Le 15/02/2022 à 18:54, Zebediah Figura a écrit :
[1] claims the following:
"It's not valid to access an object of one type using an lvalue of another. In simple cases GCC diagnoses violations of this requirement by -Wstrict-aliasing. -Warray-bounds doesn't detect aliasing violations but it does detect a subset of the problem that's apparent when the size of the lvalue's type is greater than the size of the object. The object must be big enough for the whole lvalue, even if the accessed member is within the bounds of the smaller object."
I can't easily find this in the C specification, but it's also not the easiest thing to search for, and it wouldn't surprise me if it's true.
I think you mean this (from C89/90 ; 6.3 expressions)
Anobjectshallhaveitsstoredvalueaccessedonlybyanlvaluethathasoneofthefollowing types?” -thedeclaredtypeoftheobject, -a qualifiedversionofthedeclaredtypeoftheobject. -a typethatis thesignedorunsignedtypecorrespondingtothedeclaredtypeoftheobject, -a typethatisthesignedorunsignedtypecorrespondingtoa qualifiedversionofthedeclared typeoftheobject. -anaggregateoruniontypethatincludesoneoftheaforementionedtypesamongitsmembers (including.recursively.a memberofa hubaggregateorcontainedunion).or -a charactertype
Still, I'm not convinced we really want to change our usage accordingly. I'm guessing this is one of those things that may be undefined behaviour but always works in practice, and from the looks of things we have more false positives from -Warray-bounds than true positives.
I'm not proposing to find and fix all UB in Wine code [1]; I'm just trying to reduce the warnigs generated by our default settings : roughly 200 warnings for a full Wine compilation for gcc (mingw 11.2 cross compiler) [2]
very few true positive indeed (IMO the years of coverity and other tools did clean up a lot of them) (only one found [3])
But if we really do want -Warray-bounds, I'd rather just allocate the whole VIDEOINFO structure here.
I think it's still useful for detecting bad allocations in future code
I'll resubmit with the whole allocation then
thanks for looking into it
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103292
P.S. can you please remember to keep wine-devel in CC?
sorry, my bad
[1] even though having ubsan and asan running on Wine's PE code would be nice (mingw clang may provide it)
[2] it maybe I get nauseous about those warnings because of all the full recompilations for the 'long types' thingie
[3] the funny story is that the first code committed was correct but it has been (badly) changed to fix a Coverity report <g>
https://source.winehq.org/git/wine.git/?a=commit;h=14f8089dc1793a6c7d91d3dfe...