x86-64: Allow alternative patching in the vDSO

This code is short enough and different enough from the module
loader that it's not worth trying to share anything.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/e73112e4381fff29e31b882c2d0856822edaea53.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/vma.c

@@ -23,11 +23,44 @@ extern unsigned short vdso_sync_cpuid;
 static struct page **vdso_pages;
 static unsigned vdso_size;
 
+static void __init patch_vdso(void *vdso, size_t len)
+{
+	Elf64_Ehdr *hdr = vdso;
+	Elf64_Shdr *sechdrs, *alt_sec = 0;
+	char *secstrings;
+	void *alt_data;
+	int i;
+
+	BUG_ON(len < sizeof(Elf64_Ehdr));
+	BUG_ON(memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0);
+
+	sechdrs = (void *)hdr + hdr->e_shoff;
+	secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
+
+	for (i = 1; i < hdr->e_shnum; i++) {
+		Elf64_Shdr *shdr = &sechdrs[i];
+		if (!strcmp(secstrings + shdr->sh_name, ".altinstructions")) {
+			alt_sec = shdr;
+			goto found;
+		}
+	}
+
+	/* If we get here, it's probably a bug. */
+	pr_warning("patch_vdso: .altinstructions not found\n");
+	return;  /* nothing to patch */
+
+found:
+	alt_data = (void *)hdr + alt_sec->sh_offset;
+	apply_alternatives(alt_data, alt_data + alt_sec->sh_size);
+}
+
 static int __init init_vdso_vars(void)
 {
 	int npages = (vdso_end - vdso_start + PAGE_SIZE - 1) / PAGE_SIZE;
 	int i;
 
+	patch_vdso(vdso_start, vdso_end - vdso_start);
+
 	vdso_size = npages << PAGE_SHIFT;
 	vdso_pages = kmalloc(sizeof(struct page *) * npages, GFP_KERNEL);
 	if (!vdso_pages)