powerpc/32: Add warning on misaligned copy_page() or clear_page()

copy_page() and clear_page() expect page aligned destination, and
use dcbz instruction to clear entire cache lines based on the
assumption that the destination is cache aligned.

As shown during analysis of a bug in BTRFS filesystem, a misaligned
copy_page() can create bugs that are difficult to locate (see Link).

Add an explicit WARNING when copy_page() or clear_page() are called
with misaligned destination.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=204371
Link: https://lore.kernel.org/r/c6cea38f90480268d439ca44a645647e260fff09.1565941808.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/page_32.h

@@ -40,6 +40,8 @@ typedef unsigned long long pte_basic_t;
 typedef unsigned long pte_basic_t;
 #endif
 
+#include <asm/bug.h>
+
 /*
  * Clear page using the dcbz instruction, which doesn't cause any
  * memory traffic (except to write out any cache lines which get
@@ -49,6 +51,8 @@ static inline void clear_page(void *addr)
 {
 	unsigned int i;
 
+	WARN_ON((unsigned long)addr & (L1_CACHE_BYTES - 1));
+
 	for (i = 0; i < PAGE_SIZE / L1_CACHE_BYTES; i++, addr += L1_CACHE_BYTES)
 		dcbz(addr);
 }

arch/powerpc/kernel/misc_32.S

@@ -452,7 +452,12 @@ END_FTR_SECTION_IFSET(CPU_FTR_COHERENT_ICACHE)
 	stwu	r9,16(r3)
 
 _GLOBAL(copy_page)
+	rlwinm	r5, r3, 0, L1_CACHE_BYTES - 1
 	addi	r3,r3,-4
+
+0:	twnei	r5, 0	/* WARN if r3 is not cache aligned */
+	EMIT_BUG_ENTRY 0b,__FILE__,__LINE__, BUGFLAG_WARNING
+
 	addi	r4,r4,-4
 
 	li	r5,4