2019-08-16 07:53:45

by Christophe Leroy

Subject: [PATCH] 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 <[email protected]>
Cc: Erhard F. <[email protected]>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=204371
---
arch/powerpc/include/asm/page_32.h | 4 ++++
arch/powerpc/kernel/misc_32.S | 5 +++++
2 files changed, 9 insertions(+)

diff --git a/arch/powerpc/include/asm/page_32.h b/arch/powerpc/include/asm/page_32.h
index 683dfbc67ca8..d64dfe3ac712 100644
--- a/arch/powerpc/include/asm/page_32.h
+++ b/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);
}
diff --git a/arch/powerpc/kernel/misc_32.S b/arch/powerpc/kernel/misc_32.S
index fe4bd321730e..02d90e1ebf65 100644
--- a/arch/powerpc/kernel/misc_32.S
+++ b/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
--
2.13.3

2019-08-22 17:09:12

by Michael Ellerman

Subject: Re: [PATCH] powerpc/32: Add warning on misaligned copy_page() or clear_page()

On Fri, 2019-08-16 at 07:52:20 UTC, Christophe Leroy wrote:
> 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 <[email protected]>
> Cc: Erhard F. <[email protected]>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=204371

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/7ab0b7cb8951d4095d73e203759b74d41916e455

cheers