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

[James Clark <james.clark@arm.com>]

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.


> 
> $ 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
>