[PATCH v3 0/1] ARM: Thumb-2: Symbol manipulation macros for function body copying

Dave Martin dave.martin at linaro.org
Thu Jan 13 17:41:42 EST 2011

For at least one board (omap3), some functions are copied from
their link-time location into other memory at run-time.

This is a plausible thing to do if, for example, the board
might need to do something like manipulating the SDRAM 
controller configuration during power management operations.
Such code may not be able to execute from the SDRAM itself.

In Thumb-2, copying function bodies is not straightforward:
for Thumb symbols, bit 0 is set by the toolchain, and so
a function symbol can't be used directly as a base address
for memcpy: this leads to an off-by-one error, resulting in
garbage instructions in the destination buffer.

The obvious solution is to mask off this bit when calling
memcpy() and then insert the bit into the address of the
target buffer, in order to derive a pointer which can be
used to call the copied function in the correct instruction
set.  However, in practice the compiler may optimise this
operation away.  This seems wrong, but having discussed this
with compiler folks I believe it's not a compiler bug: rather,
C doesn't specifiy what happens when casting function pointers
and attempting to do arithmetic on them.  So some surprising
optimisations can happen.

To make it easier to deal with cases like this, I've had a
go at writing some macros to make copying function bodies
easier, while being robust for ARM and Thumb-2.

In particular, the required type-casts are implemented as
empty asm() blocks, to ensure that the compiler makes no
assumptions about the result.

This patch provides a fncpy() macro which resembles memcpy().
It can be used as in this example:

    extern int scary_function(int a, char *b);
    extern const int size_of_scary_function;
    extern void *scary_memory_buf;

    int (*runtime_scary_function)(int a, char *b);

    runtime_scary_function = fncpy(scary_memory_buf,

This is quite a lot more readable than the explicit code,
and should give the correct result.

fncpy() calls flush_icache_range() as necessary.

It's not possible to determine the size of a function from
C code.  This must be done by other means, such as adding
extra symbols in the assembler code where scary_function is

Signed-off-by: Dave Martin <dave.martin at linaro.org>
KernelVersion: v2.6.37

Dave Martin (1):
  ARM: Thumb-2: Symbol manipulation macros for function body copying

 arch/arm/include/asm/fncpy.h |  110 ++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 110 insertions(+), 0 deletions(-)
 create mode 100644 arch/arm/include/asm/fncpy.h

More information about the linux-arm-kernel mailing list