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

[Kent Overstreet <kent.overstreet@linux.dev>]

On Mon, May 15, 2023 at 04:45:42AM +0000, Christophe Leroy wrote:
> 
> 
> 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;
> }

Yeah.

I'm looking at the bpf code now.

<RANT MODE, YOU ARE WARNED>

Can I just say, for the record - god damn this situation is starting to
piss me off? This really nicely encapsulates everything I hate about
kernel development processes and culture and the fscking messes that get
foisted upon people as a result.

All I'm trying to do is write a fucking filesystem here people, I've got
enough on my plate. Dealing with the fallout of a kernel interface going
away without a proper replacement was NOT WHAT I FUCKING HAD IN MIND?

5% performance regression without this. That's just not acceptable, I
can't produce a filesystem that people will in the end want to use by
leaving performance on the table, it's death of a thousand cuts if I
take that attitude. Every 1% needs to be accounted for, a 5% performance
regression is flat out not going to happen.

And the real icing on this motherfucking turd sandwich of a cake, is
that I'm not the first person to have to solve this particular technical
problem.

BPF has the code I need.

But, in true kernel fashion, did they recognize that this was a
subproblem they could write as a library, both making their code more
modular and easier to understand, as well as, oh I don't know, not
leaving a giant steaming turd for the next person to come along?

Nope.

I'd be embarassed if I was responsible for this.