kernel features MIB

[Kris Kennaway]

John Baldwin wrote:
> One of the things we have at work is a kern.features sysctl MIB that contains 
> nodes to indicate if a named feature is present.  For example, on i386 we 
> have kern.features.pae and we auto enable -DPAE for kernel modules if the 
> currently running kernel is using PAE using that sysctl.
> 
> One of the patches I want to commit soon is support for handling 
> shm_open/shm_unlink directly in the kernel via swap-backed VM objects (the 
> long-heralded memfd stuff).  I would like to have the sysctl MIB so that 
> libc's for older releases (e.g. libc.so.6) could use the syscalls if they are 
> available so that shm segments are shared between compat apps (e.g. 4.x or 
> 6.x) and up-to-date apps.
> 
> At work we don't have a pretty API for this at all, but I'm thinking for 
> FreeBSD we can do this:
> 
> FEATURE(foo, "description of foo")
> 
> which is a macro to create the 'kern.features.foo' node and set it to 1.  Then 
> we could have a routine in libc:
> 
> int	feature_present(const char *name);
> 
> That returns a boolean to indicate if a given feature is present or not by 
> invoking sysctlbyname(3), etc.
> 
> Any objections to the idea?
> 

I have wanted something like this for a long time.  In ports land they 
often need to know this kind of thing, e.g. is compat4x support enabled 
in the kernel, etc.

Kris