[PATCH] aarch64: always map VDSO at worst case alignment

[Kyle McMartin <kyle@redhat.com>]

Currently on ARM64 with 4K pages set, GDB fails to load the VDSO with
the error "Failed to read a valid object file image from memory" as it
is applying the phdr alignment to the vma, and attempting to read below
where the VDSO is mapped. This is because our segment alignment is 64K
in the ELF headers, but the VDSO only has PAGE_SIZE alignment from
get_unmapped_area.

Work around this by calling vm_unmapped_area directly, and specifying
the worst case alignment (64K) directly.

With this patch applied, I no longer have issues loading the VDSO in
gdb (and no more error message every time I run a program inside it.)

Signed-off-by: Kyle McMartin <kyle@redhat.com>

--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -163,7 +163,18 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
 	vdso_mapping_len = (vdso_pages + 1) << PAGE_SHIFT;
 
 	down_write(&mm->mmap_sem);
-	vdso_base = get_unmapped_area(NULL, 0, vdso_mapping_len, 0, 0);
+	{
+		/* the VDSO must be worst-case aligned to 64K */
+		struct vm_unmapped_area_info info =
+			{
+				.flags = 0,
+				.length = vdso_mapping_len,
+				.low_limit = mm->mmap_base,
+				.high_limit = TASK_SIZE,
+				.align_mask = (1 << 16) - 1,
+			};
+		vdso_base = vm_unmapped_area(&info);
+	}
 	if (IS_ERR_VALUE(vdso_base)) {
 		ret = vdso_base;
 		goto up_fail;