Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

Vincent Donnefort
2024-05-09T11:05:40Z

On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote:
> On Tue, 30 Apr 2024 12:13:51 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
>
> > +#ifdef CONFIG_MMU
> > +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> > + struct vm_area_struct *vma)
> > +{
> > + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
> > + unsigned int subbuf_pages, subbuf_order;
> > + struct page **pages;
> > + int p = 0, s = 0;
> > + int err;
> > +
> > + /* Refuse MP_PRIVATE or writable mappings */
> > + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> > + !(vma->vm_flags & VM_MAYSHARE))
> > + return -EPERM;
> > +
> > + /*
> > + * Make sure the mapping cannot become writable later. Also tell the VM
> > + * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND). Finally,
> > + * prevent migration, GUP and dump (VM_IO).
> > + */
> > + vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
>
> Do we really need the VM_IO?
>
> When testing this in gdb, I would get:
>
> (gdb) p tmap->map->subbuf_size
> Cannot access memory at address 0x7ffff7fc2008
>
> It appears that you can't ptrace IO memory. When I removed that flag,
> gdb has no problem reading that memory.

Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.
But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are
enough protection?

I don't see how anything could use GUP there and as David pointed-out on the
previous version, it doesn't event prevent the GUP-fast path.

>
> I think we should drop that flag.
>
> Can you send a v23 with that removed, Shuah's update, and also the
> change below:

Ack. …