Messages in this thread | | | Date | Wed, 26 Aug 2009 15:01:08 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 08/12] add trace events for each syscall entry/exit |
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* Christoph Hellwig (hch@infradead.org) wrote: > On Wed, Aug 26, 2009 at 02:41:16PM -0400, Christoph Hellwig wrote: > > We do not support system calls from kernelspace anymore. All the > > macros to do real system calls are gone, and there are very very few > > places left calling sys_foo as normal function calls. > > > > An how exactly is calling sys_foo as a normal function call different > > from calling do_foo or vfs_foo? >
Not very different, no. But the fact is that we would not be instrumenting do_foo nor vfs_foo because we would somehow expect all callers to go through a system call, which ain't always true.
> And if you really need to trace direct callers of sys_foo just put > a jprobe on it. >
Yep, that would do it. Getting the arguments of the function upon entry and the return value on exit is pretty much all we need.
But I would like to ensure that we do not duplicate the instrumentation done by the generic syscall instrumentation neither, so we don't end up having:
syscall_entry X (args) do_X (args) do_X return (return value) syscall_exit X (return value)
If I am not mistakened, the current execution paths are:
From userland:
syscall -> sys_X() -> do_X()
From the kernel:
do_X()
Adding a trampoline taken only when the kernel is doing the call might be useful there to selectively trace calls made by the kernel.
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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