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

[Milian Wolff <milian.wolff@kdab.com>]

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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?

> > $ 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

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