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

John Johansen
2025-11-26T19:33:39Z

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);
else if (th.td_flags == YYTD_DATA16)
UNPACK_ARRAY(table->td_data, blob, th.td_lolen,
u16, __be16, be16_to_cpu);