* [PATCH] make the default alignment trap behavior configurable for user space
[not found] ` <alpine.LFD.2.00.0812110915230.14328@xanadu.home>
@ 2010-03-05 15:01 ` Uwe Kleine-König
2010-03-05 21:01 ` Russell King - ARM Linux
0 siblings, 1 reply; 2+ messages in thread
From: Uwe Kleine-König @ 2010-03-05 15:01 UTC (permalink / raw)
To: linux-arm-kernel
Hello,
>
> Here's the revised patch:
>
> From: Nicolas Pitre <nico@cam.org>
> Date: Tue, 9 Dec 2008 23:06:00 -0500
> Subject: [PATCH] [ARM] make the default alignment trap behavior configurable for user space
>
> For more than a decade, gcc's reliance on the ARM unaligned access
> behavior to optimize code on old ARM CPU versions has been deprecated.
> Still, the default align trap behavior with user space unaligned accesses
> is to let them go through to remain compatible with old binaries from
> before that era.
>
> However, in the EABI case, there is simply no valid reason for those
> silent unaligned accesses. They just cannot possibly provide expected
> results anymore since EABI was defined way after the gcc unaligned access
> optimization has been deprecated, and when they occur then silent
> corruptions and/or subtle bugs do result.
>
> Let's have the default behavior configurable for user space, and default
> that default to fixing up unaligned accesses for EABI systems.
>
> Signed-off-by: Nicolas Pitre <nico@marvell.com>
I like that patch. (And I'd choose SIGBUS.)
Is it still considered?
Best regards
Uwe
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 9722f8b..e8df97f 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -934,6 +934,55 @@ config ALIGNMENT_TRAP
> correct operation of some network protocols. With an IP-only
> configuration it is safe to say N, otherwise say Y.
>
> +choice
> + prompt "Default behavior for user space unaligned memory access"
> + depends on ALIGNMENT_TRAP && (CPU_32v3 || CPU_32v4 || CPU_32v4T || CPU_32v5)
> + default ALIGNMENT_TRAP_USER_FIXUP if AEABI
> + help
> + ARM processors prior ARMv6 cannot fetch/store information which
> + isnot naturally aligned on the bus, i.e., a 4 byte fetch must
> + start at an address divisible by 4. This option determines what
> + is the default behavior when such unaligned accesses are performed
> + by user space programs. This behavior can be changed at run time
> + through /proc/cpu/alignment -- see Documentation/arm/mem_alignment
> + for the details on that interface.
> +
> +config ALIGNMENT_TRAP_USER_IGNORE
> + bool "keep hardware defined behavior"
> + help
> + Unaligned memory accesses are architecturally defined to rotate
> + right the word-aligned data transferred by a non word-aligned
> + address, by 1, 2 or 3 bytes depending on the value of the two
> + least significant address bits.
> +
> + This is usually not what you want, except if you have to be
> + compatible with ancient user space binaries compiled for ARMv3
> + with a gcc from around 1994 which used this property to optimize
> + some sub-word accesses.
> +
> +config ALIGNMENT_TRAP_USER_SIGBUS
> + bool "kill with a SIGBUS signal"
> + help
> + User space fetch/store instructions using a misaligned memory
> + address will send a SIGBUS signal to the corresponding process
> + by default with this choice. Normally, that process will be
> + terminated, and a core dump generated if resource limits allow it.
> + Running such a program from gdb, or providing gdb with the
> + generated core dump, should identify the exact location in the code
> + responsible for the unaligned access so the code could be fixed.
> +
> +config ALIGNMENT_TRAP_USER_FIXUP
> + bool "fixup by software emulation"
> + help
> + User space fetch/store instructions using a misaligned memory
> + address will be emulated in software by default with this choice.
> + This provides the highest level of compatibility for programs
> + written with no considerations for memory alignment restrictions,
> + but with a severe performance impact if a significant number
> + of such memory accesses are performed.
> +
> +endchoice
> +
> endmenu
>
> menu "Boot options"
> diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
> index 2d5884c..c0c366e 100644
> --- a/arch/arm/mm/alignment.c
> +++ b/arch/arm/mm/alignment.c
> @@ -800,6 +800,12 @@ static int __init alignment_init(void)
> res->write_proc = proc_alignment_write;
> #endif
>
> +#if defined(CONFIG_ALIGNMENT_TRAP_USER_FIXUP)
> + ai_usermode = UM_FIXUP;
> +#elif defined(CONFIG_ALIGNMENT_TRAP_USER_SIGBUS)
> + ai_usermode = UM_SIGNAL;
> +#endif
> +
> /*
> * ARMv6 and later CPUs can perform unaligned accesses for
> * most single load and store instructions up to word size.
> @@ -816,6 +822,10 @@ static int __init alignment_init(void)
> ai_usermode = UM_FIXUP;
> }
>
> +#ifdef CONFIG_DEBUG_USER
> + ai_usermode |= UM_WARN;
> +#endif
> +
> hook_fault_code(1, do_alignment, SIGILL, "alignment exception");
> hook_fault_code(3, do_alignment, SIGILL, "alignment exception");
>
>
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--
Pengutronix e.K. | Uwe Kleine-K?nig |
Industrial Linux Solutions | http://www.pengutronix.de/ |
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