https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108060
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:4d0baeae315ebe7d0ec7682ea3e7c0516027c2b8 commit r13-6593-g4d0baeae315ebe7d0ec7682ea3e7c0516027c2b8 Author: Marek Polacek <pola...@redhat.com> Date: Wed Mar 8 09:15:07 2023 -0500 ubsan: missed -fsanitize=bounds for compound ops [PR108060] In this PR we are dealing with a missing .UBSAN_BOUNDS, so the out-of-bounds access in the test makes the program crash before a UBSan diagnostic was emitted. In C and C++, c_genericize gets a[b] = a[b] | c; but in C, both a[b] are one identical shared tree (not in C++ because cp_fold/ARRAY_REF created two same but not identical trees). Since ubsan_walk_array_refs_r keeps a pset, in C we produce a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] = a[b] | c; because the LHS is walked before the RHS. Since r7-1900, we gimplify the RHS before the LHS. So the statement above gets gimplified into _1 = a[b]; c.0_2 = c; b.1 = b; .UBSAN_BOUNDS (0B, b.1, 8); With this patch we produce: a[b] = a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] | c; which gets gimplified into: b.0 = b; .UBSAN_BOUNDS (0B, b.0, 8); _1 = a[b.0]; therefore we emit a runtime error before making the bad array access. I think it's OK that only the RHS gets a .UBSAN_BOUNDS, as in few lines above: the instrumented array access dominates the array access on the LHS, and I've verified that b = 0; a[b] = (a[b], b = -32768, a[0] | c); works as expected: the inner a[b] is OK but we do emit an error for the a[b] on the LHS. For GCC 14, we could apply <https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613687.html> since the copy_node doesn't seem to be needed. PR sanitizer/108060 PR sanitizer/109050 gcc/c-family/ChangeLog: * c-gimplify.cc (ubsan_walk_array_refs_r): For a MODIFY_EXPR, instrument the RHS before the LHS. gcc/testsuite/ChangeLog: * c-c++-common/ubsan/bounds-17.c: New test. * c-c++-common/ubsan/bounds-18.c: New test. * c-c++-common/ubsan/bounds-19.c: New test. * c-c++-common/ubsan/bounds-20.c: New test. * c-c++-common/ubsan/bounds-21.c: New test.
