Messages in this thread | | | From | Christophe Leroy <> | Subject | Re: [PATCH 07/32] mm: Bring back vmalloc_exec | Date | Mon, 15 May 2023 04:45:42 +0000 |
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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
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