> Mathieu Desnoyers wrote:
>>
>> Steve ? I just noticed this :
>>
>>
>> ftrace_modify_code(unsigned long ip, unsigned char *old_code,
>> unsigned char *new_code)
>> {
>> unsigned replaced;
>> unsigned old = *(unsigned *)old_code;
>> unsigned new = *(unsigned *)new_code;
>> int faulted = 0;
>>
>> /*
>> * Note: Due to modules and __init, code can
>> * disappear and change, we need to protect against faulting
>> * as well as code changing.
>> *
>> * No real locking needed, this code is run through
>> * kstop_machine.
>> */
>> asm volatile (
>> "1: lwz %1, 0(%2)\n"
>> " cmpw %1, %5\n"
>> " bne 2f\n"
>> " stwu %3, 0(%2)\n"
>> "2:\n"
>> ".section .fixup, \"ax\"\n"
>> "3: li %0, 1\n"
>> " b 2b\n"
>> ".previous\n"
>> ".section __ex_table,\"a\"\n"
>> _ASM_ALIGN "\n"
>> _ASM_PTR "1b, 3b\n"
>> ".previous"
>> : "=r"(faulted), "=r"(replaced)
>> : "r"(ip), "r"(new),
>> "0"(faulted), "r"(old)
>> : "memory");
>>
>> if (replaced != old && replaced != new)
>> faulted = 2;
>>
>> if (!faulted)
>> flush_icache_range(ip, ip + 8);
>>
>> return faulted;
>> }
>>
>> What happens if you are really unlucky and get the following execution
>> sequence ?
>>
>>
>> Load module A at address 0xddfc3e00
>> Populate ftrace function table while the system runs
>> Unload module A
>> Load module B at address 0xddfc3e00
>> Activate ftrace
>> -> At that point, since there is another module loaded in the same
>> address space, it won't fault. If there happens to be an instruction
>> which looks exactly like the instruction we are replacing at the same
>> location, we are introducing a bug. True both on x86 ans powerpc...
>>
>>
>
> Hi Mathieu,
>
> Yep I know of this issue, and it is very unlikely it would happen. But
> that's not good enough, so this is why I did this:
>
>
http://marc.info/?l=linux-kernel&m=121876928124384&w=2
>
http://marc.info/?l=linux-kernel&m=121876938524523&w=2
>
> ;-)
>
> On powerpc it would be less likely an issue since the code segments are all
> 4 bytes aligned. And a call being replaced would be a call to mcount
> (relative jump). A call to mcount would be the same for both. Then again,
> we could be replacing a nop, but most likely that wouldn't hurt either,
> since nops are seldom used, and if we did call mcount, it would probably
> not hurt. But it would be an issue if Module B happen to put a data section
> that matched the code to jmp to mcount or a nop.
>
