BPF limits stack usage by MAX_BPF_STACK bytes across all call frames,
however this is enforced by function check_max_stack_depth() which is
executed after do_check_{subprogs,main}().
This means that when check_ids() is executed the maximal stack depth is not
yet verified, thus in theory the number of stack spills might be
MAX_CALL_FRAMES * MAX_BPF_STACK / BPF_REG_SIZE.
However, any program with stack usage deeper than
MAX_BPF_STACK / BPF_REG_SIZE would be rejected by verifier.
Hence save some memory by reducing the BPF_ID_MAP_SIZE.
This is a follow up for
https://lore.kernel.org/bpf/CAEf4BzYN1JmY9t03pnCHc4actob80wkBz2vk90ihJCBzi8CT9w@xxxxxxxxxxxxxx/
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
include/linux/bpf_verifier.h | 4 ++--
kernel/bpf/verifier.c | 6 ++++--
2 files changed, 6 insertions(+), 4 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 53d175cbaa02..da72e16f1dee 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -274,8 +274,8 @@ struct bpf_id_pair {
};
#define MAX_CALL_FRAMES 8
-/* Maximum number of register states that can exist at once */
-#define BPF_ID_MAP_SIZE ((MAX_BPF_REG + MAX_BPF_STACK / BPF_REG_SIZE) * MAX_CALL_FRAMES)
+/* Maximum number of register states that can exist at once in a valid program */
+#define BPF_ID_MAP_SIZE (MAX_BPF_REG * MAX_CALL_FRAMES + MAX_BPF_STACK / BPF_REG_SIZE)
struct bpf_verifier_state {
/* call stack tracking */
struct bpf_func_state *frame[MAX_CALL_FRAMES];
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a5255a0dcbb6..fb040516a946 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12951,8 +12951,10 @@ static bool check_ids(u32 old_id, u32 cur_id, struct bpf_id_pair *idmap)
if (idmap[i].old == old_id)
return idmap[i].cur == cur_id;
}
- /* We ran out of idmap slots, which should be impossible */
- WARN_ON_ONCE(1);
+ /* Run out of slots in idmap, conservatively return false, cached
+ * state will not be reused. The BPF_ID_MAP_SIZE is sufficiently
+ * large to fit all valid programs.
+ */
return false;
}
--
2.38.2
