Messages in this thread | | | Date | Tue, 19 Sep 2006 13:54:05 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Linux Kernel Markers |
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* Martin Bligh (mbligh@google.com) wrote: > How about we combine all three ideas together ... > > 1. Load modified copy of the function in question. > 2. overwrite the first instruction of the routine with an int3 that > does what you say (atomically) > 3. Then overwrite the second instruction with a jump that's faster > 4. Now atomically overwrite the int3 with a nop, and let the jump > take over. >
Very good idea.. However, overwriting the second instruction with a jump could be dangerous on preemptible and SMP kernels, because we never know if a thread has an IP in any of its contexts that would return exactly at the middle of the jump. I think it would be doable to overwrite a 5+ bytes instruction with a NOP non-atomically in all cases, but as the instructions nin the prologue seems to be smaller :
prologue on x86 0: 55 push %ebp 1: 89 e5 mov %esp,%ebp epilogue on x86 3: 5d pop %ebp 4: c3 ret
Then is can be a problem. Ideas are welcome.
Mathieu
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