Re: [PATCH 07/32] mm: Bring back vmalloc_exec

From: Christophe Leroy
Date: Sun May 14 2023 - 14:39:11 EST

Le 13/05/2023 à 15:25, Lorenzo Stoakes a écrit :
> On Tue, May 09, 2023 at 02:12:41PM -0700, Lorenzo Stoakes wrote:
>> On Tue, May 09, 2023 at 01:46:09PM -0700, Christoph Hellwig wrote:
>>> On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote:
>>>> From: Kent Overstreet <kent.overstreet@xxxxxxxxx>
>>>> This is needed for bcachefs, which dynamically generates per-btree node
>>>> unpack functions.
>>> No, we will never add back a way for random code allocating executable
>>> memory in kernel space.
>> Yeah I think I glossed over this aspect a bit as it looks ostensibly like simply
>> reinstating a helper function because the code is now used in more than one
>> place (at lsf/mm so a little distracted :)
>> But it being exported is a problem. Perhaps there's another way of acheving the
>> same aim without having to do so?
> Just to be abundantly clear, my original ack was a mistake (I overlooked
> the _exporting_ of the function being as significant as it is and assumed
> in an LSF/MM haze that it was simply a refactoring of _already available_
> functionality rather than newly providing a means to allocate directly
> executable kernel memory).
> Exporting this is horrible for the numerous reasons expounded on in this
> thread, we need a different solution.
> Nacked-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>

I addition to that, I still don't understand why you bring back
vmalloc_exec() instead of using module_alloc().

As reminded in a previous response, some architectures like powerpc/32s
cannot allocate exec memory in vmalloc space. On powerpc this is because
exec protection is performed on 256Mbytes segments and vmalloc space is
flagged non-exec. Some other architectures have a constraint on distance
between kernel core text and other text.

Today you have for instance kprobes in the kernel that need dynamic exec
memory. It uses module_alloc() to get it. On some architectures you also
have ftrace that gets some exec memory with module_alloc().

So, I still don't understand why you cannot use module_alloc() and need
vmalloc_exec() instead.