Re: [PATCH bpf-next v6 03/11] bpf: Add write access to tcp_sock and sock fields

[Alexei Starovoitov]

On Fri, Jan 19, 2018 at 05:45:40PM -0800, Lawrence Brakmo wrote:
> 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 

Acked-by: Alexei Starovoitov 

that is really clever way of doing inline writes into fields.
I suspect we will be using this approach in other places.

[PATCH bpf-next v6 03/11] bpf: Add write access to tcp_sock and sock fields

[Lawrence Brakmo]

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 
---
 include/linux/filter.h |  9 +
 include/net/tcp.h  |  2 +-
 net/core/filter.c  | 48 
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 425056c..daa5a67 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 6939e69..108d16a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -2010,7 +2010,7 @@ static inline int tcp_call_bpf(struct sock *sk, int op)
struct bpf_sock_ops_kern sock_ops;
int ret;
 
-   memset(_ops, 0, sizeof(sock_ops));
+   memset(_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 292bda8..1ff36ca 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4486,6 +4486,54 @@ 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,