Subject: Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()

From: Mike Rapoport <[email-masked]>
Date: Sat, 20 Apr 2024 07:22:50 +0300

On Fri, Apr 19, 2024 at 02:42:16PM -0700, Song Liu wrote:
> On Fri, Apr 19, 2024 at 1:00 PM Mike Rapoport <[email-masked]> wrote:
> >
> > On Fri, Apr 19, 2024 at 10:32:39AM -0700, Song Liu wrote:
> > > On Fri, Apr 19, 2024 at 10:03 AM Mike Rapoport <[email-masked]> wrote:
> > > [...]
> > > > > >
> > > > > > [1] https://lore.kernel.org/all/[email-masked]
> > > > >
> > > > > For the ROX to work, we need different users (module text, kprobe, etc.) to have
> > > > > the same execmem_range. From [1]:
> > > > >
> > > > > static void *execmem_cache_alloc(struct execmem_range *range, size_t size)
> > > > > {
> > > > > ...
> > > > >        p = __execmem_cache_alloc(size);
> > > > >        if (p)
> > > > >                return p;
> > > > >       err = execmem_cache_populate(range, size);
> > > > > ...
> > > > > }
> > > > >
> > > > > We are calling __execmem_cache_alloc() without range. For this to work,
> > > > > we can only call execmem_cache_alloc() with one execmem_range.
> > > >
> > > > Actually, on x86 this will "just work" because everything shares the same
> > > > address space :)
> > > >
> > > > The 2M pages in the cache will be in the modules space, so
> > > > __execmem_cache_alloc() will always return memory from that address space.
> > > >
> > > > For other architectures this indeed needs to be fixed with passing the
> > > > range to __execmem_cache_alloc() and limiting search in the cache for that
> > > > range.
> > >
> > > I think we at least need the "map to" concept (initially proposed by Thomas)
> > > to get this work. For example, EXECMEM_BPF and EXECMEM_KPROBE
> > > maps to EXECMEM_MODULE_TEXT, so that all these actually share
> > > the same range.
> >
> > Why?
> 
> IIUC, we need to update __execmem_cache_alloc() to take a range pointer as
> input. module text will use "range" for EXECMEM_MODULE_TEXT, while kprobe
> will use "range" for EXECMEM_KPROBE. Without "map to" concept or sharing
> the "range" object, we will have to compare different range parameters to check
> we can share cached pages between module text and kprobe, which is not
> efficient. Did I miss something?

We can always share large ROX pages as long as they are within the correct
address space. The permissions for them are ROX and the alignment
differences are due to KASAN and this is handled during allocation of the
large page to refill the cache. __execmem_cache_alloc() only needs to limit
the search for the address space of the range.

And regardless, they way we deal with sharing of the cache can be sorted
out later.

> Thanks,
> Song

-- 
Sincerely yours,
Mike.