Compiler lacks asm-goto support on Fedora 28

I couldn't find an rpm for v4l2loopback so I tried to compile it on a fresh Fedora 28, but it failed :

$ LANG=en_US.utf8 make
Building v4l2-loopback driver...
make -C /lib/modules/`uname -r`/build M=/opt/v4l2loopback modules
make[1]: Entering directory '/usr/src/kernels/4.17.6-200.fc28.x86_64'
arch/x86/Makefile:184: *** Compiler lacks asm-goto support..  Stop.
make[1]: Leaving directory '/usr/src/kernels/4.17.6-200.fc28.x86_64'
make: *** [Makefile:43: v4l2loopback.ko] Error 2

I found that asm-goto has been supported since gcc 4.5, and I have 8.1.1 installed :

$ LANG=en_US.utf8 gcc -v
Using built-in specs.
COLLECT_GCC=cc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable- languages=c,c++,fortran,objc,obj-c++,ada,go,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl --enable-libmpx --enable-offload-targets=nvptx-none --without-cuda-driver --enable-gnu-indirect-function --enable-cet --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 8.1.1 20180712 (Red Hat 8.1.1-5) (GCC)

I managed to compile the module by simply commenting the following check in /usr/src/kernels/4.17.6-200.fc28.x86_64/arch/x86/Makefile :

# ifndef CC_HAVE_ASM_GOTO
#  $(error Compiler lacks asm-goto support.)
# endif

but obviously this is a dirty workaround. Yet, the Makefile CFLAGS actually had -DCC_HAVE_ASM_GOTO set. I also tried to set CC_HAV_ASM_GOTO in the Makefile but it didn't help.

What's wrong here ? Is it the Makefile, or Fedora, or am I missing something ? I lost familiarity with Makefiles…

I got the same problem. The caches of Makefile were staled/dirty. In my case, it seems because I terminated make on the fly.

Inspired from this old patch https://lkml.org/lkml/2018/3/19/1474, I changed from the shell-cached to shell in Makefile in base dir to avoid caching and it works:

-ifeq ($(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y)
+ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y)

Then clean to make sure we don't have bad cache: