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From: Song Liu <song@kernel.org>
Date: Fri, 19 Apr 2024 14:42:16 -0700
Message-ID: <CAPhsuW4KRM4O4RFbYQrt=Coqyh9w29WiF2YF=8soDfauLFsKBA@mail.gmail.com>
Subject: Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()
To: Mike Rapoport <rppt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>, Peter Zijlstra <peterz@infradead.org>, 
	linux-kernel@vger.kernel.org, Alexandre Ghiti <alexghiti@rivosinc.com>, 
	Andrew Morton <akpm@linux-foundation.org>, Bjorn Topel <bjorn@kernel.org>, 
	Catalin Marinas <catalin.marinas@arm.com>, Christophe Leroy <christophe.leroy@csgroup.eu>, 
	"David S. Miller" <davem@davemloft.net>, Dinh Nguyen <dinguyen@kernel.org>, 
	Donald Dutile <ddutile@redhat.com>, Eric Chanudet <echanude@redhat.com>, 
	Heiko Carstens <hca@linux.ibm.com>, Helge Deller <deller@gmx.de>, Huacai Chen <chenhuacai@kernel.org>, 
	Kent Overstreet <kent.overstreet@linux.dev>, Luis Chamberlain <mcgrof@kernel.org>, 
	Michael Ellerman <mpe@ellerman.id.au>, Nadav Amit <nadav.amit@gmail.com>, 
	Palmer Dabbelt <palmer@dabbelt.com>, Puranjay Mohan <puranjay12@gmail.com>, 
	Rick Edgecombe <rick.p.edgecombe@intel.com>, Russell King <linux@armlinux.org.uk>, 
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On Fri, Apr 19, 2024 at 1:00=E2=80=AFPM Mike Rapoport <rppt@kernel.org> wro=
te:
>
> On Fri, Apr 19, 2024 at 10:32:39AM -0700, Song Liu wrote:
> > On Fri, Apr 19, 2024 at 10:03=E2=80=AFAM Mike Rapoport <rppt@kernel.org=
> wrote:
> > [...]
> > > > >
> > > > > [1] https://lore.kernel.org/all/20240411160526.2093408-1-rppt@ker=
nel.org
> > > >
> > > > 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 =3D __execmem_cache_alloc(size);
> > > >        if (p)
> > > >                return p;
> > > >       err =3D execmem_cache_populate(range, size);
> > > > ...
> > > > }
> > > >
> > > > We are calling __execmem_cache_alloc() without range. For this to w=
ork,
> > > > 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 s=
pace.
> > >
> > > For other architectures this indeed needs to be fixed with passing th=
e
> > > 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 Th=
omas)
> > 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 c=
heck
we can share cached pages between module text and kprobe, which is not
efficient. Did I miss something?

Thanks,
Song