[PATCH] D33910: [ubsan] Detect invalid unsigned pointer index expression (clang)

[Vedant Kumar via Phabricator via cfe-commits]

vsk created this revision.

Adding an unsigned offset to a base pointer has undefined behavior if
the result of the expression would precede the base. An example from
@regehr:

  int foo(char *p, unsigned offset) {
    return p + offset >= p; // This may be optimized to '1'.
  }
  
  foo(p, -1); // UB.

This patch extends the pointer overflow check in ubsan to detect invalid
unsigned pointer index expressions. It changes the instrumentation to
only permit non-negative offsets in pointer index expressions when all
of the GEP indices are unsigned.

Aside: If anyone has a better name for this type of bug, I'm all ears.
Using "unsigned pointer index expression" could be a problem, because it
sounds like an indexing expression with an _unsigned pointer_.


https://reviews.llvm.org/D33910

Files:
  lib/CodeGen/CGExpr.cpp
  lib/CodeGen/CGExprScalar.cpp
  lib/CodeGen/CodeGenFunction.h
  test/CodeGen/ubsan-pointer-overflow.m

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