bpf: Introduce function-by-function verification

New llvm and old llvm with libbpf help produce BTF that distinguish global and
static functions. Unlike arguments of static function the arguments of global
functions cannot be removed or optimized away by llvm. The compiler has to use
exactly the arguments specified in a function prototype. The argument type
information allows the verifier validate each global function independently.
For now only supported argument types are pointer to context and scalars. In
the future pointers to structures, sizes, pointer to packet data can be
supported as well. Consider the following example:

static int f1(int ...)
{
  ...
}

int f3(int b);

int f2(int a)
{
  f1(a) + f3(a);
}

int f3(int b)
{
  ...
}

int main(...)
{
  f1(...) + f2(...) + f3(...);
}

The verifier will start its safety checks from the first global function f2().
It will recursively descend into f1() because it's static. Then it will check
that arguments match for the f3() invocation inside f2(). It will not descend
into f3(). It will finish f2() that has to be successfully verified for all
possible values of 'a'. Then it will proceed with f3(). That function also has
to be safe for all possible values of 'b'. Then it will start subprog 0 (which
is main() function). It will recursively descend into f1() and will skip full
check of f2() and f3(), since they are global. The order of processing global
functions doesn't affect safety, since all global functions must be proven safe
based on their arguments only.

Such function by function verification can drastically improve speed of the
verification and reduce complexity.

Note that the stack limit of 512 still applies to the call chain regardless whether
functions were static or global. The nested level of 8 also still applies. The
same recursion prevention checks are in place as well.

The type information and static/global kind is preserved after the verification
hence in the above example global function f2() and f3() can be replaced later
by equivalent functions with the same types that are loaded and verified later
without affecting safety of this main() program. Such replacement (re-linking)
of global functions is a subject of future patches.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200110064124.1760511-3-ast@kernel.org

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 26e40de9ef55..5406e6e96585 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -304,11 +304,13 @@ struct bpf_insn_aux_data {
 	u64 map_key_state; /* constant (32 bit) key tracking for maps */
 	int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
 	int sanitize_stack_off; /* stack slot to be cleared */
-	bool seen; /* this insn was processed by the verifier */
+	u32 seen; /* this insn was processed by the verifier at env->pass_cnt */
 	bool zext_dst; /* this insn zero extends dst reg */
 	u8 alu_state; /* used in combination with alu_limit */
-	bool prune_point;
+
+	/* below fields are initialized once */
 	unsigned int orig_idx; /* original instruction index */
+	bool prune_point;
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
@@ -379,6 +381,7 @@ struct bpf_verifier_env {
 		int *insn_stack;
 		int cur_stack;
 	} cfg;
+	u32 pass_cnt; /* number of times do_check() was called */
 	u32 subprog_cnt;
 	/* number of instructions analyzed by the verifier */
 	u32 prev_insn_processed, insn_processed;
@@ -428,4 +431,7 @@ bpf_prog_offload_replace_insn(struct bpf_verifier_env *env, u32 off,
 void
 bpf_prog_offload_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt);
 
+int check_ctx_reg(struct bpf_verifier_env *env,
+		  const struct bpf_reg_state *reg, int regno);
+
 #endif /* _LINUX_BPF_VERIFIER_H */