bpf: Add write access to tcp_sock and sock fields

This patch adds a macro, SOCK_OPS_SET_FIELD, for writing to
struct tcp_sock or struct sock fields. This required adding a new
field "temp" to struct bpf_sock_ops_kern for temporary storage that
is used by sock_ops_convert_ctx_access. It is used to store and recover
the contents of a register, so the register can be used to store the
address of the sk. Since we cannot overwrite the dst_reg because it
contains the pointer to ctx, nor the src_reg since it contains the value
we want to store, we need an extra register to contain the address
of the sk.

Also adds the macro SOCK_OPS_GET_OR_SET_FIELD that calls one of the
GET or SET macros depending on the value of the TYPE field.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

3 files changed, 58 insertions, 1 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 425056c7f96c..daa5a676335f 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1007,6 +1007,15 @@ struct bpf_sock_ops_kern {
                 u32 replylong[4];
         };
         u32     is_fullsock;
+        u64     temp;                   /* temp and everything after is not
+                                         * initialized to 0 before calling
+                                         * the BPF program. New fields that
+                                         * should be initialized to 0 should
+                                         * be inserted before temp.
+                                         * temp is scratch storage used by
+                                         * sock_ops_convert_ctx_access
+                                         * as temporary storage of a register.
+                                         */
 };
 
 #endif /* __LINUX_FILTER_H__ */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5a1d26a18599..6092eaff61cf 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -2011,7 +2011,7 @@ static inline int tcp_call_bpf(struct sock *sk, int op)
         struct bpf_sock_ops_kern sock_ops;
         int ret;
 
-        memset(&sock_ops, 0, sizeof(sock_ops));
+        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
         if (sk_fullsock(sk)) {
                 sock_ops.is_fullsock = 1;
                 sock_owned_by_me(sk);
diff --git a/net/core/filter.c b/net/core/filter.c
index dbb6d2f60680..c356ec02b1a5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4491,6 +4491,54 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
                                       offsetof(OBJ, OBJ_FIELD));              \
         } while (0)
 
+/* Helper macro for adding write access to tcp_sock or sock fields.
+ * The macro is called with two registers, dst_reg which contains a pointer
+ * to ctx (context) and src_reg which contains the value that should be
+ * stored. However, we need an additional register since we cannot overwrite
+ * dst_reg because it may be used later in the program.
+ * Instead we "borrow" one of the other register. We first save its value
+ * into a new (temp) field in bpf_sock_ops_kern, use it, and then restore
+ * it at the end of the macro.
+ */
+#define SOCK_OPS_SET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ)                         \
+        do {                                                                  \
+                int reg = BPF_REG_9;                                          \
+                BUILD_BUG_ON(FIELD_SIZEOF(OBJ, OBJ_FIELD) >                   \
+                             FIELD_SIZEOF(struct bpf_sock_ops, BPF_FIELD));   \
+                if (si->dst_reg == reg || si->src_reg == reg)                 \
+                        reg--;                                                \
+                if (si->dst_reg == reg || si->src_reg == reg)                 \
+                        reg--;                                                \
+                *insn++ = BPF_STX_MEM(BPF_DW, si->dst_reg, reg,               \
+                                      offsetof(struct bpf_sock_ops_kern,      \
+                                               temp));                        \
+                *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(                       \
+                                                struct bpf_sock_ops_kern,     \
+                                                is_fullsock),                 \
+                                      reg, si->dst_reg,                       \
+                                      offsetof(struct bpf_sock_ops_kern,      \
+                                               is_fullsock));                 \
+                *insn++ = BPF_JMP_IMM(BPF_JEQ, reg, 0, 2);                    \
+                *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(                       \
+                                                struct bpf_sock_ops_kern, sk),\
+                                      reg, si->dst_reg,                       \
+                                      offsetof(struct bpf_sock_ops_kern, sk));\
+                *insn++ = BPF_STX_MEM(BPF_FIELD_SIZEOF(OBJ, OBJ_FIELD),       \
+                                      reg, si->src_reg,                       \
+                                      offsetof(OBJ, OBJ_FIELD));              \
+                *insn++ = BPF_LDX_MEM(BPF_DW, reg, si->dst_reg,               \
+                                      offsetof(struct bpf_sock_ops_kern,      \
+                                               temp));                        \
+        } while (0)
+
+#define SOCK_OPS_GET_OR_SET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ, TYPE)            \
+        do {                                                                  \
+                if (TYPE == BPF_WRITE)                                        \
+                        SOCK_OPS_SET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ);        \
+                else                                                          \
+                        SOCK_OPS_GET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ);        \
+        } while (0)
+
         case offsetof(struct bpf_sock_ops, snd_cwnd):
                 SOCK_OPS_GET_FIELD(snd_cwnd, snd_cwnd, struct tcp_sock);
                 break;