Hi,
> Ah, you are not using the ftrace framework nor relayfs? Mmiotrace
used to be relayfs at one point and then converted to ftrace.
used to be relayfs at one point and then converted to ftrace.
Yes, I considered these when I started working on KernelStrider but finally borrowed ideas from Perf and implemented them. A mmapped ring buffer does its job well and has a higher throughput than Ftrace in my case.
> Are you saying that you intercept function calls, and *never* rely
> on page faulting?
The system intercepts both function calls *and* memory operations made by the driver itself. Yes, it never relies on page faulting.
> Does that mean that if a driver does the ugly thing and
> dereferences an iomem pointer directly, you won't catch that?
It will be caught.
What my system actually does is as follows.
When the target kernel module has been loaded into memory but before it has begun its initialization, KernelStrider processes it, function after function. It creates an instrumented variant of each function in the module mapping space and places a jump at the beginning of the original function to point to the instrumented one. After instrumentation is done, the target driver may start executing.
If some original function of the driver contained, say,
mov 0xabcd (%rax), %rsi
mov %rbx, 0xbeeffeed (%rsi)
that will be transformed to something like
lea 0xabcd (%rax), %rbx
mov %rbx, <local_storage1>
mov 0xabcd (%rax), %rsi
lea 0xbeeffeed (%rsi), %rbx
mov %rbx, <local_storage2>
mov %rbx, 0xbeeffeed (%rsi)
...
<send the local_storage to the output system>
That is, the address which is about to be accessed is determined and stored in 'local_storage', a special memory structure. At the end of the block of instructions, the information from the local storage is sent to the output system. So the addresses and sizes of the accessed memory areas as well as the types of the accesses (read/write/update) will be available for reading from the user space.
It is actually more complex than that (KernelStrider has to deal with register allocation, relocations and other things) but the principle is as I described.
The function calls are processed too so that we can set our own handlers to execute at the beginning of a function and right before its exit.
Yes, the functions like read[bwql]() and write[bwlq]() are usually inline but they pose no problem: on x86 they compile to ordinary MOV instructions and the like which are handled as I described above.
The instrumented code will access the ioremapped area the same way as the original code would, no need for single-stepping or emulation in this case.
What I wrote in my previous letter is that there is a special case when the target driver uses some non-inline function provided by the kernel proper or by another driver and that function accesses the ioremapped memory area of interest.
KernelStrider needs to track all such functions in order not to miss some memory accesses to that ioremapped area. Perhaps, that's manageable. There are not too many such functions, aren't they?
> I don't really know. I guess everything could be possible in
> proprietary drivers, but you can look at the instruction decoding
> code in mmiotrace, which digs up the type and size of access and
> the value. That has been enough so far.
Yes, I will take a closer look on that part of MmioTrace, thanks for the point.
Regards,
Eugene
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