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

[Christophe Leroy]

Le 15/05/2023 à 01:43, Kent Overstreet a écrit :
> On Sun, May 14, 2023 at 06:39:00PM +0000, Christophe Leroy wrote:
>> 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.
> 
> Because I didn't know about it :)
> 
> Looks like that is indeed the appropriate interface (if a bit poorly
> named), I'll switch to using that, thanks.
> 
> It'll still need to be exported, but it looks like the W|X attribute
> discussion is not really germane here since it's what other in kernel
> users are using, and there's nothing particularly special about how
> bcachefs is using it compared to them.

The W|X subject is applicable.

If you look into powerpc's module_alloc(), you'll see that when 
CONFIG_STRICT_MODULE_RWX is selected, module_alloc() allocate 
PAGE_KERNEL memory. It is then up to the consumer to change it to RO-X.

See for instance in arch/powerpc/kernel/kprobes.c:

void *alloc_insn_page(void)
{
	void *page;

	page = module_alloc(PAGE_SIZE);
	if (!page)
		return NULL;

	if (strict_module_rwx_enabled())
		set_memory_rox((unsigned long)page, 1);

	return page;
}


Christophe