[cfe-dev] segmentation fault while building dominator tree in clang

Kristóf Umann via cfe-dev cfe-dev at lists.llvm.org
Sat Apr 6 10:04:26 PDT 2019


Hi!

I recently fiddled around this part of the code as well when trying to
implement an improvement for my checker in the StaticAnalyzer. For the
following invocation:

clang -cc1 -analyze -analyzer-checker=debug.DumpDominators (clang
repository)test/Analysis/cxx-uninitialized-object-unguarded-access.cpp

I received a segfault. I eventually figured that Clang's CFG contains
nullpointers, and the following patch on LLVM fixed the issue:

diff --git a/include/llvm/Support/GenericDomTreeConstruction.h
b/include/llvm/Support/GenericDomTreeConstruction.h
index ccceba88171..a4a238c310b 100644
--- a/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/include/llvm/Support/GenericDomTreeConstruction.h
@@ -235,6 +235,9 @@ struct SemiNCAInfo {
       constexpr bool Direction = IsReverse != IsPostDom;  // XOR.
       for (const NodePtr Succ :
            ChildrenGetter<Direction>::Get(BB, BatchUpdates)) {
+        if (!Succ)
+          continue;
         const auto SIT = NodeToInfo.find(Succ);
         // Don't visit nodes more than once but remember to collect
         // ReverseChildren.

However, I'm not sure whether the CFG is supposed to have nullpointers --
logically, maybe this isn't where we should fix this issue. An assert
wouldn't hurt though.

Good luck!
Kristóf

On Sat, 6 Apr 2019 at 15:57, Abu Naser Masud via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello,
>
> This is my first post in this list. I am building an analysis tool in
> ClangTool.I am getting segmentation fault while building a dominator tree
> in clang. The sample code that I am using to build the dominator tree is
> the following:
>
> *const* Decl* D=*static_cast*<Decl *>(f);     // FunctionDecl f
>
> AnalysisDeclContextManager  *analDeclCtxMgr=*new *
> AnalysisDeclContextManager(context);
>
> *if*(AnalysisDeclContext  *analDeclCtx=analDeclCtxMgr->getContext(D)){
>
>    DominatorTree domTree;
>
>    domTree.buildDominatorTree(*analDeclCtx);
>
> }
>
>
> The input function for my tool is the following code from perlbench(CPU
> 2017)
>
>
> *static* *bool*
>
> S_adjust_index(pTHX_ AV *av, *const* MAGIC *mg, SSize_t *keyp)
>
> {
>
>     *bool* adjust_index = 1;
>
>     *if* (mg) {
>
> */* Handle negative array indices 20020222 MJD */*
>
> SV * *const* ref = SvTIED_obj(MUTABLE_SV(av), mg);
>
> SvGETMAGIC(ref);
>
> *if* (SvROK(ref) && SvOBJECT(SvRV(ref))) {
>
>     SV * *const* * *const* negative_indices_glob =
>
> hv_fetchs(SvSTASH(SvRV(ref)), NEGATIVE_INDICES_VAR, 0);
>
>
>     *if* (negative_indices_glob && isGV(*negative_indices_glob)
>
>     && SvTRUE(GvSV(*negative_indices_glob)))
>
> adjust_index = 0;
>
> }
>
>     }
>
>
>     *if* (adjust_index) {
>
> *keyp += AvFILL(av) + 1;
>
> *if* (*keyp < 0)
>
>     *return* *FALSE*;
>
>     }
>
>     *return* *TRUE*;
>
> }
>
>
> Would you please let me know where the problem is?
>
>
> Thanks,
>
> Masud
>
>
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