[llvm-bugs] [Bug 32630] New: UBSan: No warning for misaligned load of extern

[via llvm-bugs]

http://bugs.llvm.org/show_bug.cgi?id=32630

            Bug ID: 32630
           Summary: UBSan: No warning for misaligned load of extern
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vsk at apple.com
                CC: llvm-bugs at lists.llvm.org

Presently, UBSan does not diagnose misaligned loads/stores from extern
variables. Example:

// a.c
struct S { int I; }
extern S global_S;
int getI() { return global_S.I; }

// b.c
char ...;
char global_S[4];
char ...;

UBSan can emit an alignment check for loads from DeclRefExprs. However, this
isn't enough to catch the issue, because the IRBuilder constant folds the
alignment test away to 'true': (and (ptrtoint (gep &global_S ...)) 3) => 0.
This makes the test ineffective.

We might try and fix this by disabling constant folding when emitting UBSan
checks.

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