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Re: [PATCH] Linux Kernel Markers
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Martin Bligh <mbligh at google dot com>
- Cc: 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, Alan Cox <alan at lxorguk dot ukuu dot org dot uk>
- Date: Tue, 19 Sep 2006 14:11:01 -0400
- Subject: Re: [PATCH] Linux Kernel Markers
- References: <20060919081124.GA30394@elte.hu> <451008AC.6030006@google.com> <20060919154612.GU3951@redhat.com> <4510151B.5070304@google.com> <20060919093935.4ddcefc3.akpm@osdl.org> <45101DBA.7000901@google.com> <20060919063821.GB23836@in.ibm.com> <45102641.7000101@google.com> <20060919175405.GC26339@Krystal> <4510308A.1070401@google.com>
* Martin Bligh (mbligh@google.com) wrote:
> Mathieu Desnoyers wrote:
> >* Martin Bligh (mbligh@google.com) wrote:
> >
> >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 not in 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.
>
> Ugh, yes that's somewhat problematic. It does seem rather unlikely that
> there's a function call in the function prologue when we're busy
> offloading stuff onto the stack, but still ...
>
A function call is not the cause of the problem : an interrupt/trap is.
> For the cases where we're prepared to overwrite the call instruction in
> the caller, rather than insert an extra jump in the callee, can we not
> do that atomically by overwriting the address we're jumping to (the
> call is obviously there already)? Doesn't fix function pointers, etc,
> but might work well for the simple case at least.
>
I don't think we have any guarantee that the function pointer in the call is
aligned, so I guess it would not be an atomic replacement.
Mathieu
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