linux-kernel - Re: [PATCH v3 1/2] bpf: add __weak hook for allocating executable memory

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Message-ID: <a3e95c22-fcae-3550-2d3c-4d07eaad95ae@iogearbox.net>
Date:   Fri, 23 Nov 2018 22:07:11 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-kernel@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jessica Yu <jeyu@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 1/2] bpf: add __weak hook for allocating executable
 memory

On 11/23/2018 10:41 AM, Ard Biesheuvel wrote:
> By default, BPF uses module_alloc() to allocate executable memory,
> but this is not necessary on all arches and potentially undesirable
> on some of them.
> 
> So break out the module_alloc() and module_memfree() calls into __weak
> functions to allow them to be overridden in arch code.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  kernel/bpf/core.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 1a796e0799ec..572dd74c26e3 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -609,6 +609,16 @@ static void bpf_jit_uncharge_modmem(u32 pages)
>  	atomic_long_sub(pages, &bpf_jit_current);
>  }
>  
> +void *__weak bpf_jit_alloc_exec(unsigned long size)
> +{
> +	return module_alloc(size);
> +}
> +
> +void __weak bpf_jit_free_exec(const void *addr)
> +{
> +	module_memfree(addr);
> +}
> +
>  struct bpf_binary_header *
>  bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>  		     unsigned int alignment,
> @@ -626,7 +636,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>  
>  	if (bpf_jit_charge_modmem(pages))
>  		return NULL;
> -	hdr = module_alloc(size);
> +	hdr = bpf_jit_alloc_exec(size);
>  	if (!hdr) {
>  		bpf_jit_uncharge_modmem(pages);
>  		return NULL;
> @@ -650,7 +660,7 @@ void bpf_jit_binary_free(struct bpf_binary_header *hdr)
>  {
>  	u32 pages = hdr->pages;
>  
> -	module_memfree(hdr);
> +	bpf_jit_free_exec(hdr);
>  	bpf_jit_uncharge_modmem(pages);

The const needs to be removed, this generates the following warning:

# make -j8 kernel/bpf/
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  CC      kernel/bpf/core.o
  CC      kernel/bpf/syscall.o
  CC      kernel/bpf/verifier.o
  CC      kernel/bpf/hashtab.o
  CC      kernel/bpf/helpers.o
  CC      kernel/bpf/inode.o
  CC      kernel/bpf/arraymap.o
  CC      kernel/bpf/lpm_trie.o
kernel/bpf/core.c: In function ‘bpf_jit_free_exec’:
kernel/bpf/core.c:619:17: warning: passing argument 1 of ‘module_memfree’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  module_memfree(addr);
                 ^~~~
In file included from kernel/bpf/core.c:28:0:
./include/linux/moduleloader.h:30:6: note: expected ‘void *’ but argument is of type ‘const void *’
 void module_memfree(void *module_region);
      ^~~~~~~~~~~~~~
  CC      kernel/bpf/local_storage.o
[...]

Please respin with that fixed.

Thanks,
Daniel