linux-kernel - Re: [PATCH 07/32] mm: Bring back vmalloc

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Message-ID: <ZGPzocRpSlg+4vgN@moria.home.lan>
Date:   Tue, 16 May 2023 17:20:33 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Kees Cook <keescook@...omium.org>
Cc:     Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-bcachefs@...r.kernel.org" <linux-bcachefs@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

On Tue, May 16, 2023 at 02:02:11PM -0700, Kees Cook wrote:
> For something that small, why not use the text_poke API?

This looks like it's meant for patching existing kernel text, which
isn't what I want - I'm generating new functions on the fly, one per
btree node.

I'm working up a new allocator - a (very simple) slab allocator where
you pass a buffer, and it gives you a copy of that buffer mapped
executable, but not writeable.

It looks like we'll be able to convert bpf, kprobes, and ftrace
trampolines to it; it'll consolidate a fair amount of code (particularly
in bpf), and they won't have to burn a full page per allocation anymore.

bpf has a neat trick where it maps the same page in two different
locations, one is the executable location and the other is the writeable
location - I'm stealing that.

external api will be:

void *jit_alloc(void *buf, size_t len, gfp_t gfp);
void jit_free(void *buf);
void jit_update(void *buf, void *new_code, size_t len); /* update an existing allocation */