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Re: [PATCH v2] qemu-trace-stap: changing SYSTEMTAP_TAPSET considered har

From: Gerd Hoffmann
Subject: Re: [PATCH v2] qemu-trace-stap: changing SYSTEMTAP_TAPSET considered harmful.
Date: Fri, 9 Apr 2021 13:43:04 +0200

On Fri, Apr 09, 2021 at 12:17:28PM +0100, Daniel P. Berrangé wrote:
> On Fri, Apr 09, 2021 at 12:58:10PM +0200, Gerd Hoffmann wrote:
> > Setting SYSTEMTAP_TAPSET to some value other than
> > /usr/share/systemtap/tapsets results in systemtap not finding the
> > standard tapset library any more, which in turn breaks tracing because
> > pid() and other standard systemtap functions are not available any more.
> > 
> > So using SYSTEMTAP_TAPSET to point systemtap to the qemu probes will
> > only work for the prefix=/usr installs because both qemu and system
> > tapsets in the same directory then.  All other prefixes are broken.
> > 
> > Fix that by using the "-I $tapsetdir" command line switch instead.
> Do you know if "-I tapsetdir" appends or prepends to the search path ?
> We need it to prepend to ensure that we override any QEMU stp files
> that might be already present in /usr/share from an RPM provided
> QEMU binary.

Dunno, "man stap" isn't clear on that one.  strace shows the files in
the SYSTEMTAP_TAPSET directory are read first.  That doesn't imply those
probes have a higher priority though.

take care,

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