Re: [PATCH 0/2] apparmor unaligned memory fixes

Helge Deller
2025-11-26T15:12:29Z

* david laight <david.laight@runbox.com>:
> On Wed, 26 Nov 2025 12:03:03 +0100
> Helge Deller <deller@gmx.de> wrote:
>
> > On 11/26/25 11:44, david laight wrote:
> ...
> > >> diff --git a/security/apparmor/match.c b/security/apparmor/match.c
> > >> index 26e82ba879d44..3dcc342337aca 100644
> > >> --- a/security/apparmor/match.c
> > >> +++ b/security/apparmor/match.c
> > >> @@ -71,10 +71,10 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
> > >> u8, u8, byte_to_byte);
> > >
> > > Is that that just memcpy() ?
> >
> > No, it's memcpy() only on big-endian machines.
>
> You've misread the quoting...
> The 'data8' case that was only half there is a memcpy().
>
> > On little-endian machines it converts from big-endian
> > 16/32-bit ints to little-endian 16/32-bit ints.
> >
> > But I see some potential for optimization here:
> > a) on big-endian machines just use memcpy()
>
> true
>
> > b) on little-endian machines use memcpy() to copy from possibly-unaligned
> > memory to then known-to-be-aligned destination. Then use a loop with
> > be32_to_cpu() instead of get_unaligned_xx() as it's faster.
>
> There is a function that does a loop byteswap of a buffer - no reason
> to re-invent it.

I assumed there must be something, but I did not see it. Which one?

> But I doubt it is always (if ever) faster to do a copy and then byteswap.
> The loop control and extra memory accesses kill performance.

Yes, you are probably right.

> Not that I've seen a fast get_unaligned() - I don't think gcc or clang
> generate optimal code - For LE I think it is something like:
> low = *(addr & ~3);
> high = *((addr + 3) & ~3);
> shift = (addr & 3) * 8;
> value = low << shift | high >> (32 - shift);
> Note that it is only 2 aligned memory reads - even for 64bit.

Ok, then maybe we should keep it simple like this patch:

[PATCH v2] apparmor: Optimize table creation from possibly unaligned memory

Source blob may come from userspace and might be unaligned.
Try to optize the copying process by avoiding unaligned memory accesses.

Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/security/apparmor/include/match.h b/security/apparmor/include/match.h
index 1fbe82f5021b..386da2023d50 100644
--- a/security/apparmor/include/match.h
+++ b/security/apparmor/include/match.h
@@ -104,16 +104,20 @@ struct aa_dfa {
struct table_header *tables[YYTD_ID_TSIZE];
};

-#define byte_to_byte(X) (X)
+#define byte_to_byte(X) (*(X))

#define UNPACK_ARRAY(TABLE, BLOB, LEN, TTYPE, BTYPE, NTOHX) \
do { \
typeof(LEN) __i; \
TTYPE *__t = (TTYPE *) TABLE; \
BTYPE *__b = (BTYPE *) BLOB; \
- for (__i = 0; __i < LEN; __i++) { \
- __t[__i] = NTOHX(__b[__i]); \
- } \
+ BUILD_BUG_ON(sizeof(TTYPE) != sizeof(BTYPE)); \
+ if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) || sizeof(BTYPE) == 1) \
+ memcpy(__t, __b, (LEN) * sizeof(BTYPE)); \
+ else /* copy & convert convert from big-endian */ \
+ for (__i = 0; __i < LEN; __i++) { \
+ __t[__i] = NTOHX(&__b[__i]); \
+ } \
} while (0)

static inline size_t table_size(size_t len, size_t el_size)
diff --git a/security/apparmor/match.c b/security/apparmor/match.c
index c5a91600842a..13e2f6873329 100644
--- a/security/apparmor/match.c
+++ b/security/apparmor/match.c
@@ -15,6 +15,7 @@
#include <linux/vmalloc.h>
#include <linux/err.h>
#include <linux/kref.h>
+#include <linux/unaligned.h>

#include "include/lib.h"
#include "include/match.h"
@@ -70,10 +71,10 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
u8, u8, byte_to_byte);
else if (th.td_flags == YYTD_DATA16)
UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
- u16, __be16, be16_to_cpu);
+ u16, __be16, get_unaligned_be16);
else if (th.td_flags == YYTD_DATA32)
UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
- u32, __be32, be32_to_cpu);
+ u32, __be32, get_unaligned_be32);
else
goto fail;
/* if table was vmalloced make sure the page tables are synced