powerpc/vdso: Don't map VDSO at a fixed address on PPC32

PPC64 removed default mapping address from VDSO in
commit 30d0b36 ("powerpc: Move 64bit VDSO to improve context
switch performance").

Do like PPC64 and let get_unmapped_area() place the VDSO mapping
at the address it wants, don't force a default address.

This allows randomisation of VDSO address.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cba76f5a5b01fcc49415e632d92c11c1c5998cab.1660843877.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/vdso.h

@@ -2,9 +2,6 @@
 #ifndef _ASM_POWERPC_VDSO_H
 #define _ASM_POWERPC_VDSO_H
 
-/* Default map addresses for 32bit vDSO */
-#define VDSO32_MBASE	0x100000
-
 #define VDSO_VERSION_STRING	LINUX_2.6.15
 
 #ifndef __ASSEMBLY__

arch/powerpc/kernel/vdso.c

@@ -200,28 +200,19 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
 	if (is_32bit_task()) {
 		vdso_spec = &vdso32_spec;
 		vdso_size = &vdso32_end - &vdso32_start;
-		vdso_base = VDSO32_MBASE;
 	} else {
 		vdso_spec = &vdso64_spec;
 		vdso_size = &vdso64_end - &vdso64_start;
-		/*
-		 * On 64bit we don't have a preferred map address. This
-		 * allows get_unmapped_area to find an area near other mmaps
-		 * and most likely share a SLB entry.
-		 */
-		vdso_base = 0;
 	}
 
 	mappings_size = vdso_size + vvar_size;
 	mappings_size += (VDSO_ALIGNMENT - 1) & PAGE_MASK;
 
 	/*
-	 * pick a base address for the vDSO in process space. We try to put it
-	 * at vdso_base which is the "natural" base for it, but we might fail
-	 * and end up putting it elsewhere.
+	 * Pick a base address for the vDSO in process space.
 	 * Add enough to the size so that the result can be aligned.
 	 */
-	vdso_base = get_unmapped_area(NULL, vdso_base, mappings_size, 0, 0);
+	vdso_base = get_unmapped_area(NULL, 0, mappings_size, 0, 0);
 	if (IS_ERR_VALUE(vdso_base))
 		return vdso_base;