[libcamera-devel] [PATCH v2 4/4] libcamera: base: backtrace: Fallback to libunwind for symbolic names

[Laurent Pinchart]

Hi Kieran,

On Tue, Oct 12, 2021 at 11:57:11AM +0100, Kieran Bingham wrote:
> Quoting Laurent Pinchart (2021-10-03 23:36:06)
> > 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 0aafc6a366c5..d93e5518670d 100644
> > --- a/src/libcamera/base/backtrace.cpp
> > +++ b/src/libcamera/base/backtrace.cpp
> > @@ -202,6 +202,12 @@ 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) {
> > @@ -210,6 +216,29 @@ 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);
> > +
> > +               entry << "+0x" << std::hex << offset << "\n";
> > +               backtraceText_.emplace_back(entry.str());
> > +#endif
> >         } while (unw_step(&cursor) > 0);
> >  
> >         return true;
> > @@ -245,9 +274,15 @@ 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), "");
> 
> Oh I see. Took a bit of looking to realise that this is concatenating
> all of the std::strings in a vector into a single string, while skipping
> the number of levels. Neat, but a little bit obtuse to read at first.

The C++20 ranges library is supposed to enable more expressive syntaxes,
such as

	return backtraceText_ | std::views::drop(skipLevels) | std::views::join("");

(won't work as-is, it doesn't compile and I don't want to spend time
figuring out what's wrong :-)).

The concept is interesting, but it seems that in real life scenari it's
actually difficult to use.

> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> > +       }
> > +
> >  #if HAVE_DW
> >         DwflParser dwfl;
> >  

-- 
Regards,

Laurent Pinchart