Re: [PATCH v7 6/7] mseal, system mappings: uprobe mapping

[Lorenzo Stoakes]

On Wed, Feb 26, 2025 at 07:01:36PM +0100, Oleg Nesterov wrote:
> On 02/26, Lorenzo Stoakes wrote:
> >
> > On Wed, Feb 26, 2025 at 05:26:04PM +0100, Oleg Nesterov wrote:
> > > On 02/24, jeffxu@xxxxxxxxxxxx wrote:
> > > >
> > > > Unlike other system mappings, the uprobe mapping is not
> > > > established during program startup. However, its lifetime is the same
> > > > as the process's lifetime. It could be sealed from creation.
> > >
> > > Agreed, VM_SEALED should be always for the "[uprobes]" vma, regardless
> > > of config options.
> >
> > If you think this ought to be the case generally, then perhaps we should
> > drop this patch from the commit and just do this separately as a
> > permanent-on thing, if you are sure this is fine + want it?
>
> See below...
>
> > An aside - we _definitely_ cannot allow this -system mapping stuff- to be
> > enabled without a config option,
>
> This is clear.
>
> But as for uprobes in particular I do think that VM_SEALED is always fine.
>
> Do we really want it? I dunno. If a task unmaps its "[uprobes]" vma it
> will crash when it hits the uprobes bp next time. Unless the probed insn
> can be emulated and it is not ret-probe. Do we really care? Again, I don't
> know.
>
> Should this change come as a separate patch? I don't understand why it should
> but I am fine either way.
>
> In short. please do what you think is right,  VM_SEALED can't hurt uprobes ;)
>
> > > 	#ifdef CONFIG_64BIT
> > > 	/* VM is sealed, in vm_flags */
> > > 	#define VM_SEALED	_BITUL(63)
> > > +	#else
> > > +	#define VM_SEALED	0
> > > 	#endif
> >
> > This has been raised a few times. Jeff objects to this
>
> OK,
>
> > > and then simply
> > >
> > > 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
> > > -				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO,
> > > +				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO|VM_SEALED,
> > >
> > > ?
> >
> > Nah you'd have to do:
> >
> >
> > > 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
> > 				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO
> > #ifdef CONFIG_64BIT
> > 				VM_SEALED
> > #endif
> > 				,
>
> Why??? With the proposed change above VM_SEALED == 0 if !CONFIG_64BIT.
>

Like I said, Jeff opposes the change. I disagree with him, and agree with you,
because this is very silly.

But I don't want to hold up this series with that discussion (this is for his
sake...)

> Oleg.
>

Jeff - perhaps drop this and let's return to it in a follow up so this series
isn't held up?

Thanks.