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

[Stanislav Fomichev <sdf@fomichev.me>]

On 07/01, Yonghong Song wrote:
> 
> 
> On 7/1/19 9:04 AM, Stanislav Fomichev wrote:
> > On 07/01, Andrii Nakryiko wrote:
> >> On Sat, Jun 29, 2019 at 10:53 PM Yonghong Song <yhs@fb.com> wrote:
> >>>
> >>>
> >>>
> >>> 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.
> >>
> >> But I guess it's still possible for clang to optimize two consecutive
> >> 4-byte reads into single 8-byte read in some circumstances? If that's
> >> the case, maybe it's a good idea to have corresponding read checks as
> >> well?
> > I guess clang can do those kinds of optimizations. I can put it on my
> > todo and address later (or when we actually see it out in the wild).
> 
> Okay, I find a Facebook internal app. does trying to read the 4 bytes
> and compare to a predefined loopback address. We may need to handle
> read cases as well. But this can be a followup after actual tryout.
Sounds good, will follow up on that.

> > 
> >> But overall this looks good to me:
> >>
> >> Acked-by: Andrii Nakryiko <andriin@fb.com>
> > Thanks for a review!
> > 
> >>>
> >>> 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);  \
> >>>>