[LLVMdev] Debug Info and DFSan

[Peter Collingbourne]

On Tue, Oct 07, 2014 at 10:04:30AM -0700, David Blaikie wrote:
> Hi Peter,
> 
> After discovering several bugs in ArgumentPromotion and
> DeadArgumentElimination where llvm::Functions were replaced with similar
> functions (with the same name) to transform their type in some way, I
> started looking at all calls to llvm::Function::takeName to see if there
> were any other debug info quality bugs in similar callers.
> 
> One such caller is the DataFlowSanitizer, and I don't see any debug info
> tests for this so I'm wondering what /should/ happen here.
> 
> Is DFSan+DebugInfo something that matters? I assume so.

It may be important in the future, but at the moment the dfsan runtime library
does not make use of debug info. The debug info could still be useful for
regular debugging tasks though.

> It looks like DFSan is creating wrappers (in/around
> DataFlowSanitizer.cpp:680-700) - when it does this, should it update the
> debug info for these functions? Or are these internal instrumentation
> functions & nothing to do with the code the user wrote? I can't quite tell
> from the code.

The functions created by that part of the code replace the original functions,
so they should inherit the debug info for those functions.

But the code below that can also create wrapper functions which do not need
debug info (lines 712-746). Wrappers normally show up for uninstrumented
functions (e.g. main and many libc functions).

> Could you provide any C/C++ source examples whis part of DFSan fires
> reliably, so I could experiment with some examples and see how the debug
> info looks?

This is an example of a program that exercises the replacement function and
wrapper features.

--------------------------------------------------------------------------------
#include <stddef.h>
#include <string.h>

size_t len(size_t (*strlen_ptr)(const char *), const char *str) {
  return strlen_ptr(str);
}

int main(void) {
  return len(strlen, "foo");
}
--------------------------------------------------------------------------------

In this example, 'len' is rewritten to 'dfs$len', 'main' keeps its original
name (the pass treats it as an uninstrumented function), and wrappers are
created for 'main' and 'strlen' (the wrapper for 'main' is unused as the
C runtime calls the regular 'main' function directly).

I compile this with '-O0 -g'. A 'break main'/'run'/'break strlen'/'cont'
gives a relevant stack trace:

#0  __strlen_sse2_pminub () at ../sysdeps/x86_64/multiarch/strlen-sse2-pminub.S:33
#1  0x00005555555587ff in __dfsw_strlen (s=0x55555556fe17 "foo", s_label=<optimized out>, ret_label=0x7fffffffddee)
    at llvm/projects/compiler-rt/lib/dfsan/dfsan_custom.cc:203
#2  0x000055555556bbdc in dfsw$strlen ()
#3  0x000055555556bb51 in len (strlen_ptr=0x55555556bbc0 <dfsw$strlen>, str=0x55555556fe17 "foo") at strlen.c:5
#4  0x000055555556bb96 in main ()

In this stack trace, #2 is the compiler-generated wrapper function for strlen.

It looks like the debug info for 'len' is preserved correctly, but I don't
know why the debug info for 'main' is missing.

Thanks,
-- 
Peter