powerpc/vmlinux.lds: Add an explicit symbol for the SRWX boundary

Currently __init_begin is used as the boundary for strict RWX between
executable/read-only text and data, and non-executable (after boot) code
and data.

But that's a little subtle, so add an explicit symbol to document that
the SRWX boundary lies there, and add a comment making it clear that
__init_begin must also begin there.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220916131422.318752-2-mpe@ellerman.id.au

arch/powerpc/include/asm/sections.h

@@ -13,6 +13,7 @@ typedef struct func_desc func_desc_t;
 #include <asm-generic/sections.h>
 
 extern char __head_end[];
+extern char __srwx_boundary[];
 
 /* Patch sites */
 extern s32 patch__call_flush_branch_caches1;

arch/powerpc/kernel/vmlinux.lds.S

@@ -214,11 +214,16 @@ SECTIONS
 	}
 #endif
 
+	/*
+	 * Various code relies on __init_begin being at the strict RWX boundary.
+	 */
+	. = ALIGN(STRICT_ALIGN_SIZE);
+	__srwx_boundary = .;
+	__init_begin = .;
+
 /*
  * Init sections discarded at runtime
  */
-	. = ALIGN(STRICT_ALIGN_SIZE);
-	__init_begin = .;
 	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
 		_sinittext = .;
 		INIT_TEXT

arch/powerpc/mm/book3s32/mmu.c

@@ -158,7 +158,7 @@ static unsigned long __init __mmu_mapin_ram(unsigned long base, unsigned long to
 unsigned long __init mmu_mapin_ram(unsigned long base, unsigned long top)
 {
 	unsigned long done;
-	unsigned long border = (unsigned long)__init_begin - PAGE_OFFSET;
+	unsigned long border = (unsigned long)__srwx_boundary - PAGE_OFFSET;
 	unsigned long size;
 
 	size = roundup_pow_of_two((unsigned long)_einittext - PAGE_OFFSET);

arch/powerpc/mm/book3s64/radix_pgtable.c

@@ -259,8 +259,8 @@ print_mapping(unsigned long start, unsigned long end, unsigned long size, bool e
 static unsigned long next_boundary(unsigned long addr, unsigned long end)
 {
 #ifdef CONFIG_STRICT_KERNEL_RWX
-	if (addr < __pa_symbol(__init_begin))
-		return __pa_symbol(__init_begin);
+	if (addr < __pa_symbol(__srwx_boundary))
+		return __pa_symbol(__srwx_boundary);
 #endif
 	return end;
 }