Messages in this thread | | | Date | Tue, 19 Sep 2006 21:09:43 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Linux Kernel Markers |
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* Alan Cox (alan@lxorguk.ukuu.org.uk) wrote: > Ar Maw, 2006-09-19 am 13:54 -0400, ysgrifennodd Mathieu Desnoyers: > > 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. > > No: on x86 it is the *same* case for all of these even writing an int3. > One byte or a megabyte, > > You MUST ensure that every CPU executes a serializing instruction before > it hits code that was modified by another processor. Otherwise you get > CPU errata and the CPU produces results which vendors like to describe > as "undefined". > > Thus you have to serialize, and if you are serializing it really doesn't > matter if you write a byte, a paragraph or a page. > Hi Alan,
What I am trying to address is not "code patching with INT3", but "code patching with a 5 bytes JMP". The errata you point to applies to both and kprobes mechanism already takes care of this with the serialization method you describe.
However, there is a supplemental problem with the fact that a JMP is 5 bytes, not 1. You are right about saying that overwriting code with any amount of *int3* does not matter, but what happens when you put one or more 5 bytes long jumps instead ?
Think about it : if you are replacing 1-2-3 or 4 bytes long instruction and, unluckily, on any stack of any thread preempted from any CPU, you have a current instruction pointer pointing at the middle of the region where you want to put the 5 bytes JMP, the processor will likely trigger an illegal instruction fault when this particular thread is scheduled back.
Mathieu
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