On Tue, Dec 3, 2019 at 5:42 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > Quoting Nick Desaulniers (2019-12-02 19:18:20) > > On Sat, Nov 23, 2019 at 12:05 PM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > Quoting Nathan Chancellor (2019-11-23 19:53:22) > > > > -Wtautological-compare was recently added to -Wall in LLVM, which > > > > exposed an if statement in i915 that is always false: > > > > > > > > ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning: > > > > result of comparison of constant 576460752303423487 with expression of > > > > type 'unsigned int' is always false > > > > [-Wtautological-constant-out-of-range-compare] > > > > if (unlikely(remain > N_RELOC(ULONG_MAX))) > > > > ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ > > > > > > > > Since remain is an unsigned int, it can never be larger than UINT_MAX, > > > > which is less than ULONG_MAX / sizeof(struct drm_i915_gem_relocation_entry). > > > > Remove this statement to fix the warning. > > > > > > The check should remain as we do want to document the overflow > > > calculation, and it should represent the types used -- it's much easier > > > > What do you mean "represent the types used?" Are you concerned that > > the type of drm_i915_gem_exec_object2->relocation_count might change > > in the future? > > We may want to change the restriction, yes. > > > > to review a stub than trying to find a missing overflow check. If the > > > overflow cannot happen as the types are wide enough, no problem, the > > > compiler can remove the known false branch. > > > > What overflow are you trying to protect against here? > > These values are under user control, our validation steps should be > clear and easy to check. If we have the types wrong, if the checks are > wrong, we need to fix them. If the code is removed because it can be > evaluated by the compiler to be redundant, it is much harder for us to > verify that we have tried to validate user input. > > > > Tautology here has a purpose for conveying information to the reader. > > > > Well leaving a warning unaddressed is also not a solution. Either > > replace it with a comment or turn off the warning for your subdir. > > My personal preference would be to use a bunch of central macros for the > various type/kmalloc overflows, and have the warnings suppressed there > since they are very much about documenting user input validation. > -Chris Is kmalloc_array what you're looking for? Looks like it has the `check_mul_overflow` call in it. -- Thanks, ~Nick Desaulniers _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
