Re: [PATCH 1/4] mm/vmalloc: allow arch-specific vmalloc_node overrides

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Christoph Hellwig <hch@infradead.org>,
	Maxwell Bland <mbland@motorola.com>
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Subject: Re: [PATCH 1/4] mm/vmalloc: allow arch-specific vmalloc_node overrides
Date: Wed, 21 Feb 2024 07:38:32 +0000	[thread overview]
Message-ID: <e508c3e0-8644-40f1-aee2-90625237b01c@csgroup.eu> (raw)
In-Reply-To: <ZdWNalbmABYDuFHE@infradead.org>

Le 21/02/2024 à 06:43, Christoph Hellwig a écrit :
> 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.

We have MODULES_VADDR and MODULES_END that are used by arm, arm64, 
loongarcg, powerpc, riscv, s390, sparc, x86_64

is_vmalloc_or_module_addr() is using MODULES_VADDR so I guess this 
function fails on mips ?

> 
>>
>> 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.