mmap kernel chunk into user space

[John Baldwin]

On 7/21/19 6:21 PM, Laurie Jennings wrote:
> 
> Im wondering if there have been changes to the api since FreeBSD 9 as I can't get some code I'm porting to work.
> I have a block of kernel memory wired down and I want to map it to user space. Its just a big structure that has stats and other volatile info. In 9.x I was able to simply do:
> // kptr has the kernel address obtained from an ioctl call
> 
> fd=open("/dev/kmem",O_RDWR);memp=mmap(0,size,PROT_READ|PROT_WRITE,MAP_SHARED,fd,(off_t)kptr);
> 
> And it just worked. In 11.3 it fails, and I havent been able to get ANYTHING to work with this method. I'm open to another method; I read something about mmap no longer supportinjgkmem. In which case, what can I do?

Supporting arbitrary mmap of /dev/kmem is not safe as the user mappings
weren't updated if the kernel mappings changed.  To do what you want,
you would create a dedicated device in /dev that you can mmap to access
your data structure.  The simplest way is to just have your device's
d_mmap callback use the offset as an offset into your in-kernel pointer
and then use pmap_kextract() to get the physical address to return.

A fancier version would be to build an sglist describing your buffer and
create an OBJT_SG VM object that you returned from the d_mmap_single
callback, but if you only ever have a single object that is never freed,
the simple version will work fine.

-- 
John Baldwin