sk-filter: Add ability to get socket filter program (v2)

The SO_ATTACH_FILTER option is set only. I propose to add the get
ability by using SO_ATTACH_FILTER in getsockopt. To be less
irritating to eyes the SO_GET_FILTER alias to it is declared. This
ability is required by checkpoint-restore project to be able to
save full state of a socket.

There are two issues with getting filter back.

First, kernel modifies the sock_filter->code on filter load, thus in
order to return the filter element back to user we have to decode it
into user-visible constants. Fortunately the modification in question
is interconvertible.

Second, the BPF_S_ALU_DIV_K code modifies the command argument k to
speed up the run-time division by doing kernel_k = reciprocal(user_k).
Bad news is that different user_k may result in same kernel_k, so we
can't get the original user_k back. Good news is that we don't have
to do it. What we need to is calculate a user2_k so, that

  reciprocal(user2_k) == reciprocal(user_k) == kernel_k

i.e. if it's re-loaded back the compiled again value will be exactly
the same as it was. That said, the user2_k can be calculated like this

  user2_k = reciprocal(kernel_k)

with an exception, that if kernel_k == 0, then user2_k == 1.

The optlen argument is treated like this -- when zero, kernel returns
the amount of sock_fprog elements in filter, otherwise it should be
large enough for the sock_fprog array.

changes since v1:
* Declared SO_GET_FILTER in all arch headers
* Added decode of vlan-tag codes

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 136 insertions, 0 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 5a114d41bf11..c23543cba132 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -760,3 +760,133 @@ int sk_detach_filter(struct sock *sk)
         return ret;
 }
 EXPORT_SYMBOL_GPL(sk_detach_filter);
+
+static void sk_decode_filter(struct sock_filter *filt, struct sock_filter *to)
+{
+        static const u16 decodes[] = {
+                [BPF_S_ALU_ADD_K]       = BPF_ALU|BPF_ADD|BPF_K,
+                [BPF_S_ALU_ADD_X]       = BPF_ALU|BPF_ADD|BPF_X,
+                [BPF_S_ALU_SUB_K]       = BPF_ALU|BPF_SUB|BPF_K,
+                [BPF_S_ALU_SUB_X]       = BPF_ALU|BPF_SUB|BPF_X,
+                [BPF_S_ALU_MUL_K]       = BPF_ALU|BPF_MUL|BPF_K,
+                [BPF_S_ALU_MUL_X]       = BPF_ALU|BPF_MUL|BPF_X,
+                [BPF_S_ALU_DIV_X]       = BPF_ALU|BPF_DIV|BPF_X,
+                [BPF_S_ALU_MOD_K]       = BPF_ALU|BPF_MOD|BPF_K,
+                [BPF_S_ALU_MOD_X]       = BPF_ALU|BPF_MOD|BPF_X,
+                [BPF_S_ALU_AND_K]       = BPF_ALU|BPF_AND|BPF_K,
+                [BPF_S_ALU_AND_X]       = BPF_ALU|BPF_AND|BPF_X,
+                [BPF_S_ALU_OR_K]        = BPF_ALU|BPF_OR|BPF_K,
+                [BPF_S_ALU_OR_X]        = BPF_ALU|BPF_OR|BPF_X,
+                [BPF_S_ALU_XOR_K]       = BPF_ALU|BPF_XOR|BPF_K,
+                [BPF_S_ALU_XOR_X]       = BPF_ALU|BPF_XOR|BPF_X,
+                [BPF_S_ALU_LSH_K]       = BPF_ALU|BPF_LSH|BPF_K,
+                [BPF_S_ALU_LSH_X]       = BPF_ALU|BPF_LSH|BPF_X,
+                [BPF_S_ALU_RSH_K]       = BPF_ALU|BPF_RSH|BPF_K,
+                [BPF_S_ALU_RSH_X]       = BPF_ALU|BPF_RSH|BPF_X,
+                [BPF_S_ALU_NEG]         = BPF_ALU|BPF_NEG,
+                [BPF_S_LD_W_ABS]        = BPF_LD|BPF_W|BPF_ABS,
+                [BPF_S_LD_H_ABS]        = BPF_LD|BPF_H|BPF_ABS,
+                [BPF_S_LD_B_ABS]        = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_PROTOCOL]    = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_PKTTYPE]     = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_IFINDEX]     = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_NLATTR]      = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_NLATTR_NEST] = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_MARK]        = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_QUEUE]       = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_HATYPE]      = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_RXHASH]      = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_CPU]         = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_ALU_XOR_X]   = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_SECCOMP_LD_W] = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_VLAN_TAG]    = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_ANC_VLAN_TAG_PRESENT] = BPF_LD|BPF_B|BPF_ABS,
+                [BPF_S_LD_W_LEN]        = BPF_LD|BPF_W|BPF_LEN,
+                [BPF_S_LD_W_IND]        = BPF_LD|BPF_W|BPF_IND,
+                [BPF_S_LD_H_IND]        = BPF_LD|BPF_H|BPF_IND,
+                [BPF_S_LD_B_IND]        = BPF_LD|BPF_B|BPF_IND,
+                [BPF_S_LD_IMM]          = BPF_LD|BPF_IMM,
+                [BPF_S_LDX_W_LEN]       = BPF_LDX|BPF_W|BPF_LEN,
+                [BPF_S_LDX_B_MSH]       = BPF_LDX|BPF_B|BPF_MSH,
+                [BPF_S_LDX_IMM]         = BPF_LDX|BPF_IMM,
+                [BPF_S_MISC_TAX]        = BPF_MISC|BPF_TAX,
+                [BPF_S_MISC_TXA]        = BPF_MISC|BPF_TXA,
+                [BPF_S_RET_K]           = BPF_RET|BPF_K,
+                [BPF_S_RET_A]           = BPF_RET|BPF_A,
+                [BPF_S_ALU_DIV_K]       = BPF_ALU|BPF_DIV|BPF_K,
+                [BPF_S_LD_MEM]          = BPF_LD|BPF_MEM,
+                [BPF_S_LDX_MEM]         = BPF_LDX|BPF_MEM,
+                [BPF_S_ST]              = BPF_ST,
+                [BPF_S_STX]             = BPF_STX,
+                [BPF_S_JMP_JA]          = BPF_JMP|BPF_JA,
+                [BPF_S_JMP_JEQ_K]       = BPF_JMP|BPF_JEQ|BPF_K,
+                [BPF_S_JMP_JEQ_X]       = BPF_JMP|BPF_JEQ|BPF_X,
+                [BPF_S_JMP_JGE_K]       = BPF_JMP|BPF_JGE|BPF_K,
+                [BPF_S_JMP_JGE_X]       = BPF_JMP|BPF_JGE|BPF_X,
+                [BPF_S_JMP_JGT_K]       = BPF_JMP|BPF_JGT|BPF_K,
+                [BPF_S_JMP_JGT_X]       = BPF_JMP|BPF_JGT|BPF_X,
+                [BPF_S_JMP_JSET_K]      = BPF_JMP|BPF_JSET|BPF_K,
+                [BPF_S_JMP_JSET_X]      = BPF_JMP|BPF_JSET|BPF_X,
+        };
+        u16 code;
+
+        code = filt->code;
+
+        to->code = decodes[code];
+        to->jt = filt->jt;
+        to->jf = filt->jf;
+
+        if (code == BPF_S_ALU_DIV_K) {
+                /*
+                 * When loaded this rule user gave us X, which was
+                 * translated into R = r(X). Now we calculate the
+                 * RR = r(R) and report it back. If next time this
+                 * value is loaded and RRR = r(RR) is calculated
+                 * then the R == RRR will be true.
+                 *
+                 * One exception. X == 1 translates into R == 0 and
+                 * we can't calculate RR out of it with r().
+                 */
+
+                if (filt->k == 0)
+                        to->k = 1;
+                else
+                        to->k = reciprocal_value(filt->k);
+
+                BUG_ON(reciprocal_value(to->k) != filt->k);
+        } else
+                to->k = filt->k;
+}
+
+int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf, unsigned int len)
+{
+        struct sk_filter *filter;
+        int i, ret;
+
+        lock_sock(sk);
+        filter = rcu_dereference_protected(sk->sk_filter,
+                        sock_owned_by_user(sk));
+        ret = 0;
+        if (!filter)
+                goto out;
+        ret = filter->len;
+        if (!len)
+                goto out;
+        ret = -EINVAL;
+        if (len < filter->len)
+                goto out;
+
+        ret = -EFAULT;
+        for (i = 0; i < filter->len; i++) {
+                struct sock_filter fb;
+
+                sk_decode_filter(&filter->insns[i], &fb);
+                if (copy_to_user(&ubuf[i], &fb, sizeof(fb)))
+                        goto out;
+        }
+
+        ret = filter->len;
+out:
+        release_sock(sk);
+        return ret;
+}
diff --git a/net/core/sock.c b/net/core/sock.c
index 0a023b8daa55..06286006a2cc 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1077,6 +1077,12 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
         case SO_BINDTODEVICE:
                 v.val = sk->sk_bound_dev_if;
                 break;
+        case SO_GET_FILTER:
+                len = sk_get_filter(sk, (struct sock_filter __user *)optval, len);
+                if (len < 0)
+                        return len;
+
+                goto lenout;
         default:
                 return -ENOPROTOOPT;
         }