[LLVMdev] Debug Info and DFSan
David Blaikie
dblaikie at gmail.com
Tue Oct 7 15:30:49 PDT 2014
On Tue, Oct 7, 2014 at 2:51 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> Looks good, thanks!
>
> Can you write the test case, please? You probably have more experience
> writing debug info tests than I do.
>
Sure - though how would I get the pre-dfsan .ll file to produce this
behavior? I've tried compiling to a .ll file without dfsan, then feeling
that .ll through opt -dfsan, and I got different output. Instead of
wrapping main, it just replaced it. Is there some attribute or other thing
the frontend is adding to ensure main is wrapped, rather than converted?
>
> 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>
> > > > >>>
> > > > >>> 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$len', 'main' keeps
> its
> > > > >>> original
> > > > >>> name (the pass treats it as an uninstrumented function), and
> > > wrappers are
> > > > >>> created for 'main' and 'strlen' (the wrapper for 'main' is
> unused as
> > > the
> > > > >>> C runtime calls the regular 'main' function directly).
> > > > >>>
> > > > >>
> > > > >> OK, so when you say wrappers are created - where does the name go?
> > > "main"
> > > > >> keeps the name? (it's important which way this transformation is
> done
> > > - if
> > > > >> the guts of main are moved to a new function and the name moved as
> > > well,
> > > > >> that's not the same as keeping the same function as far as debug
> info
> > > > >> metadata is concerned)
> > >
> > > In this case the function keeps its original name and the wrapper is
> > > created
> > > separately.
> > >
> > > > >>> I compile this with '-O0 -g'. A 'break main'/'run'/'break
> > > strlen'/'cont'
> > > > >>> gives a relevant stack trace:
> > > > >>>
> > > > >>> #0 __strlen_sse2_pminub () at
> > > > >>> ../sysdeps/x86_64/multiarch/strlen-sse2-pminub.S:33
> > > > >>> #1 0x00005555555587ff in __dfsw_strlen (s=0x55555556fe17 "foo",
> > > > >>> s_label=<optimized out>, ret_label=0x7fffffffddee)
> > > > >>> at llvm/projects/compiler-rt/lib/dfsan/dfsan_custom.cc:203
> > > > >>> #2 0x000055555556bbdc in dfsw$strlen ()
> > > > >>> #3 0x000055555556bb51 in len (strlen_ptr=0x55555556bbc0
> > > <dfsw$strlen>,
> > > > >>> str=0x55555556fe17 "foo") at strlen.c:5
> > > > >>> #4 0x000055555556bb96 in main ()
> > > > >>>
> > > > >>> In this stack trace, #2 is the compiler-generated wrapper
> function
> > > for
> > > > >>> strlen.
> > > > >>>
> > > > >>> It looks like the debug info for 'len' is preserved correctly,
> but I
> > > > >>> don't
> > > > >>> know why the debug info for 'main' is missing.
> > > > >>>
> > > > >>
> > > > >> Yeah - not quite sure either. I'll poke around with it for a
> little
> > > bit.
> > > > >>
> > > > >
> > > > > DataFlowSanitizer.cpp:719: F.replaceAllUsesWith(WrappedFnCst);
> > > > >
> > > > > replaces the debug info metadata's pointer to main with a pointer
> to
> > > > > dfsw$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("__dfsan_arg_tls", 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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