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Date: Tue, 20 Feb 2024 21:43:06 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Maxwell Bland <mbland@motorola.com>
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Subject: Re: [PATCH 1/4] mm/vmalloc: allow arch-specific vmalloc_node
 overrides
Message-ID: <ZdWNalbmABYDuFHE@infradead.org>
References: <20240220203256.31153-1-mbland@motorola.com>
 <20240220203256.31153-2-mbland@motorola.com>
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In-Reply-To: <20240220203256.31153-2-mbland@motorola.com>

On Tue, Feb 20, 2024 at 02:32:53PM -0600, Maxwell Bland wrote:
> Present non-uniform use of __vmalloc_node and __vmalloc_node_range makes
> enforcing appropriate code and data seperation untenable on certain
> microarchitectures, as VMALLOC_START and VMALLOC_END are monolithic
> while the use of the vmalloc interface is non-monolithic: in particular,
> appropriate randomness in ASLR makes it such that code regions must fall
> in some region between VMALLOC_START and VMALLOC_end, but this
> necessitates that code pages are intermingled with data pages, meaning
> code-specific protections, such as arm64's PXNTable, cannot be
> performantly runtime enforced.

That's not actually true.  We have MODULE_START/END to separate them,
which is used by mips only for now.

> 
> The solution to this problem allows architectures to override the
> vmalloc wrapper functions by enforcing that the rest of the kernel does
> not reimplement __vmalloc_node by using __vmalloc_node_range with the
> same parameters as __vmalloc_node or provides a __weak tag to those
> functions using __vmalloc_node_range with parameters repeating those of
> __vmalloc_node.

I'm really not too happy about overriding the functions.  Especially
as the separation is a generally good idea and it would be good to
move everyone (or at least all modern architectures) over to a scheme
like this.