lttng-modules 2.10.8-1ubuntu2~18.04.3 fail to build with linux-kvm in Bionic

[Kleber Sacilotto de Souza]

Testing failed on:
    amd64: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-bionic/bionic/amd64/l/lttng-modules/20201127_124844_8cd8d@/log.gz

Build error:
  CC [M] /var/lib/dkms/lttng-modules/2.10.8/build/lttng-abi.o
In file included from /var/lib/dkms/lttng-modules/2.10.8/build/lttng-abi.c:47:0:
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h:99:6: warning: "LTTNG_KERNEL_RANGE" is not defined, evaluates to 0 [-Wundef]
   || LTTNG_KERNEL_RANGE(5,4,28, 5,5,0) \
      ^~~~~~~~~~~~~~~~~~
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h:99:24: error: missing binary operator before token "("
   || LTTNG_KERNEL_RANGE(5,4,28, 5,5,0) \
                        ^
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h: In function ‘wrapper_vmalloc_sync_mappings’:
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h:120:9: error: implicit declaration of function ‘vmalloc_sync_all’; did you mean ‘vmalloc_to_pfn’? [-Werror=implicit-function-declaration]
  return vmalloc_sync_all();
         ^~~~~~~~~~~~~~~~
         vmalloc_to_pfn
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h:120:9: warning: ‘return’ with a value, in function returning void
  return vmalloc_sync_all();
         ^~~~~~~~~~~~~~~~~~
/var/lib/dkms/lttng-modules/2.10.8/build/wrapper/vmalloc.h:118:6: note: declared here
 void wrapper_vmalloc_sync_mappings(void)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
scripts/Makefile.build:330: recipe for target '/var/lib/dkms/lttng-modules/2.10.8/build/lttng-abi.o' failed

For some reason this dkms module is failing to build only with linux-kvm on Bionic, it builds successfully with all the other flavors/derivatives.

[Guilherme G. Piccoli (gpiccoli)]

This is caused due to another missing config option in linux-kvm, CONFIG_KALLSYMS. Lttng seems to have a bug in which if this symbol is not present, it doesn't include <linux/vmalloc.h>, but...some other code portion make use of that, so voilá hehe