Re: [PATCH bpf-next 1/2] bpf: allow wide (u64) aligned stores for some fields of bpf_sock_addr

[Yonghong Song <yhs@fb.com>]

On 6/28/19 4:10 PM, Stanislav Fomichev wrote:
> Since commit cd17d7770578 ("bpf/tools: sync bpf.h") clang decided
> that it can do a single u64 store into user_ip6[2] instead of two
> separate u32 ones:
> 
>   #  17: (18) r2 = 0x100000000000000
>   #  ; ctx->user_ip6[2] = bpf_htonl(DST_REWRITE_IP6_2);
>   #  19: (7b) *(u64 *)(r1 +16) = r2
>   #  invalid bpf_context access off=16 size=8
> 
>  From the compiler point of view it does look like a correct thing
> to do, so let's support it on the kernel side.
> 
> Credit to Andrii Nakryiko for a proper implementation of
> bpf_ctx_wide_store_ok.
> 
> Cc: Andrii Nakryiko <andriin@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Fixes: cd17d7770578 ("bpf/tools: sync bpf.h")
> Reported-by: kernel test robot <rong.a.chen@intel.com>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>

The change looks good to me with the following nits:
   1. could you add a cover letter for the patch set?
      typically if the number of patches is more than one,
      it would be a good practice with a cover letter.
      See bpf_devel_QA.rst .
   2. with this change, the comments in uapi bpf.h
      are not accurate any more.
         __u32 user_ip6[4];      /* Allows 1,2,4-byte read an 4-byte write.
                                  * Stored in network byte order. 

                                  */
         __u32 msg_src_ip6[4];   /* Allows 1,2,4-byte read an 4-byte write.
                                  * Stored in network byte order.
                                  */
      now for stores, aligned 8-byte write is permitted.
      could you update this as well?

 From the typical usage pattern, I did not see a need
for 8-tye read of user_ip6 and msg_src_ip6 yet. So let
us just deal with write for now.

With the above two nits,
Acked-by: Yonghong Song <yhs@fb.com>

> ---
>   include/linux/filter.h |  6 ++++++
>   net/core/filter.c      | 22 ++++++++++++++--------
>   2 files changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 340f7d648974..3901007e36f1 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -746,6 +746,12 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
>   	return size <= size_default && (size & (size - 1)) == 0;
>   }
>   
> +#define bpf_ctx_wide_store_ok(off, size, type, field)			\
> +	(size == sizeof(__u64) &&					\
> +	off >= offsetof(type, field) &&					\
> +	off + sizeof(__u64) <= offsetofend(type, field) &&		\
> +	off % sizeof(__u64) == 0)
> +
>   #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0]))
>   
>   static inline void bpf_prog_lock_ro(struct bpf_prog *fp)
> diff --git a/net/core/filter.c b/net/core/filter.c
> index dc8534be12fc..5d33f2146dab 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6849,6 +6849,16 @@ static bool sock_addr_is_valid_access(int off, int size,
>   			if (!bpf_ctx_narrow_access_ok(off, size, size_default))
>   				return false;
>   		} else {
> +			if (bpf_ctx_wide_store_ok(off, size,
> +						  struct bpf_sock_addr,
> +						  user_ip6))
> +				return true;
> +
> +			if (bpf_ctx_wide_store_ok(off, size,
> +						  struct bpf_sock_addr,
> +						  msg_src_ip6))
> +				return true;
> +
>   			if (size != size_default)
>   				return false;
>   		}
> @@ -7689,9 +7699,6 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>   /* SOCK_ADDR_STORE_NESTED_FIELD_OFF() has semantic similar to
>    * SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF() but for store operation.
>    *
> - * It doesn't support SIZE argument though since narrow stores are not
> - * supported for now.
> - *
>    * In addition it uses Temporary Field TF (member of struct S) as the 3rd
>    * "register" since two registers available in convert_ctx_access are not
>    * enough: we can't override neither SRC, since it contains value to store, nor
> @@ -7699,7 +7706,7 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>    * instructions. But we need a temporary place to save pointer to nested
>    * structure whose field we want to store to.
>    */
> -#define SOCK_ADDR_STORE_NESTED_FIELD_OFF(S, NS, F, NF, OFF, TF)		       \
> +#define SOCK_ADDR_STORE_NESTED_FIELD_OFF(S, NS, F, NF, SIZE, OFF, TF)	       \
>   	do {								       \
>   		int tmp_reg = BPF_REG_9;				       \
>   		if (si->src_reg == tmp_reg || si->dst_reg == tmp_reg)	       \
> @@ -7710,8 +7717,7 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>   				      offsetof(S, TF));			       \
>   		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(S, F), tmp_reg,	       \
>   				      si->dst_reg, offsetof(S, F));	       \
> -		*insn++ = BPF_STX_MEM(					       \
> -			BPF_FIELD_SIZEOF(NS, NF), tmp_reg, si->src_reg,	       \
> +		*insn++ = BPF_STX_MEM(SIZE, tmp_reg, si->src_reg,	       \
>   			bpf_target_off(NS, NF, FIELD_SIZEOF(NS, NF),	       \
>   				       target_size)			       \
>   				+ OFF);					       \
> @@ -7723,8 +7729,8 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>   						      TF)		       \
>   	do {								       \
>   		if (type == BPF_WRITE) {				       \
> -			SOCK_ADDR_STORE_NESTED_FIELD_OFF(S, NS, F, NF, OFF,    \
> -							 TF);		       \
> +			SOCK_ADDR_STORE_NESTED_FIELD_OFF(S, NS, F, NF, SIZE,   \
> +							 OFF, TF);	       \
>   		} else {						       \
>   			SOCK_ADDR_LOAD_NESTED_FIELD_SIZE_OFF(		       \
>   				S, NS, F, NF, SIZE, OFF);  \
>