netdev - [PATCH v2 bpf-next 3/3] bpf: add faked "ending" subprog

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Date:   Wed,  2 May 2018 16:17:19 -0400
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     alexei.starovoitov@...il.com, borkmann@...earbox.net
Cc:     john.fastabend@...il.com, ecree@...arflare.com,
        netdev@...r.kernel.org, oss-drivers@...ronome.com,
        Jiong Wang <jiong.wang@...ronome.com>
Subject: [PATCH v2 bpf-next 3/3] bpf: add faked "ending" subprog

There are quite a few code snippet like the following in verifier:

       subprog_start = 0;
       if (env->subprog_cnt == cur_subprog + 1)
               subprog_end = insn_cnt;
       else
               subprog_end = env->subprog_info[cur_subprog + 1].start;

The reason is there is no marker in subprog_info array to tell the end of
it.

We could resolve this issue by introducing a faked "ending" subprog.
The special "ending" subprog is with "insn_cnt" as start offset, so it is
serving as the end mark whenever we iterate over all subprogs.

Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
---
 kernel/bpf/verifier.c | 34 ++++++++++++++--------------------
 1 file changed, 14 insertions(+), 20 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9764b9b..65a6e2e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -766,7 +766,7 @@ static int add_subprog(struct bpf_verifier_env *env, int off)
 	ret = find_subprog(env, off);
 	if (ret >= 0)
 		return 0;
-	if (env->subprog_cnt > BPF_MAX_SUBPROGS) {
+	if (env->subprog_cnt >= BPF_MAX_SUBPROGS) {
 		verbose(env, "too many subprograms\n");
 		return -E2BIG;
 	}
@@ -807,16 +807,18 @@ static int check_subprogs(struct bpf_verifier_env *env)
 			return ret;
 	}
 
+	/* Add a fake 'exit' subprog which could simplify subprog iteration
+	 * logic. 'subprog_cnt' should not be increased.
+	 */
+	subprog[env->subprog_cnt].start = insn_cnt;
+
 	if (env->log.level > 1)
 		for (i = 0; i < env->subprog_cnt; i++)
 			verbose(env, "func#%d @%d\n", i, subprog[i].start);
 
 	/* now check that all jumps are within the same subprog */
-	subprog_start = 0;
-	if (env->subprog_cnt == cur_subprog + 1)
-		subprog_end = insn_cnt;
-	else
-		subprog_end = subprog[cur_subprog + 1].start;
+	subprog_start = subprog[cur_subprog].start;
+	subprog_end = subprog[cur_subprog + 1].start;
 	for (i = 0; i < insn_cnt; i++) {
 		u8 code = insn[i].code;
 
@@ -840,11 +842,9 @@ static int check_subprogs(struct bpf_verifier_env *env)
 				verbose(env, "last insn is not an exit or jmp\n");
 				return -EINVAL;
 			}
-			cur_subprog++;
 			subprog_start = subprog_end;
-			if (env->subprog_cnt == cur_subprog + 1)
-				subprog_end = insn_cnt;
-			else
+			cur_subprog++;
+			if (cur_subprog < env->subprog_cnt)
 				subprog_end = subprog[cur_subprog + 1].start;
 		}
 	}
@@ -1499,7 +1499,6 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
 	int depth = 0, frame = 0, idx = 0, i = 0, subprog_end;
 	struct bpf_subprog_info *subprog = env->subprog_info;
 	struct bpf_insn *insn = env->prog->insnsi;
-	int insn_cnt = env->prog->len;
 	int ret_insn[MAX_CALL_FRAMES];
 	int ret_prog[MAX_CALL_FRAMES];
 
@@ -1514,10 +1513,7 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
 		return -EACCES;
 	}
 continue_func:
-	if (env->subprog_cnt == idx + 1)
-		subprog_end = insn_cnt;
-	else
-		subprog_end = subprog[idx + 1].start;
+	subprog_end = subprog[idx + 1].start;
 	for (; i < subprog_end; i++) {
 		if (insn[i].code != (BPF_JMP | BPF_CALL))
 			continue;
@@ -5070,7 +5066,8 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len
 
 	if (len == 1)
 		return;
-	for (i = 0; i < env->subprog_cnt; i++) {
+	/* NOTE: fake 'exit' subprog should be updated as well. */
+	for (i = 0; i <= env->subprog_cnt; i++) {
 		if (env->subprog_info[i].start < off)
 			continue;
 		env->subprog_info[i].start += len - 1;
@@ -5268,10 +5265,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 
 	for (i = 0; i < env->subprog_cnt; i++) {
 		subprog_start = subprog_end;
-		if (env->subprog_cnt == i + 1)
-			subprog_end = prog->len;
-		else
-			subprog_end = env->subprog_info[i + 1].start;
+		subprog_end = env->subprog_info[i + 1].start;
 
 		len = subprog_end - subprog_start;
 		func[i] = bpf_prog_alloc(bpf_prog_size(len), GFP_USER);
-- 
2.7.4