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

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Message-ID: <20251126212309.5b21edca@pumpkin>
Date: Wed, 26 Nov 2025 21:23:09 +0000
From: david laight <david.laight@...box.com>
To: Helge Deller <deller@...nel.org>
Cc: Helge Deller <deller@....de>, John Johansen
 <john.johansen@...onical.com>, John Paul Adrian Glaubitz
 <glaubitz@...sik.fu-berlin.de>, linux-kernel@...r.kernel.org,
 apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org,
 linux-parisc@...r.kernel.org
Subject: Re: [PATCH 0/2] apparmor unaligned memory fixes

On Wed, 26 Nov 2025 16:12:23 +0100
Helge Deller <deller@...nel.org> wrote:

> * david laight <david.laight@...box.com>:
> > On Wed, 26 Nov 2025 12:03:03 +0100
> > Helge Deller <deller@....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?

I can't find it either - just some functions to do an in-place swap.
(Which aren't usually a good idea)

> 
> > 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.

Not sure that reads right.
'Allow for misaligned data from userspace when byteswapping source blob' ?

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

It's a bit of a shame you need something for the above...

	David


>  
>  #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