[PATCH v4] ARM: Thumb-2: Symbol manipulation macros for function body copying

[Jean Pihet]

On Fri, Jan 14, 2011 at 10:17 PM, Dave Martin <dave.martin at linaro.org> wrote:
> In low-level board support code, there is sometimes a need to
> copy a function body to another location at run-time.
>
> A straightforward call to memcpy doesn't work in Thumb-2,
> because bit 0 of external Thumb function symbols is set to 1,
> indicating that the function is Thumb.  Without corrective
> measures, this will cause an off-by-one copy, and the copy
> may be called using the wrong instruction set.
>
> This patch adds an fncpy() macro to help with such copies.
>
> Particular care is needed, because C doesn't guarantee any
> defined behaviour when casting a function pointer to any other
> type.  This has been observed to lead to strange optimisation
> side-effects when doing the arithmetic which is required in
> order to copy/move function bodies correctly in Thumb-2.
>
> Thanks to Russell King and Nicolas Pitre for their input
> on this patch.
>
> Signed-off-by: Dave Martin <dave.martin at linaro.org>
> Tested-by: Jean Pihet <j-pihet at ti.com>
> ---
>  arch/arm/include/asm/fncpy.h |   96 ++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 96 insertions(+), 0 deletions(-)
>  create mode 100644 arch/arm/include/asm/fncpy.h
>
> diff --git a/arch/arm/include/asm/fncpy.h b/arch/arm/include/asm/fncpy.h
> new file mode 100644
> index 0000000..8b94b5f
> --- /dev/null
> +++ b/arch/arm/include/asm/fncpy.h
> @@ -0,0 +1,96 @@
> +/*
> + * arch/arm/include/asm/fncpy.h - helper macros for function body copying
> + *
> + * Copyright (C) 2011 Linaro Limited
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> + */
> +
> +/*
> + * These macros are intended for use when there is a need to copy a low-level
> + * function body into special memory.
> + *
> + * For example, when reconfiguring the SDRAM controller, the code doing the
> + * reconfiguration may need to run from SRAM.
> + *
> + * NOTE: that the copied function body must be entirely self-contained and
> + * position-independent in order for this to work properly.
> + *
> + * NOTE: in order for embedded literals and data to get referenced correctly,
> + * the alignment of functions must be preserved when copying.  To ensure this,
> + * the source and destination addresses for fncpy() must be aligned to a
> + * multiple of 8 bytes: you will be get a BUG() if this condition is not met.
> + * You will typically need a ".align 3" directive in the assembler where the
> + * function to be copied is defined, and ensure that your allocator for the
> + * destination buffer returns 8-byte-aligned pointers.

Note that aligning the source and destination pointers to a multiple
of 8 bytes has an impact on the behavio(u)r and so must be carefully
thought and tested on OMAP1/2/3 platforms.

Regards,
Jean

> + *
> + * Typical usage example:
> + *
> + * extern int f(args);
> + * extern uint32_t size_of_f;
> + * int (*copied_f)(args);
> + * void *sram_buffer;
> + *
> + * copied_f = fncpy(sram_buffer, &f, size_of_f);
> + *
> + * ... do any required D-side/I-side synchronisation ...
> + *
> + * ... later, call the function: ...
> + *
> + * copied_f(args);
> + *
> + * The size of the function to be copied can't be determined from C:
> + * this must be determined by other means, such as adding assmbler directives
> + * in the file where f is defined.
> + */
> +
> +#ifndef __ASM_FNCPY_H
> +#define __ASM_FNCPY_H
> +
> +#include <linux/types.h>
> +#include <linux/string.h>
> +
> +#include <asm/bug.h>
> +#include <asm/cacheflush.h>
> +
> +/*
> + * Minimum alignment requirement for the source and destination addresses
> + * for function copying.
> + */
> +#define FNCPY_ALIGN 8
> +
> +#define fncpy(dest_buf, funcp, size) ({                                        \
> +       uintptr_t __funcp_address;                                      \
> +       typeof(funcp) __result;                                         \
> +                                                                       \
> +       asm("" : "=r" (__funcp_address) : "0" (funcp));                 \
> +                                                                       \
> +       /*                                                              \
> +        * Ensure alignment of source and destination addresses,        \
> +        * disregarding the function's Thumb bit:                       \
> +        */                                                             \
> +       BUG_ON((uintptr_t)(dest_buf) & (FNCPY_ALIGN - 1) ||             \
> +               (__funcp_address & ~(uintptr_t)1 & (FNCPY_ALIGN - 1))); \
> +                                                                       \
> +       memcpy(dest_buf, (void const *)(__funcp_address & ~1), size);   \
> +       flush_icache_range((unsigned long)(dest_buf),                   \
> +               (unsigned long)(dest_buf) + (size));                    \
> +                                                                       \
> +       asm("" : "=r" (__result)                                        \
> +               : "0" ((uintptr_t)(dest_buf) | (__funcp_address & 1))); \
> +                                                                       \
> +       __result;                                                       \
> +})
> +
> +#endif /* !__ASM_FNCPY_H */
> --
> 1.7.1
>
> --
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