close_range is not available on older systems, preventing the building of a UML kernel

[Glenn Washburn]

Hi all,

I'm wanting to build a UML kernel on Debian bullseye, which is at
kernel version 5.10, which does not support close_range(). It appears
as though close_range() support is required on the host since commit
32e8eaf263d ("um: use execveat to create userspace MMs"). Was there a
conscious decision to break support for kernels less than 5.11, when
close_range() was added. What is the policy on support of the host
kernel? Since 5.4 is still supported as a longterm release. I think it
would be nice to at least support 5.4. Is there any interest in
removing this restriction?

Also, this system is at glibc 2.31, which does not have execveat()
support, which is required by that same commit above. Since execveat()
has been supported since 3.19, this kernel should support it. Perhaps
the issue can be resolved by defining a syscall wrapper in the UM
headers if support is not found in glibc.

Glenn