[PATCH bpf v2 1/2] bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs

[PATCH bpf v2 1/2] bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs

[Toke Høiland-Jørgensen]

The verifier allows programs to call global functions as long as their
argument types match, using BTF to check the function arguments. One of the
allowed argument types to such global functions is PTR_TO_CTX; however the
check for this fails on BPF_PROG_TYPE_EXT functions because the verifier
uses the wrong type to fetch the vmlinux BTF ID for the program context
type. This failure is seen when an XDP program is loaded using
libxdp (which loads it as BPF_PROG_TYPE_EXT and attaches it to a global XDP
type program).

Fix the issue by passing in the target program type instead of the
BPF_PROG_TYPE_EXT type to bpf_prog_get_ctx() when checking function
argument compatibility.

The first Fixes tag refers to the latest commit that touched the code in
question, while the second one points to the code that first introduced
the global function call verification.

v2:
- Use resolve_prog_type()

Fixes: 3363bd0cfbb8 ("bpf: Extend kfunc with PTR_TO_CTX, PTR_TO_MEM argument support")
Fixes: 51c39bb1d5d1 ("bpf: Introduce function-by-function verification")
Reported-by: Simon Sundberg <simon.sundberg@kau.se>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 kernel/bpf/btf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 7bccaa4646e5..63d0ac7dfe2f 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6054,6 +6054,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    struct bpf_reg_state *regs,
 				    bool ptr_to_mem_ok)
 {
+	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	struct bpf_verifier_log *log = &env->log;
 	u32 i, nargs, ref_id, ref_obj_id = 0;
 	bool is_kfunc = btf_is_kernel(btf);
@@ -6171,7 +6172,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				return -EINVAL;
 			}
 			/* rest of the arguments can be anything, like normal kfunc */
-		} else if (btf_get_prog_ctx_type(log, btf, t, env->prog->type, i)) {
+		} else if (btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
 			/* If function expects ctx type in BTF check that caller
 			 * is passing PTR_TO_CTX.
 			 */
-- 
2.36.1

[PATCH bpf v2 2/2] selftests/bpf: Add selftest for calling global functions from freplace

[Toke Høiland-Jørgensen]

Add a selftest that calls a global function with a context object parameter
from an freplace function to check that the program context type is
correctly converted to the freplace target when fetching the context type
from the kernel BTF.

v2:
- Trim includes
- Get rid of global function
- Use __noinline

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 .../selftests/bpf/prog_tests/fexit_bpf2bpf.c   | 14 ++++++++++++++
 .../selftests/bpf/progs/freplace_global_func.c | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/freplace_global_func.c

diff --git a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
index d9aad15e0d24..02bb8cbf9194 100644
--- a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
+++ b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
@@ -395,6 +395,18 @@ static void test_func_map_prog_compatibility(void)
 				     "./test_attach_probe.o");
 }
 
+static void test_func_replace_global_func(void)
+{
+	const char *prog_name[] = {
+		"freplace/test_pkt_access",
+	};
+
+	test_fexit_bpf2bpf_common("./freplace_global_func.o",
+				  "./test_pkt_access.o",
+				  ARRAY_SIZE(prog_name),
+				  prog_name, false, NULL);
+}
+
 /* NOTE: affect other tests, must run in serial mode */
 void serial_test_fexit_bpf2bpf(void)
 {
@@ -416,4 +428,6 @@ void serial_test_fexit_bpf2bpf(void)
 		test_func_replace_multi();
 	if (test__start_subtest("fmod_ret_freplace"))
 		test_fmod_ret_freplace();
+	if (test__start_subtest("func_replace_global_func"))
+		test_func_replace_global_func();
 }
diff --git a/tools/testing/selftests/bpf/progs/freplace_global_func.c b/tools/testing/selftests/bpf/progs/freplace_global_func.c
new file mode 100644
index 000000000000..96cb61a6ce87
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/freplace_global_func.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+__noinline
+int test_ctx_global_func(struct __sk_buff *skb)
+{
+	volatile int retval = 1;
+	return retval;
+}
+
+SEC("freplace/test_pkt_access")
+int new_test_pkt_access(struct __sk_buff *skb)
+{
+	return test_ctx_global_func(skb);
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.36.1

Re: [PATCH bpf v2 1/2] bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Mon,  6 Jun 2022 09:52:51 +0200 you wrote:
> The verifier allows programs to call global functions as long as their
> argument types match, using BTF to check the function arguments. One of the
> allowed argument types to such global functions is PTR_TO_CTX; however the
> check for this fails on BPF_PROG_TYPE_EXT functions because the verifier
> uses the wrong type to fetch the vmlinux BTF ID for the program context
> type. This failure is seen when an XDP program is loaded using
> libxdp (which loads it as BPF_PROG_TYPE_EXT and attaches it to a global XDP
> type program).
> 
> [...]

Here is the summary with links:
  - [bpf,v2,1/2] bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs
    https://git.kernel.org/bpf/bpf/c/f858c2b2ca04
  - [bpf,v2,2/2] selftests/bpf: Add selftest for calling global functions from freplace
    https://git.kernel.org/bpf/bpf/c/2cf7b7ffdae5

You are awesome, thank you!
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