Re: perf probe: adding probe on C++ member/namespace function or versioned symbols

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Dec 15, 2021 at 02:10:33PM +0100, Milian Wolff escreveu:
> On Donnerstag, 9. Dezember 2021 11:47:52 CET James Clark wrote:
> > On 08/12/2021 20:08, Milian Wolff wrote:
> > > Hey there,
> > > 
> > > how can one escape function names for `perf probe`? It seems like the
> > > current parser is confused by the `:` which is common in C++ symbol
> > > names.
> > > 
> > > ```
> > > $ perf probe -F --exec /usr/lib/libQt5Widgets.so | grep syncBacking
> > > QWidgetPrivate::syncBackingStore
> > > QWidgetPrivate::syncBackingStore
> > 
> > It might not be obvious, but you have to get the mangled name with 'nm -D',
> > and then pass the mangled name with --no-demangle to perf and then it works:
> > 
> >   sudo ./perf probe --no-demangle --exec libQt5Widgets.so.5 --add
> > '_ZN14QWidgetPrivate16syncBackingStoreEv'
> > 
> > Maybe the docs could be updated to make it clearer, or print a warning
> > message, or get perf to output mangled symbols in addition to demangled
> > with -F.
> 
> Thank you, that indeed works. But it's _very_ unintuitive. Generally, I think 
> there are many languages out there which produce demangled names containing 
> colons - could we come up with a way to allow specifying a demangled symbol 
> including colons (and maybe even @@), that does not conflict with the existing 
> `:` for the line suffix? For C++ (and Rust?) at least, we could handle two 
> colons differently from just one. What do you think?

I agree, we need to lift this limitation.

- Arnaldo
 
> > > $ perf probe --exec /usr/lib/libQt5Widgets.so --add
> > > QWidgetPrivate::syncBackingStore
> > > Semantic error :There is non-digit char in line number.
> > > ...
> > > ```
> > > 
> > > Additionally, it is also not possible to pass a versioned symbol either:
> > > ```
> > > $ nm -aD /usr/lib/libQt5Widgets.so | grep syncBacking
> > > 0000000000197480 T
> > > _ZN14QWidgetPrivate16syncBackingStoreERK7QRegion@@Qt_5_PRIVATE_API
> > > 00000000001973b0 T
> > > _ZN14QWidgetPrivate16syncBackingStoreEv@@Qt_5_PRIVATE_API Semantic error
> > > :SRC@SRC is not allowed.
> > 
> > If adding mangling fix doesn't work for this one, you could try removing
> > some of the name processing in probe-event.c like:
> > 
> > 	/* Cut off the dot suffixes (e.g. .const, .isra) and version suffixes 
> */
> > 	p = strpbrk(nbase, ".@");
> > 	if (p && p != nbase)
> > 		*p = '\0';
> > 
> > > ...
> > > ```
> > > 
> > > Is there a workaround available for this?
> > > 
> > > Thanks
> 
> 
> -- 
> Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
> KDAB (Deutschland) GmbH, a KDAB Group company
> Tel: +49-30-521325470
> KDAB - The Qt, C++ and OpenGL Experts



-- 

- Arnaldo