Re: [PATCH] kprobes: Enable tracing for mololithic kernel images

Luis Chamberlain
2022-06-08T20:26:39Z

On Wed, Jun 08, 2022 at 11:20:53AM -0700, Song Liu wrote:
> On Wed, Jun 8, 2022 at 9:12 AM Song Liu <song@kernel.org> wrote:
> > On Wed, Jun 8, 2022 at 7:21 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > On Wed, 8 Jun 2022 08:25:38 +0300
> > > Jarkko Sakkinen <jarkko@kernel.org> wrote:
> > > > On Wed, Jun 08, 2022 at 10:35:42AM +0800, Guo Ren wrote:
> > > > > On Wed, Jun 8, 2022 at 8:02 AM Jarkko Sakkinen <jarkko@profian.com> wrote:
> > > > > > As the result, kprobes can be used with a monolithic kernel.
> > > > > It's strange when MODULES is n, but vmlinux still obtains module_alloc.
> > > > >
> > > > > Maybe we need a kprobe_alloc, right?
> > > >
> > > > Perhaps not the best name but at least it documents the fact that
> > > > they use the same allocator.
> > > >
> > > > Few years ago I carved up something "half-way there" for kprobes,
> > > > and I used the name text_alloc() [*].
> > > >
> > > > [*] https://lore.kernel.org/all/20200724050553.1724168-1-jarkko.sakkinen@linux.intel.com/
> > >
> > > Yeah, I remember that. Thank you for updating your patch!
> > > I think the idea (split module_alloc() from CONFIG_MODULE) is good to me.
> > > If module support maintainers think this name is not good, you may be
> > > able to rename it as text_alloc() and make the module_alloc() as a
> > > wrapper of it.
> >
> > IIUC, most users of module_alloc() use it to allocate memory for text, except
> > that module code uses it for both text and data. Therefore, I guess calling it
> > text_alloc() is not 100% accurate until we change the module code (to use
> > a different API to allocate memory for data).
>
> Git history showed me
>
> 7a0e27b2a0ce mm: remove vmalloc_exec
>
> I guess we are somehow going back in time...

No, that was removed because it has only one user. The real hard work
to generalize vmalloc_exec() with all the arch special sauce was not
done.

To do this properly architectures must be able to override it. We can
use the old vmalloc_exec() or text_alloc(). I think vmalloc_exec() is
more in line with mm stuff, but it would be our first __weak mm call
from what I can tell.

Anyway patches welcomed.

Luis