linux-kernel - Re: [PATCH v3 bpf-next 5/8] bpf: use module_alloc_huge for bpf_prog

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Date:   Fri, 20 May 2022 18:00:57 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Song Liu <song@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-mm@...ck.org, ast@...nel.org, daniel@...earbox.net,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 5/8] bpf: use module_alloc_huge for
 bpf_prog_pack

On Thu, May 19, 2022 at 08:15:45PM -0700, Song Liu wrote:
> Also, remove set_vm_flush_reset_perms() from alloc_new_pack() and use
> set_memory_[nx|rw] in bpf_prog_pack_free(). This is because
> VM_FLUSH_RESET_PERMS does not work with huge pages yet. [1]
> 
> [1] https://lore.kernel.org/bpf/aeeeaf0b7ec63fdba55d4834d2f524d8bf05b71b.camel@intel.com/
> Suggested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Song Liu <song@...nel.org>
> ---

Rick,

although VM_FLUSH_RESET_PERMS is rather new my concern here is we're
essentially enabling sloppy users to grow without also addressing
what if we have to take the leash back to support VM_FLUSH_RESET_PERMS
properly? If the hack to support this on other architectures other than
x86 is as simple as the one you in vm_remove_mappings() today:

	if (flush_reset && !IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) {
		set_memory_nx(addr, area->nr_pages);
		set_memory_rw(addr, area->nr_pages);
	}

then I suppose this isn't a big deal. I'm just concerned here this being
a slippery slope of sloppiness leading to something which we will
regret later.

My intution tells me this shouldn't be a big issue, but I just want to
confirm.

  Luis

> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index cacd8684c3c4..b64d91fcb0ba 100644
> @@ -949,6 +947,8 @@ static void bpf_prog_pack_free(struct bpf_binary_header *hdr)
>  
>  	mutex_lock(&pack_mutex);
>  	if (hdr->size > bpf_prog_pack_size) {
> +		set_memory_nx((unsigned long)hdr, hdr->size / PAGE_SIZE);
> +		set_memory_rw((unsigned long)hdr, hdr->size / PAGE_SIZE);
>  		module_memfree(hdr);
>  		goto out;
>  	}
> @@ -975,6 +975,8 @@ static void bpf_prog_pack_free(struct bpf_binary_header *hdr)
>  	if (bitmap_find_next_zero_area(pack->bitmap, bpf_prog_chunk_count(), 0,
>  				       bpf_prog_chunk_count(), 0) == 0) {
>  		list_del(&pack->list);
> +		set_memory_nx((unsigned long)pack->ptr, bpf_prog_pack_size / PAGE_SIZE);
> +		set_memory_rw((unsigned long)pack->ptr, bpf_prog_pack_size / PAGE_SIZE);
>  		module_memfree(pack->ptr);
>  		kfree(pack);
>  	}
> -- 
> 2.30.2
>