There are now patches available to deal with the host of ‘objtool‘ warnings when compiling the kernel with GCC8:

...................................
drivers/video/fbdev/core/fbmem.o: warning: objtool: fb_set_var()+0x209: sibling call from callable instruction with modified stack frame
drivers/video/fbdev/core/fbmem.o: warning: objtool: do_remove_conflicting_framebuffers()+0xa6: sibling call from callable instruction with modified stack frame
drivers/video/fbdev/core/fbmem.o: warning: objtool: register_framebuffer()+0x14b: sibling call from callable instruction with modified stack frame
................... etc....

More details in this thread:  http://lkml.iu.edu/hypermail/linux/kernel/1805.1/02193.html

I have applied the 3 patches, and with Kernel 4.17 (4.17-rc5) the compile is now – relatively – free from ‘warnings’, apart from a few after executing # make xconfig..    The patches also apply cleanly to 4.16 (4.16.8) and do also remove the objtool-related warnings, but still leave a number of ‘syscall‘ and other warnings, which do not occur with 4.17..:

..................
./include/linux/compat.h:52:18: warning: ‘compat_sys_x86_clone’ alias between functions of incompatible types ‘long int(long unsigned int, long unsigned int, int *, long unsigned int, int *)’ and ‘long int(long int, long int, long int, long int, long int)’ [-Wattribute-alias]
 asmlinkage long compat_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))\
 ^~~~~~~~~~
./include/linux/compat.h:47:2: note: in expansion of macro ‘COMPAT_SYSCALL_DEFINEx’
 COMPAT_SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
 ^~~~~~~~~~~~~~~~~~~~~~
arch/x86/ia32/sys_ia32.c:240:1: note: in expansion of macro ‘COMPAT_SYSCALL_DEFINE5’
 COMPAT_SYSCALL_DEFINE5(x86_clone, unsigned long, clone_flags,
 ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/compat.h:56:18: note: aliased declaration here
 asmlinkage long compat_SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))\
 ^~~~~~~~~~
./include/linux/compat.h:47:2: note: in expansion of macro ‘COMPAT_SYSCALL_DEFINEx’
 COMPAT_SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
 ^~~~~~~~~~~~~~~~~~~~~~
arch/x86/ia32/sys_ia32.c:240:1: note: in expansion of macro ‘COMPAT_SYSCALL_DEFINE5’
 COMPAT_SYSCALL_DEFINE5(x86_clone, unsigned long, clone_flags,
..............  (repeated) ...............

So – at least with 4.17 Kernels, the GCC8 warnings have been dealt with..   These patches are still work-in-progress, but hopefully will make it to mainline soon..

Robert Gadsdon.  May 14, 2018.