[Buildroot] [PATCH] linux: forcibly disable use of gcc plugins

Yann E. MORIN yann.morin.1998 at free.fr
Tue May 12 20:33:53 UTC 2020

The soon-to-be-released linux 5.7 has changed the way it detects the
ability of gcc to use plugins, when it dropped support for gcc 4.7 or
older [0].

To detect the ability to use gcc plugins, the kernel has to check whther
the host gcc is capable enough to build them.

When we call one of the configurator for the linux kernel, we explicitly
pass a value of HOSTCC=$(HOSTCC_NOCCACHE), because there might be a
discrepancy between the ncurses headers and libraries as found by the
linux kconfig build [1] [2].

But then, when we build the kernel, we pass another value to use [3]
HOSTCC="$(HOSTCC) $(HOST_CFLAGS) $(HOST_LDFLAGS)" which boils down to
roughly: gcc -I.../host/include -L.../host/lib -Wl,-rpath,.../host/lib
This is needed so that at build time, the kernel can build host tools
that link with our openssl et al.

So, the two HOSTCC we pass to the kernel may have different behaviours.
For example, on a machine where gmp is missing in the system, it is
available in $(O)/host/ when using an internal toolchain (and under a
few other conditions).

In that case, when configuring the kernel, it decides that the host
compiler can't build plugins, so the dependencies of CONFIG_GCC_PLUGINS
are not met, and that option is not present in the linux' .config file
(neither as "=y" nor as "is not set"). But then, when we build the
kernel, the host compiler suddenly becomes capable of building the
plugins, and the internal syncconfig run by the kernel will notice that
the dependencies of CONFIG_GCC_PLUGINS are now met, and that the user
shall decide on its value. And this blocks a build on an interactive
console (abbreviated):

    * Restart config...
    * GCC plugins
    GCC plugins (GCC_PLUGINS) [Y/n/?] (NEW) _

But most problematic is the behaviour when run in a shell that is not
interactiove (e.g. a CI job or such) (abbreviated):

    * Restart config...
    * GCC plugins
    GCC plugins (GCC_PLUGINS) [Y/n/?] (NEW)
    Error in reading or end of file.
      Generate some entropy during boot and runtime (GCC_PLUGIN_LATENT_ENTROPY) [N/y/?] (NEW)
    Error in reading or end of file.
      Randomize layout of sensitive kernel structures (GCC_PLUGIN_RANDSTRUCT) [N/y/?] (NEW)
    Error in reading or end of file.
    * Memory initialization
    Initialize kernel stack variables at function entry
    > 1. no automatic initialization (weakest) (INIT_STACK_NONE)
      2. zero-init structs marked for userspace (weak) (GCC_PLUGIN_STRUCTLEAK_USER) (NEW)
      3. zero-init structs passed by reference (strong) (GCC_PLUGIN_STRUCTLEAK_BYREF) (NEW)
      4. zero-init anything passed by reference (very strong) (GCC_PLUGIN_STRUCTLEAK_BYREF_ALL) (NEW)
    Error in reading or end of file.
    Poison kernel stack before returning from syscalls (GCC_PLUGIN_STACKLEAK) [N/y/?] (NEW)
    Error in reading or end of file.
    Enable heap memory zeroing on allocation by default (INIT_ON_ALLOC_DEFAULT_ON) [N/y/?] n
    Enable heap memory zeroing on free by default (INIT_ON_FREE_DEFAULT_ON) [N/y/?] n

The most obvious and simple solution would be to unconditionally disable
gcc plugins alltogether, in the KCONFIG_FIXUP hook. But that can't work
either, because after applying the fixups, we call olddefconfig (or the
likes) with the incapable HOSTCC, so the disabled option would be removed
anyway, and we'd be back to square one.

So, in addition to the above, we also forcibly hack the same call just
before actually building the kernel.

Note that the two are needed: the one in the fixups is needed for those
that have a system that already allows building gcc plugins, and the
second is needed in the other case, where the system does not allow it
but would work with our additional headers and libs in $(O)/host/. The
two ensure there is a very similar experience in the two situations.

Forcibly disabling the use of gcc plugins is not a regression on our
side: it has never been possible to do so so far. We're now making sure
that can't work by accident.

Reported-by: Ganesh <ganesh45in at gmail.com>,
Reported-by: Heiko Thiery <heiko.thiery at gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998 at free.fr>
Cc: Michael Walle <michael.walle at kontron.com>
Cc: Peter Korsgaard <peter at korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni at bootlin.com>
Cc: Arnout Vandecappelle <arnout at mind.be>
 linux/linux.mk | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/linux/linux.mk b/linux/linux.mk
index 69bbad99e1..b9f2052ee7 100644
--- a/linux/linux.mk
+++ b/linux/linux.mk
@@ -363,6 +363,7 @@ define LINUX_KCONFIG_FIXUP_CMDS
@@ -423,7 +424,10 @@ endif
 # '$(LINUX_TARGET_NAME)' targets separately because calling them in
 # the same $(MAKE) invocation has shown to cause parallel build
 # issues.
+# The call to disable gcc-plugins is a stop-gap measure:
+#   http://lists.busybox.net/pipermail/buildroot/2020-May/282727.html
 	$(foreach dts,$(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH)), \
 		cp -f $(dts) $(LINUX_ARCH_PATH)/boot/dts/

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