help with inline assembly code?
cfriesen at nortel.com
Sat Apr 25 04:06:16 EST 2009
Scott Wood wrote:
> Chris Friesen wrote:
>> I've got a function that is used to overwrite opcodes in order to create
>> self-modifying code. It worked just fine with previous compilers, but
>> with gcc 4.3 it seems like it sometimes (but not always) causes problems
>> when inlined. If I force it to never be inlined, it works fine.
>> First, here's the code:
>> void alter_opcode(unsigned long *addr, unsigned long opcode)
>> asm volatile(
>> "stw %1,0(%0) \n\t"
>> "dcbf 0,%0 \n\t"
>> "sync \n\t"
>> "icbi 0,%0, \n\t"
>> "isync \n\t"
>> :: "r" (addr), "r" (opcode): "memory");
>> The symptom of the problem is a segfault on the "stw" instruction. I've
>> verified that the address it's trying to write to is the expected
> Verified by looking at the address in "addr", or by looking at the
> reported faulting address?
Verified by running it in userspace under gdb, then looking at the
registers listed in the disassembly and comparing it to the process maps.
>> and that the opcode being written is the expected opcode.
>> I assume I've mixed up the registers or constraints or
>> something...anyone want to take a crack at it?
> Is the compiler assigning r0 to addr? That will be treated as a literal
> zero instead. Try changing "r" (addr) to "b" (addr), or use stwx.
Bingo! Is there a constraint to tell the compiler to not use r0 for addr?
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