| Message ID | 20220617150922.1878926-1-arnd@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] Kbuild: change CONFIG_FRAME_WARN for 32-bit | expand |
Hello Arnd, your To: was empty, I wonder if that was on purpose?! On Fri, Jun 17, 2022 at 05:08:53PM +0200, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.de> > > The introduction of CONFIG_GCC_PLUGIN_LATENT_ENTROPY raised the > warning limit for 32-bit architectures to a much higher value in > 2016. Initially this had no effect for Arm machines as their kernels > tend to be cross-compiled, and the feature detection for the plugin did > not work with common cross compilers. > > I could not find the original regression report that led to the warning > limit getting raised, but I have been unable to reproduce this with gcc-12 > and linux-5.18 -- all frame sizes appear to be be below the normal 1024 > byte limit on at least arm32 and i386. > > However, the KASAN feature introduced later on does raise the frame size > of a number of functions above the warning limit, in almost all cases to > somewhere below 1280 bytes. The effect is that an arm allmodconfig build > produces no warnings because of the check for the latent entropy plugin, > but a normal KASAN based build with gcc does produce build warnings or > failures when CONFIG_WERROR is set. > > The 1280 byte limit is an arbitrary choice here, as it disables almost > all warnings but leaves a few outliers that are probably better addressed > with code changes. There are a number of functions that are just below the > normal 1024 byte limit at the moment, and an extra 256 bytes for those > is within the expectation, but more than that is probably an indication > of a bad design or a real bug. > > Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> > Cc: Masahiro Yamada <masahiroy@kernel.org> > Cc: Kees Cook <keescook@chromium.org> > Cc: linux-kbuild@vger.kernel.org > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > --- > lib/Kconfig.debug | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index 3da5f9acb966..8a3afd837e99 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -387,9 +387,9 @@ endif # DEBUG_INFO > config FRAME_WARN > int "Warn for stack frames larger than" > range 0 8192 > - default 2048 if GCC_PLUGIN_LATENT_ENTROPY > default 2048 if PARISC > default 1536 if (!64BIT && XTENSA) > + default 1280 if (!64BIT && KASAN_STACK) > default 1024 if !64BIT > default 2048 if 64BIT > help I wasn't aware this was configurable with a Kconfig symbol. Thanks for addressing that issue, the reasoning looks good and right. I just now noticed you explained your findings earlier in irc, only found them because I digged my irc logs for my original report to you. Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Best regards Uwe
Hi Arnd, On Fri, Jun 17, 2022 at 05:08:53PM +0200, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.de> > > The introduction of CONFIG_GCC_PLUGIN_LATENT_ENTROPY raised the > warning limit for 32-bit architectures to a much higher value in > 2016. Initially this had no effect for Arm machines as their kernels > tend to be cross-compiled, and the feature detection for the plugin did > not work with common cross compilers. > > I could not find the original regression report that led to the warning > limit getting raised, but I have been unable to reproduce this with gcc-12 > and linux-5.18 -- all frame sizes appear to be be below the normal 1024 > byte limit on at least arm32 and i386. > > However, the KASAN feature introduced later on does raise the frame size > of a number of functions above the warning limit, in almost all cases to > somewhere below 1280 bytes. The effect is that an arm allmodconfig build > produces no warnings because of the check for the latent entropy plugin, > but a normal KASAN based build with gcc does produce build warnings or > failures when CONFIG_WERROR is set. > > The 1280 byte limit is an arbitrary choice here, as it disables almost > all warnings but leaves a few outliers that are probably better addressed > with code changes. There are a number of functions that are just below the > normal 1024 byte limit at the moment, and an extra 256 bytes for those > is within the expectation, but more than that is probably an indication > of a bad design or a real bug. > > Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> > Cc: Masahiro Yamada <masahiroy@kernel.org> > Cc: Kees Cook <keescook@chromium.org> > Cc: linux-kbuild@vger.kernel.org > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > --- > lib/Kconfig.debug | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index 3da5f9acb966..8a3afd837e99 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -387,9 +387,9 @@ endif # DEBUG_INFO > config FRAME_WARN > int "Warn for stack frames larger than" > range 0 8192 > - default 2048 if GCC_PLUGIN_LATENT_ENTROPY > default 2048 if PARISC > default 1536 if (!64BIT && XTENSA) > + default 1280 if (!64BIT && KASAN_STACK) For what it's worth, if this was changed to just KASAN, this would improve the situation for clang and ARCH=arm allmodconfig as well. It leaves just two -Wframe-larger-than warnings in drivers/gpu/drm/amd/display, which can be worked around with a kernel side change: https://github.com/ClangBuiltLinux/linux/issues/1455#issuecomment-995337569 Cheers, Nathan
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 3da5f9acb966..8a3afd837e99 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -387,9 +387,9 @@ endif # DEBUG_INFO config FRAME_WARN int "Warn for stack frames larger than" range 0 8192 - default 2048 if GCC_PLUGIN_LATENT_ENTROPY default 2048 if PARISC default 1536 if (!64BIT && XTENSA) + default 1280 if (!64BIT && KASAN_STACK) default 1024 if !64BIT default 2048 if 64BIT help