Re: [PATCH] Linux Kernel Markers

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* 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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