[LLVMdev] Debug Info and DFSan (original) (raw)

David Blaikie dblaikie at gmail.com
Tue Oct 7 12:10:13 PDT 2014

On Tue, Oct 7, 2014 at 11:48 AM, Peter Collingbourne <peter at pcc.me.uk> wrote:

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.

Yeah - that'd be the baseline. I assume some people probably care about being able to debug a dfsan binary. Not sure if your users have specifically highlighted this situation or come across the bugs I'm speculating about.

> 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.

Yep, that won't happen for free - we have to stitch it back into the debug info.

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> sizet len(sizet (*strlenptr)(const char *), const char *str) { return strlenptr(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).

OK, so when you say wrappers are created - where does the name go? "main" keeps the name? (it's important which way this transformation is done - if the guts of main are moved to a new function and the name moved as well, that's not the same as keeping the same function as far as debug info metadata is concerned)

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

_#0 strlensse2pminub () at ../sysdeps/x8664/multiarch/strlen-sse2-pminub.S:33 _#1 0x00005555555587ff in dfswstrlen (s=0x55555556fe17 "foo", slabel=, retlabel=0x7fffffffddee) at llvm/projects/compiler-rt/lib/dfsan/dfsancustom.cc:203 #2 0x000055555556bbdc in dfsw$strlen () #3 0x000055555556bb51 in len (strlenptr=0x55555556bbc0 <dfsw$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.

Yeah - not quite sure either. I'll poke around with it for a little bit.

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