[libsanitizer] Handle assembler-generated dwarf info in backtrace

[vries]

Summary

[ This patch attempts to fix a problem, for which another patch was submitted earlier ( https://reviews.llvm.org/D34149 ). ]

Consider this test-case, which generates a segmentation fault:
...
int
main (void)
{

void *ptr = (void *)0x01;
int value = *((int *) ptr);
return 0;

}
...

Using LD_PRELOAD we can see how libasan handles the segfault:
...
$ LD_PRELOAD=install/lib64/libasan.so ./a.out
...

Focusing just on the backtrace part for main we have:
...

#0 0x4004a6 in main (a.out+0x4004a6)

...
and when compiled with -g, we have instead instead:
...

#0 0x4004a6 in main test.c:5

...

When we compile the test-case to assembly without -g, but compile the
assembly with -g like this:
...
$ gcc test.c -S
$ gcc test.s -gdwarf-2 -c
$ gcc test.o
...
we get this, similar to the case without -g:
...

#0 0x400481 in main (a.out+0x400481)

...

In this case, the executable contains DWARF file and line number information
for the assembly file, but it's not used because the function name is
missing in the DWARF .debug_info.

This patch fixes this, allowing us to get:
...

#0 0x400481 in main test.s:14

...

[dennis13]

@vries Thanks for reopen this problem,
by the way,
this is a really hard to face in real world, you have to compile by gcc without debuginfo or write on assembly, and then add debuginfo by "as".

[vries]

Ping.

libsanitizer/sanitizer_common/sanitizer_symbolizer_libbacktrace.cc

                                        const char *filename, int lineno,
                                        const char *function) {
   SymbolizeCodeCallbackArg *cdata = (SymbolizeCodeCallbackArg *)vdata;
-  if (function) {
-    AddressInfo *info = cdata->get_new_frame(addr);
+  AddressInfo *info = cdata->get_new_frame(addr);
+  if (filename)
+    info->file = internal_strdup(filename);
+  info->line = lineno;
+  if (function)
     info->function = DemangleAlloc(function, /*always_alloc*/ true);
-    if (filename)
-      info->file = internal_strdup(filename);
-    info->line = lineno;
-    cdata->frames_symbolized++;
-  }
+  cdata->frames_symbolized++;
   return 0;
 }
 

   data.frames_symbolized = 0;
   backtrace_pcinfo((backtrace_state *)state_, addr, SymbolizeCodePCInfoCallback,
                    ErrorCallback, &data);
-  if (data.frames_symbolized > 0)
+  if (data.frames_symbolized == 1 && data.last->info.function == 0)
+    /* Augment the frame by trying to fill in the missing function with
+       backtrace_syminfo.  */
+    data.frames_symbolized = 0;
+  else if (data.frames_symbolized > 0)
     return true;
   backtrace_syminfo((backtrace_state *)state_, addr, SymbolizeCodeCallback,
                     ErrorCallback, &data);