[PATCH bpf-next v3 07/17] bpf: Perform CFG walk for exception callback

Kumar Kartikeya Dwivedi
2023-09-12T23:32:23Z

Since exception callbacks are not referenced using bpf_pseudo_func and
bpf_pseudo_call instructions, check_cfg traversal will never explore
instructions of the exception callback. Even after adding the subprog,
the program will then fail with a 'unreachable insn' error.

We thus need to begin walking from the start of the exception callback
again in check_cfg after a complete CFG traversal finishes, so as to
explore the CFG rooted at the exception callback.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
kernel/bpf/verifier.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ec3f22312516..863e4e6c4616 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15126,8 +15126,8 @@ static int check_cfg(struct bpf_verifier_env *env)
{
int insn_cnt = env->prog->len;
int *insn_stack, *insn_state;
- int ret = 0;
- int i;
+ int ex_insn_beg, i, ret = 0;
+ bool ex_done = false;

insn_state = env->cfg.insn_state = kvcalloc(insn_cnt, sizeof(int), GFP_KERNEL);
if (!insn_state)
@@ -15143,6 +15143,7 @@ static int check_cfg(struct bpf_verifier_env *env)
insn_stack[0] = 0; /* 0 is the first instruction */
env->cfg.cur_stack = 1;

+walk_cfg:
while (env->cfg.cur_stack > 0) {
int t = insn_stack[env->cfg.cur_stack - 1];

@@ -15169,6 +15170,16 @@ static int check_cfg(struct bpf_verifier_env *env)
goto err_free;
}

+ if (env->exception_callback_subprog && !ex_done) {
+ ex_insn_beg = env->subprog_info[env->exception_callback_subprog].start;
+
+ insn_state[ex_insn_beg] = DISCOVERED;
+ insn_stack[0] = ex_insn_beg;
+ env->cfg.cur_stack = 1;
+ ex_done = true;
+ goto walk_cfg;
+ }
+
for (i = 0; i < insn_cnt; i++) {
if (insn_state[i] != EXPLORED) {
verbose(env, "unreachable insn %d\n", i);
--
2.41.0