powerpc: Fix alignment bug within the init sections

This is a bug that causes early crashes in builds with an .exit.text
section smaller than a page and an .init.text section that ends in the
beginning of a physical page (this is kinda random, which might
explain why this wasn't really encountered before).

The init sections are ordered like this:
  .init.text
  .exit.text
  .init.data

Currently, these sections aren't page aligned.

Because the init code might become read-only at runtime and because
the .init.text section can potentially reside on the same physical
page as .init.data, the beginning of .init.data might be mapped
read-only along with .init.text.

Then when the kernel tries to modify a variable in .init.data (like
kthreadd_done, used in kernel_init()) the kernel panics.

To avoid this, make _einittext page aligned and also align .exit.text
to make sure .init.data is always seperated from the text segments.

Fixes: 060ef9d89d ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210102201156.10805-1-ariel.marcovitch@gmail.com

arch/powerpc/kernel/vmlinux.lds.S

@@ -187,6 +187,12 @@ SECTIONS
 	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
 		_sinittext = .;
 		INIT_TEXT
+
+		/*
+		 *.init.text might be RO so we must ensure this section ends on
+		 * a page boundary.
+		 */
+		. = ALIGN(PAGE_SIZE);
 		_einittext = .;
 #ifdef CONFIG_PPC64
 		*(.tramp.ftrace.init);
@@ -200,6 +206,8 @@ SECTIONS
 		EXIT_TEXT
 	}
 
+	. = ALIGN(PAGE_SIZE);
+
 	INIT_DATA_SECTION(16)
 
 	. = ALIGN(8);