decoding of multi-byte nops in dtrace

[Fabian Keil]

Andriy Gapon <avg at FreeBSD.org> wrote:

> on 10/06/2012 16:27 Fabian Keil said the following:
> > Andriy Gapon <avg at FreeBSD.org> wrote:
> > 
> >> It seems that the $subj is missing :-)
> >> In my environment that causes many functions to not have fbt return probe,
> >> because function body decoding fails before 'ret' is found.
> >>
> >> Here is my attempt at fixing the problem:
> >> http://people.freebsd.org/~avg/fbt-nop.patch
> >> Reviews and suggestions are welcome.
> > 
> > The patch seems to reduce the number of missing
> > fbt return probes by about 50% for me.
> > 
> > Without the patch:
> > 
> > fk at r500 /usr/src $sudo dtrace -ln fbt::: | grep -c entry
> > 23395
> > fk at r500 /usr/src $sudo dtrace -ln fbt::: | grep -c return
> > 16739
> > 
> > With the patch (and updated kernel sources):
> > 
> > fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c entry
> > 23409
> > fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c return
> > 19879
> 
> Interesting observations, thank you.
> Do you use -O2 or higher optimization for kernel/modules build?

Yes, I kept the default -O2.

> I use only -O1.

With -O1 (and your patch) I get:

fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c entry
23421
fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c return
22621

> Here are some stats from my system:
> $ dtrace -ln fbt::: | fgrep -c entry
> 16876
> $ dtrace -ln fbt::: | fgrep -c return
> 16729
> 
> So, 147 functions without return probe.
> >From a quick look at them they all seem to really never return.  Either they are
> noreturn type such panic, or functions that always call the functions of the
> first type, or functions with endless loops in them such as top level functions
> of many system threads.

I looked at a couple of the functions that still lack return
probes and the ones I looked at don't seem to belong into these
categories.

For example I get no return probes for g_eli_crypto_decrypt()
and g_eli_crypto_encrypt(). Both return the return code of
g_eli_crypto_cipher() for which I get a return probe.

Fabian
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