[libcamera-devel] [PATCH v1 4/4] libcamera: base: backtrace: Fallback to libunwind for symbolic names

[Laurent Pinchart]

Hi Nicolas,

On Mon, Sep 27, 2021 at 10:06:40AM -0400, Nicolas Dufresne wrote:
> Le vendredi 24 septembre 2021 à 13:23 +0300, Laurent Pinchart a écrit :
> > libunwind has an API to provide symbolic names for functions. It's less
> > optimal than using backtrace_symbols() or libdw, as it doesn't allow
> > deferring the symbolic names lookup, but it can be usefull as a fallback
> > if no other option is available.
> > 
> > A sample backtrace when falling back to libunwind looks like
> > 
> > libcamera::VimcCameraData::init()+0xbd
> > libcamera::PipelineHandlerVimc::match(libcamera::DeviceEnumerator*)+0x3e0
> > libcamera::CameraManager::Private::createPipelineHandlers()+0x1a7
> > libcamera::CameraManager::Private::init()+0x98
> > libcamera::CameraManager::Private::run()+0x9f
> > libcamera::Thread::startThread()+0xee
> > decltype(*(std::__1::forward<libcamera::Thread*>(fp0)).*fp()) std::__1::__invoke<void (libcamera::Thread::*)(), libcamera::Thread*, void>(void (libcamera::Thread::*&&)(), libcamera::Thread*&&)+0x77
> > void std::__1::__thread_execute<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (libcamera::Thread::*)(), libcamera::Thread*, 2ul>(std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (libcamera::Thread::*)(), libcamera::Thread*>&, std::__1::__tuple_indices<2ul>)+0x3e
> > void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (libcamera::Thread::*)(), libcamera::Thread*> >(void*)+0x62
> > start_thread+0xde
> > ???
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  include/libcamera/base/backtrace.h |  1 +
> >  src/libcamera/base/backtrace.cpp   | 37 +++++++++++++++++++++++++++++-
> >  2 files changed, 37 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/libcamera/base/backtrace.h b/include/libcamera/base/backtrace.h
> > index 58ccc14c8f81..bb77c73b67e3 100644
> > --- a/include/libcamera/base/backtrace.h
> > +++ b/include/libcamera/base/backtrace.h
> > @@ -30,6 +30,7 @@ private:
> >  	bool unwindTrace();
> >  
> >  	std::vector<void *> backtrace_;
> > +	std::vector<std::string> backtraceText_;
> >  };
> >  
> >  } /* namespace libcamera */
> > diff --git a/src/libcamera/base/backtrace.cpp b/src/libcamera/base/backtrace.cpp
> > index 40fa60d0f9bf..bbb0cc0e17da 100644
> > --- a/src/libcamera/base/backtrace.cpp
> > +++ b/src/libcamera/base/backtrace.cpp
> > @@ -196,6 +196,12 @@ bool Backtrace::unwindTrace()
> >  		return false;
> >  
> >  	do {
> > +#if HAVE_BACKTRACE || HAVE_DW
> > +		/*
> > +		 * If backtrace() or libdw is available, they will be used in
> > +		 * toString() to provide symbol information for the stack
> > +		 * frames using the IP register value.
> > +		 */
> >  		unw_word_t ip;
> >  		ret = unw_get_reg(&cursor, UNW_REG_IP, &ip);
> >  		if (ret) {
> > @@ -204,6 +210,29 @@ bool Backtrace::unwindTrace()
> >  		}
> >  
> >  		backtrace_.push_back(reinterpret_cast<void *>(ip));
> > +#else
> > +		/*
> > +		 * Otherwise, use libunwind to get the symbol information. As
> > +		 * the libunwind API uses cursors, we can't store the IP values
> > +		 * and delay symbol lookup to toString().
> > +		 */
> > +		char symbol[256];
> > +		unw_word_t offset = 0;
> > +		ret = unw_get_proc_name(&cursor, symbol, sizeof(symbol), &offset);
> > +		if (ret) {
> > +			backtraceText_.emplace_back("???\n");
> > +			continue;
> > +		}
> > +
> > +		std::ostringstream entry;
> > +
> > +		char *name = abi::__cxa_demangle(symbol, nullptr, nullptr, nullptr);
> > +		entry << (name ? name : symbol);
> > +		free(name);
> 
> Perhaps a smart pointer.

As mentioned in the reply to 2/4, I don't mind. Let's discuss it there,
and I'll update this location accordingly.

> > +
> > +		entry << "+0x" << std::hex << offset << "\n";
> > +		backtraceText_.emplace_back(entry.str());
> > +#endif
> >  	} while (unw_step(&cursor) > 0);
> >  
> >  	return true;
> > @@ -239,9 +268,15 @@ std::string Backtrace::toString(unsigned int skipLevels) const
> >  	 */
> >  	skipLevels += 2;
> >  
> > -	if (backtrace_.size() <= skipLevels)
> > +	if (backtrace_.size() <= skipLevels &&
> > +	    backtraceText_.size() <= skipLevels)
> >  		return std::string();
> >  
> > +	if (!backtraceText_.empty()) {
> > +		Span<const std::string> trace{ backtraceText_ };
> > +		return utils::join(trace.subspan(skipLevels), "");
> > +	}
> > +
> >  #if HAVE_DW
> >  	DwflParser dwfl;
> >  

-- 
Regards,

Laurent Pinchart