Here's a question that I would pose and answer if I had enough rep:

S: Linux kernel build fails with mmap error on VirtualBox shared folder

Q: When building the Linux kernel on a VirtualBox shared folder, I get almost all the way through the build, but then it fails during "SORTEX vmlinux" and reports "Could not mmap file: vmlinux". This is consistent with problems that others have reported with mmap on VirtualBox shared folders.

A: As a fix for this very specific case, I modified kernel/scripts/link-vmlinux.sh function sortextable() from this:

$(objtree)/scripts/sortextable ${1}

To this:

cp ${1} /tmp/.${1}
$(objtree)/scripts/sortextable /tmp/.${1}
cp /tmp/.${1} ${1}