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

[Laurent Pinchart]

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);
+
+		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