On 9/6/22 4:42 PM, Dave Marchevsky wrote:
On 9/1/22 5:01 PM, Joanne Koong wrote:
On Tue, Aug 30, 2022 at 11:03 AM Dave Marchevsky <davemarchevsky@xxxxxx> wrote:
Verifier logic to confirm that a callback function returns 0 or 1 was
added in commit 69c087ba6225b ("bpf: Add bpf_for_each_map_elem() helper").
At the time, callback return value was only used to continue or stop
iteration.
In order to support callbacks with a broader return value range, such as
those added further in this series, add a callback_ret_range to
bpf_func_state. Verifier's helpers which set in_callback_fn will also
set the new field, which the verifier will later use to check return
value bounds.
Default to tnum_range(0, 1) instead of using tnum_unknown as a sentinel
value as the latter would prevent the valid range (0, U64_MAX) being
used.
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
include/linux/bpf_verifier.h | 1 +
kernel/bpf/verifier.c | 4 +++-
2 files changed, 4 insertions(+), 1 deletion(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 2e3bad8640dc..9c017575c034 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -237,6 +237,7 @@ struct bpf_func_state {
*/
u32 async_entry_cnt;
bool in_callback_fn;
+ struct tnum callback_ret_range;
bool in_async_callback_fn;
/* The following fields should be last. See copy_func_state() */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9bef8b41e737..68bfa7c28048 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1745,6 +1745,7 @@ static void init_func_state(struct bpf_verifier_env *env,
state->callsite = callsite;
state->frameno = frameno;
state->subprogno = subprogno;
+ state->callback_ret_range = tnum_range(0, 1);
init_reg_state(env, state);
mark_verifier_state_scratched(env);
}
@@ -6879,6 +6880,7 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
callee->in_callback_fn = true;
+ callee->callback_ret_range = tnum_range(0, 1);
Thanks for removing this restriction for callback functions!
One quick question: is this line above needed? I think in
__check_func_call, we always call init_func_state() first before
calling set_find_vma_callback_state(), so after the init_func_state()
call, the callee->callback_ret_range will already be set to
tnum_range(0,1).
You're right, it's not strictly necessary. I think that the default range being
tnum_range(0, 1) - although necessary for backwards compat - is unintuitive. So
decided to be explicit with existing callbacks so folks didn't have to go
searching for the default to understand what the ret_range is, and it's more
obvious that callback_ret_range should be changed if existing helper code is
reused.
Maybe then it's better to keep callback_ret_range as range(0,0)
in init_func_state() to nudge/force other places to set it explicitly ?
