstrndx*ehdr->e_shentsize);
+  return (char*)ehdr + strings->sh_offset + offset;
+}
+
+static Shdr* find_section(Ehdr* ehdr, const char* name) {
+  for (int i = 0; i < ehdr->e_shnum; ++i) {
+    Shdr* hdr = (Shdr*)((char*)ehdr + ehdr->e_shoff + i*ehdr->e_shentsize);
+    if (!strcmp(find_string(ehdr, hdr->sh_name), name)) {
+      return hdr;
+    }
+  }
+  return NULL;
+}
+
+int main(void) {
+  char* vdso = (char*)getauxval(AT_SYSINFO_EHDR);
+  Ehdr* ehdr = (Ehdr*)vdso;
+  Shdr* dynsym = find_section(ehdr, ".dynsym");
+  if (!dynsym) {
+    atomic_puts("dynsym not found in VDSO");
+    return 1;
+  }
+
+  for (int si = 0; si*dynsym->sh_entsize < dynsym->sh_size; ++si) {
+    Sym* sym = (Sym*)(vdso + dynsym->sh_offset) + si;
+    // All symbols must be within the VDSO size, i.e. not the
+    // absolute address. We assume the VDSO is less than 64K.
+    test_assert(sym->st_value < 0x10000);
+  }
+
+  atomic_puts("EXIT-SUCCESS");
+  return 0;
+}