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

Helge Deller
2025-11-26T20:15:09Z

* John Johansen <john.johansen@canonical.com>:
> On 11/26/25 07:12, Helge Deller wrote:
> > * 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
>
> I think we can make one more tweak, in just not using UNPACK_ARRAY at all for the byte case
> ie.
>
> diff --git a/security/apparmor/match.c b/security/apparmor/match.c
> index 26e82ba879d44..389202560675c 100644
> --- a/security/apparmor/match.c
> +++ b/security/apparmor/match.c
> @@ -67,8 +67,7 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
> table->td_flags = th.td_flags;
> table->td_lolen = th.td_lolen;
> if (th.td_flags == YYTD_DATA8)
> - UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
> - u8, u8, byte_to_byte);
> + memcp(table->td_data, blob, th.td_lolen);

True.
Then byte_to_byte() can go away in match.h as well.
So, here is a (untested) v3:

[PATCH v3] 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..19e72b3e8f49 100644
--- a/security/apparmor/include/match.h
+++ b/security/apparmor/include/match.h
@@ -104,16 +104,18 @@ struct aa_dfa {
struct table_header *tables[YYTD_ID_TSIZE];
};

-#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)) \
+ 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..1e32c8ba14ae 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"
@@ -66,14 +67,13 @@ static struct table_header *unpack_table(char *blob, size_t bsize)
table->td_flags = th.td_flags;
table->td_lolen = th.td_lolen;
if (th.td_flags == YYTD_DATA8)
- UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
- u8, u8, byte_to_byte);
+ memcpy(table->td_data, blob, th.td_lolen);
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