[PATCH bpf-next v4 1/4] bpf: BPF token support for BPF_BTF_GET_FD_BY_ID

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

From: Mykyta Yatsenko <yatsenko@meta.com>

Currently BPF_BTF_GET_FD_BY_ID requires CAP_SYS_ADMIN, which does not
allow running it from user namespace. This creates a problem when
freplace program running from user namespace needs to query target
program BTF.
This patch relaxes capable check from CAP_SYS_ADMIN to CAP_BPF and adds
support for BPF token that can be passed in attributes to syscall.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
 include/uapi/linux/bpf.h                      |  1 +
 kernel/bpf/syscall.c                          | 21 ++++++++++++++++---
 tools/include/uapi/linux/bpf.h                |  1 +
 .../bpf/prog_tests/libbpf_get_fd_by_id_opts.c |  3 +--
 4 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index bb37897c0393..73c23daacabf 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1652,6 +1652,7 @@ union bpf_attr {
 		};
 		__u32		next_id;
 		__u32		open_flags;
+		__s32		token_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_GET_INFO_BY_FD */
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 57a438706215..eb3a31aefa70 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5137,17 +5137,32 @@ static int bpf_btf_load(const union bpf_attr *attr, bpfptr_t uattr, __u32 uattr_
 	return btf_new_fd(attr, uattr, uattr_size);
 }
 
-#define BPF_BTF_GET_FD_BY_ID_LAST_FIELD btf_id
+#define BPF_BTF_GET_FD_BY_ID_LAST_FIELD token_fd
 
 static int bpf_btf_get_fd_by_id(const union bpf_attr *attr)
 {
+	struct bpf_token *token = NULL;
+
 	if (CHECK_ATTR(BPF_BTF_GET_FD_BY_ID))
 		return -EINVAL;
 
-	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
+	if (attr->open_flags & BPF_F_TOKEN_FD) {
+		token = bpf_token_get_from_fd(attr->token_fd);
+		if (IS_ERR(token))
+			return PTR_ERR(token);
+		if (!bpf_token_allow_cmd(token, BPF_BTF_GET_FD_BY_ID))
+			goto out;
+	}
+
+	if (!bpf_token_capable(token, CAP_SYS_ADMIN))
+		goto out;
+
+	bpf_token_put(token);
 
 	return btf_get_fd_by_id(attr->btf_id);
+out:
+	bpf_token_put(token);
+	return -EPERM;
 }
 
 static int bpf_task_fd_query_copy(const union bpf_attr *attr,
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index bb37897c0393..73c23daacabf 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1652,6 +1652,7 @@ union bpf_attr {
 		};
 		__u32		next_id;
 		__u32		open_flags;
+		__s32		token_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_GET_INFO_BY_FD */
diff --git a/tools/testing/selftests/bpf/prog_tests/libbpf_get_fd_by_id_opts.c b/tools/testing/selftests/bpf/prog_tests/libbpf_get_fd_by_id_opts.c
index a3f238f51d05..976ff38a6d43 100644
--- a/tools/testing/selftests/bpf/prog_tests/libbpf_get_fd_by_id_opts.c
+++ b/tools/testing/selftests/bpf/prog_tests/libbpf_get_fd_by_id_opts.c
@@ -75,9 +75,8 @@ void test_libbpf_get_fd_by_id_opts(void)
 	if (!ASSERT_EQ(ret, -EINVAL, "bpf_link_get_fd_by_id_opts"))
 		goto close_prog;
 
-	/* BTF get fd with opts set should not work (no kernel support). */
 	ret = bpf_btf_get_fd_by_id_opts(0, &fd_opts_rdonly);
-	ASSERT_EQ(ret, -EINVAL, "bpf_btf_get_fd_by_id_opts");
+	ASSERT_EQ(ret, -ENOENT, "bpf_btf_get_fd_by_id_opts");
 
 close_prog:
 	if (fd >= 0)
-- 
2.48.1