Hi Marek, On Mon, 18 Dec 2017 11:38:20 +0100 Marek Vasut <marek.vasut@xxxxxxxxx> wrote: > On 12/18/2017 11:29 AM, Arnd Bergmann wrote: > > On Mon, Dec 18, 2017 at 10:18 AM, Marek Vasut <marek.vasut@xxxxxxxxx> wrote: > >> On 12/18/2017 10:16 AM, Arnd Bergmann wrote: > >>> On Sun, Dec 17, 2017 at 9:34 PM, Richard Weinberger <richard@xxxxxx> wrote: > >>>> Am Mittwoch, 11. Oktober 2017, 15:54:10 CET schrieb Arnd Bergmann: > >>>>> The map_word_() functions, dating back to linux-2.6.8, try to perform > >>>>> bitwise operations on a 'map_word' structure. This may have worked > >>>>> with compilers that were current then (gcc-3.4 or earlier), but end > >>>>> up being rather inefficient on any version I could try now (gcc-4.4 or > >>>>> higher). Specifically we hit a problem analyzed in gcc PR81715 where we > >>>>> fail to reuse the stack space for local variables. > > ... > >>>>> > >>>>> With the latest gcc-8 snapshot, the problem is fixed in upstream gcc, > >>>>> but nobody uses that yet, so we should still work around it in mainline > >>>>> kernels and probably backport the workaround to stable kernels as well. > >>>>> We had a couple of other functions that suffered from the same gcc bug, > >>>>> and all of those had a simpler workaround involving dummy variables > >>>>> in the inline function. Unfortunately that did not work here, the > >>>>> macro hack was the best I could come up with. > >>>>> > >>>>> It would also be helpful to have someone to a little performance testing > >>>>> on the patch, to see how much it helps in terms of CPU utilitzation. > >>>>> > >>>>> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 > >>>>> Cc: stable@xxxxxxxxxxxxxxx > >>>>> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> > >>>> > >>>> Acked-by: Richard Weinberger <richard@xxxxxx> > >>> > >>> Thanks! > >>> > >>>> Marek, I know you are not super happy with this patch but IMHO this is the > >>>> solution with the least hassle. > >>>> While functions offer better type checking I think this functions are trivial > >>>> enough to exist as macros too. > >>>> Also forcing users to upgrade/fix their compilers is only possible in a > >>>> perfect world. > >>> > >>> Right. To clarify, this is a potential security issue, as it might be used to > >>> construct a stack overflow to cause privilege escalation when combined > >>> with some other vulnerabilities. I'd definitely want this backported to > >>> stable kernels as a precaution, and I'm preparing a patch to warn > >>> about this kind of problem again in 'allmodconfig' kernels that > >>> currently disable the warning on arm64 and x86. > >> > >> Wouldn't it make more sense to fix the compiler instead ? > >> This still feels like we're fixing a bug at the wrong place ... > > > > See above: the compiler is fixed in the gcc-8.x release branch, > > which won't be out until next spring. People use all kinds of versions > > as old as gcc-4.3, even if the fix was backported to older compilers > > (which it is not), most users never rebuild their toolchains to get the > > latest bugfix releases. > > > > For instance, the Android SDK comes with prebuilt binaries of > > a gcc-4.9-prerelease version that has many known bugs that > > were fixed either by the time the official 4.9 release happened, > > or in one of the bugfix releases following it. > > But doesn't this mean we're taking the OpenSSL path (which didn't work > out well for them IIRC) ? > > I don't have a better solution for this though ... > I know you don't like this solution, but until you propose a real alternative I decided to apply it. If you come up with something better, I'll consider reverting this patch and applying yours. Regards, Boris