Re: [PATCH] netlink: move nla_put_{u8,u16,u32} out of line

[Arnd Bergmann]

On Thu, Feb 9, 2017 at 12:28 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Wed, Feb 8, 2017 at 10:18 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> When CONFIG_KASAN is enabled, the "--param asan-stack=1" causes rather large
>> stack frames in some functions. This goes unnoticed normally because
>> CONFIG_FRAME_WARN is disabled with CONFIG_KASAN by default as of commit
>> 3f181b4d8652 ("lib/Kconfig.debug: disable -Wframe-larger-than warnings with
>> KASAN=y").
>>
>> The kernelci.org build bot however has the warning enabled and that led
>> me to investigate it a little further, as every build produces these warnings:
>>
>> net/wireless/nl80211.c:4389:1: warning: the frame size of 2240 bytes is larger than 2048 bytes [-Wframe-larger-than=]
>> net/wireless/nl80211.c:1895:1: warning: the frame size of 3776 bytes is larger than 2048 bytes [-Wframe-larger-than=]
>> net/wireless/nl80211.c:1410:1: warning: the frame size of 2208 bytes is larger than 2048 bytes [-Wframe-larger-than=]
>> net/bridge/br_netlink.c:1282:1: warning: the frame size of 2544 bytes is larger than 2048 bytes [-Wframe-larger-than=]
>
>
> This may be a good thing in itself, I don't know. But I don't think we
> need to do this for KASAN.
> KASAN does increase stack frames, but it also doubles stack size. So
> at the very least we need to also double CONFIG_FRAME_WARN under KASAN
> to 4096, which would auto-fix your warnings. Enabling 4096 warning for
> KASAN may be moderately useful (catch cases of extremely large frames,
> because KASAN frame increase can be much larger than 2x). But I am
> watching these "used greatest stack depth" messages and also my test
> bots and never seen anything worryingly low.

I'm playing with this on my autobuilder some more, and so far
have only encountered a handful of functions that use more
than 2048 bytes of stack on x86-64 with KASAN enabled, and
for each one of them I could come up with a patch that improved
the object code while making the stack smaller.

A "noinline_for_kasan" annotation would be sufficient to address most
of them.

> I've built kernel with KASAN+KCOV and there is 381 case throughout the codebase:
> https://gist.githubusercontent.com/dvyukov/038426827b369f62d56e50cb39db29df/raw/96cdd758f06751339428c05fda3bc09c6beb700b/gistfile1.txt
> Changing code for each of them looks like lots of work, and can
> introduce bugs. 4096 would leave about 50 of them, but it's still
> lots.

Thanks for the list, that looks very helpful. The ones I found myself
seem to be a strict (and small) subset of those, using gcc-7.0.1 on x86-64
with allmodconfig and a few hundred randconfig builds. Which compiler
version did you use for your testing? If new versions are better than old
ones, we could start by fixing the ones that are still present in gcc-6 and
gcc-7, and making the warning conditionally on the compiler version.

Another idea might be to separate out asan_stack=1 into a separate
Kconfig option and warn if that is enabled with compilers that are known
to be relatively bad it keeping the stack small.

    Arnd