Re: [PATCH bpf-next 1/3] mm/vmalloc: introduce vmalloc_exec which allocates RO+X memory

[Christoph Hellwig]

On Wed, Jul 13, 2022 at 12:20:09PM +0200, Peter Zijlstra wrote:
> Start by adding VM_TOPDOWN_VMAP, which instead of returning the lowest
> (leftmost) vmap_area that fits, picks the higests (rightmost).
> 
> Then add module_alloc_data() that uses VM_TOPDOWN_VMAP and make
> ARCH_WANTS_MODULE_DATA_IN_VMALLOC use that instead of vmalloc (with a
> weak function doing the vmalloc).
> 
> This gets you bottom of module range is RO+X only, top is shattered
> between different !X types.
> 
> Then track the boundary between X and !X and ensure module_alloc_data()
> and module_alloc() never cross over and stay strictly separated.
> 
> Then change all module_alloc() users to expect RO+X memory, instead of
> RW.
> 
> Then make sure any extention of the X range is 2M aligned.
> 
> And presto, *everybody* always uses 2M TLB for text, modules, bpf,
> ftrace, the lot and nobody is tracking chunks.
> 
> Maybe migration can be eased by instead providing module_alloc_text()
> and ARCH_WANTS_MODULE_ALLOC_TEXT.

This all looks pretty sensible.  How are we going to do the initial
write to the executable memory, though?