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Re: [PATCH] Linux Kernel Markers
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Alan Cox <alan at lxorguk dot ukuu dot org dot uk>
- Cc: Martin Bligh <mbligh at google dot com>, prasanna at in dot ibm dot com, Andrew Morton <akpm at osdl dot org>, "Frank Ch. Eigler" <fche at redhat dot com>, Ingo Molnar <mingo at elte dot hu>, Paul Mundt <lethal at linux-sh dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, Jes Sorensen <jes at sgi dot com>, Tom Zanussi <zanussi at us dot ibm dot com>, Richard J Moore <richardj_moore at uk dot ibm dot com>, Michel Dagenais <michel dot dagenais at polymtl dot ca>, Christoph Hellwig <hch at infradead dot org>, Greg Kroah-Hartman <gregkh at suse dot de>, Thomas Gleixner <tglx at linutronix dot de>, William Cohen <wcohen at redhat dot com>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com
- Date: Tue, 19 Sep 2006 21:09:43 -0400
- Subject: Re: [PATCH] Linux Kernel Markers
- References: <20060919081124.GA30394@elte.hu> <451008AC.firstname.lastname@example.org> <20060919154612.GU3951@redhat.com> <4510151B.email@example.com> <firstname.lastname@example.org> <45101DBA.email@example.com> <20060919063821.GB23836@in.ibm.com> <firstname.lastname@example.org> <20060919175405.GC26339@Krystal> <email@example.com>
* Alan Cox (firstname.lastname@example.org) 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.
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.
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