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

Christophe Leroy
2023-05-15T04:45:54Z

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