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

Peter Collingbourne peter at pcc.me.uk
Tue Oct 7 14:51:38 PDT 2014

Looks good, thanks!

Can you write the test case, please? You probably have more experience writing debug info tests than I do.

On Tue, Oct 07, 2014 at 02:35:49PM -0700, David Blaikie wrote:

Here's a basic patch which would solve it in sort of the same way as the other optimizations I was fixing (just special case the debug info & fix it up). I can work up a test case for this as well, or you can, if you like/this seems reasonable.

On Tue, Oct 7, 2014 at 2:30 PM, Peter Collingbourne <peter at pcc.me.uk> wrote: > On Tue, Oct 07, 2014 at 12:20:55PM -0700, David Blaikie wrote: > > On Tue, Oct 7, 2014 at 12:18 PM, David Blaikie <dblaikie at gmail.com> > wrote: > > > > > > > > > > > On Tue, Oct 7, 2014 at 12:10 PM, David Blaikie <dblaikie at gmail.com> > wrote: > > > > > >> > > >> > > >> 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$len', 'main' keeps its > > >>> original > > >>> name (the pass treats it as an uninstrumented function), and > wrappers are > > >>> created for 'main' and 'strlen' (the wrapper for 'main' is unused as > the > > >>> C runtime calls the regular 'main' function directly). > > >>> > > >> > > >> OK, so when you say wrappers are created - where does the name go? > "main" > > >> keeps the name? (it's important which way this transformation is done > - if > > >> the guts of main are moved to a new function and the name moved as > well, > > >> that's not the same as keeping the same function as far as debug info > > >> metadata is concerned) > > In this case the function keeps its original name and the wrapper is > created > separately. > > > >>> I compile this with '-O0 -g'. A 'break main'/'run'/'break > strlen'/'cont' > > >>> gives a relevant stack trace: > > >>> _> > >>> #0 strlensse2pminub () at > > >>> ../sysdeps/x8664/multiarch/strlen-sse2-pminub.S:33 _> > >>> #1 0x00005555555587ff in dfswstrlen (s=0x55555556fe17 "foo", > > >>> slabel=, retlabel=0x7fffffffddee) > > >>> at llvm/projects/compiler-rt/lib/dfsan/dfsancustom.cc:203 > > >>> #2 0x000055555556bbdc in dfsw$strlen () > > >>> #3 0x000055555556bb51 in len (strlenptr=0x55555556bbc0 > <dfsw$strlen>, > > >>> str=0x55555556fe17 "foo") at strlen.c:5 > > >>> #4 0x000055555556bb96 in main () > > >>> > > >>> In this stack trace, #2 is the compiler-generated wrapper function > for > > >>> strlen. > > >>> > > >>> It looks like the debug info for 'len' is preserved correctly, but I > > >>> don't > > >>> know why the debug info for 'main' is missing. > > >>> > > >> > > >> Yeah - not quite sure either. I'll poke around with it for a little > bit. > > >> > > > > > > DataFlowSanitizer.cpp:719: F.replaceAllUsesWith(WrappedFnCst); > > > > > > replaces the debug info metadata's pointer to main with a pointer to > > > dfsw$main (this issue doesn't come up with DAE or ArgPromo because they > > > both don > > > > > > > ... they both don't use RAUW because they have to visit each call site to > > do special stuff anyway. (they're replacing one function with another of > a > > different type, RAUW isn't suitable) > > > > I /guess/ we never end up codegen'ing dfsw$main? (I haven't looked) and > > thus the debug info doesn't describe any function at all. It's possible > > that there's some other reason we don't end up describing any function at > > all... > > > > In any case if we did have "main" in the debug info describe a function, > at > > this point, it would be describing the dfsw$main, not main main. So we'd > > need to handle remapping the debug info back to the original > uninstrumented > > function anyway, I assume? (that's the expected behavior? That the > > uninstrumented function is the one described by debug info, not the > > instrumented wrapper?) > > Yes, the debug info describes 'main', not 'dfsw$main'. So I agree that > the references in the debug info would need to refer to 'main' instead of > 'dfsw$main'. Not sure if there is a good way to do that. Perhaps we could > manually RAUW and skip metadata, but I'm not sure if that would work if the > debug metadata is not a direct user. > > Thanks, > -- > Peter >

diff --git lib/Transforms/Instrumentation/DataFlowSanitizer.cpp lib/Transforms/Instrumentation/DataFlowSanitizer.cpp index 446bcf7..cb84fd6 100644 --- lib/Transforms/Instrumentation/DataFlowSanitizer.cpp +++ lib/Transforms/Instrumentation/DataFlowSanitizer.cpp @@ -51,6 +51,7 @@ #include "llvm/ADT/StringExtras.h" #include "llvm/Analysis/ValueTracking.h" #include "llvm/IR/Dominators.h" +#include "llvm/IR/DebugInfo.h" #include "llvm/IR/IRBuilder.h" #include "llvm/IR/InlineAsm.h" #include "llvm/IR/InstVisitor.h" @@ -243,6 +244,7 @@ class DataFlowSanitizer : public ModulePass { DFSanABIList ABIList; DenseMap<Value *, Function *> UnwrappedFnMap; AttributeSet ReadOnlyNoneAttrs; + DenseMap<const Function *, DISubprogram> FunctionDIs;

Value *getShadowAddress(Value *Addr, Instruction *Pos); bool isInstrumented(const Function *F); @@ -573,6 +575,8 @@ bool DataFlowSanitizer::runOnModule(Module &M) { if (ABIList.isIn(M, "skip")) return false; + FunctionDIs = makeSubprogramMap(M); + if (!GetArgTLSPtr) { Type *ArgTLSTy = ArrayType::get(ShadowTy, 64); _ArgTLS = Mod->getOrInsertGlobal("dfsanargtls", ArgTLSTy); @@ -725,6 +729,12 @@ bool DataFlowSanitizer::runOnModule(Module &M) { Value *WrappedFnCst = ConstantExpr::getBitCast(NewF, PointerType::getUnqual(FT)); F.replaceAllUsesWith(WrappedFnCst); + + // Patch the pointer to LLVM function in debug info descriptor. + auto DI = FunctionDIs.find(&F); + if (DI != FunctionDIs.end()) + DI->second.replaceFunction(&F); + UnwrappedFnMap[WrappedFnCst] = &F; *i = NewF;

-- Peter

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