[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined…

Authored by lebedev.ri on Oct 10 2019, 2:25 AM.


[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour

Quote from http://eel.is/c++draft/expr.add#4:

4     When an expression J that has integral type is added to or subtracted
      from an expression P of pointer type, the result has the type of P.
(4.1) If P evaluates to a null pointer value and J evaluates to 0,
      the result is a null pointer value.
(4.2) Otherwise, if P points to an array element i of an array object x with n
      elements ([dcl.array]), the expressions P + J and J + P
      (where J has the value j) point to the (possibly-hypothetical) array
      element i+j of x if 0≤i+j≤n and the expression P - J points to the
      (possibly-hypothetical) array element i−j of x if 0≤i−j≤n.
(4.3) Otherwise, the behavior is undefined.

Therefore, as per the standard, applying non-zero offset to nullptr
(or making non-nullptr a nullptr, by subtracting pointer's integral value
from the pointer itself) is undefined behavior. (*if* nullptr is not defined,
i.e. e.g. -fno-delete-null-pointer-checks was *not* specified.)

To make things more fun, in C (6.5.6p8), applying *any* offset to null pointer
is undefined, although Clang front-end pessimizes the code by not lowering
that info, so this UB is "harmless".

Since rL369789 (D66608 [InstCombine] icmp eq/ne (gep inbounds P, Idx..), null -> icmp eq/ne P, null)
LLVM middle-end uses those guarantees for transformations.
If the source contains such UB's, said code may now be miscompiled.
Such miscompilations were already observed:

Surprisingly, UBSan does not catch those issues
... until now. This diff teaches UBSan about these UB's.

getelementpointer inbounds is a pretty frequent instruction,
so this does have a measurable impact on performance;
I've addressed most of the obvious missing folds (and thus decreased the performance impact by ~5%),
and then re-performed some performance measurements using my RawSpeed benchmark:
(all measurements done with LLVM ToT, the sanitizer never fired.)

  • no sanitization vs. existing check: average +21.62% slowdown
  • existing check vs. check after this patch: average 22.04% slowdown
  • no sanitization vs. this patch: average 48.42% slowdown

Reviewers: vsk, filcab, rsmith, aaron.ballman, vitalybuka, rjmccall, #sanitizers

Reviewed By: rsmith

Subscribers: kristof.beyls, nickdesaulniers, nikic, ychen, dtzWill, xbolva00, dberris, arphaman, rupprecht, reames, regehr, llvm-commits, cfe-commits

Tags: #clang, #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67122

llvm-svn: 374293