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

[Kieran Bingham]

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.

Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> +       }
> +
>  #if HAVE_DW
>         DwflParser dwfl;
>  
> -- 
> Regards,
> 
> Laurent Pinchart
>