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

[David Miller]

From: Arnd Bergmann <arnd@xxxxxxxx>
Date: Wed,  8 Feb 2017 22:18:26 +0100

> 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=]
> 
> It turns out that there is a relatively simple workaround for the netlink
> users that currently use a local variable in order to do the type conversion:
> Moving the three functions (for each of the typical sizes) to lib/nlattr.c
> avoids using local variables in the caller, which drastically reduces the
> stack usage for nl80211 and br_netlink.
> 
> It would be good if we could enable the frame size check after that again,
> but that should be a separate patch and it requires some more testing
> to see which the largest acceptable frame size should be.
> 
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

You should only extern these things when KASAN is enabled.

The reason is that uninlining these routines makes attribute emission
more expensive and for some applications performance of this matters.