[PATCH bpf-next v1 1/6] bpf: Add bpf_kallsyms_lookup_name helper

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

This helper allows us to get the address of a kernel symbol from inside
a BPF_PROG_TYPE_SYSCALL prog (used by gen_loader), so that we can
relocate weak and typeless ksym vars.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf.h            |  1 +
 include/uapi/linux/bpf.h       | 14 ++++++++++++++
 kernel/bpf/syscall.c           | 24 ++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 14 ++++++++++++++
 4 files changed, 53 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index d604c8251d88..17206aae329d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2107,6 +2107,7 @@ extern const struct bpf_func_proto bpf_for_each_map_elem_proto;
 extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
 extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
 extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
+extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto;
 
 const struct bpf_func_proto *tracing_prog_func_proto(
   enum bpf_func_id func_id, const struct bpf_prog *prog);
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 6fc59d61937a..bbd0a3f4e5f6 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -4909,6 +4909,19 @@ union bpf_attr {
  *	Return
  *		The number of bytes written to the buffer, or a negative error
  *		in case of failure.
+ *
+ * long bpf_kallsyms_lookup_name(const char *name, int name_sz, int flags, u64 *res)
+ *	Description
+ *		Get the address of a kernel symbol, returned in *res*. *res* is
+ *		set to 0 if the symbol is not found.
+ *	Return
+ *		On success, zero. On error, a negative value.
+ *
+ *		**-EINVAL** if *flags* is not zero.
+ *
+ *		**-EINVAL** if string *name* is not the same size as *name_sz*.
+ *
+ *		**-ENOENT** if symbol is not found.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5089,6 +5102,7 @@ union bpf_attr {
 	FN(task_pt_regs),		\
 	FN(get_branch_snapshot),	\
 	FN(trace_vprintk),		\
+	FN(kallsyms_lookup_name),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4e50c0bfdb7d..073ca9ebe58b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4753,6 +4753,28 @@ static const struct bpf_func_proto bpf_sys_close_proto = {
 	.arg1_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_4(bpf_kallsyms_lookup_name, const char *, name, int, name_sz, int, flags, u64 *, res)
+{
+	if (flags)
+		return -EINVAL;
+
+	if (name_sz <= 1 || name[name_sz - 1])
+		return -EINVAL;
+
+	*res = kallsyms_lookup_name(name);
+	return *res ? 0 : -ENOENT;
+}
+
+const struct bpf_func_proto bpf_kallsyms_lookup_name_proto = {
+	.func		= bpf_kallsyms_lookup_name,
+	.gpl_only	= true,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_MEM,
+	.arg2_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_ANYTHING,
+	.arg4_type	= ARG_PTR_TO_LONG,
+};
+
 static const struct bpf_func_proto *
 syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -4763,6 +4785,8 @@ syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_btf_find_by_name_kind_proto;
 	case BPF_FUNC_sys_close:
 		return &bpf_sys_close_proto;
+	case BPF_FUNC_kallsyms_lookup_name:
+		return &bpf_kallsyms_lookup_name_proto;
 	default:
 		return tracing_prog_func_proto(func_id, prog);
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 6fc59d61937a..bbd0a3f4e5f6 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -4909,6 +4909,19 @@ union bpf_attr {
  *	Return
  *		The number of bytes written to the buffer, or a negative error
  *		in case of failure.
+ *
+ * long bpf_kallsyms_lookup_name(const char *name, int name_sz, int flags, u64 *res)
+ *	Description
+ *		Get the address of a kernel symbol, returned in *res*. *res* is
+ *		set to 0 if the symbol is not found.
+ *	Return
+ *		On success, zero. On error, a negative value.
+ *
+ *		**-EINVAL** if *flags* is not zero.
+ *
+ *		**-EINVAL** if string *name* is not the same size as *name_sz*.
+ *
+ *		**-ENOENT** if symbol is not found.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5089,6 +5102,7 @@ union bpf_attr {
 	FN(task_pt_regs),		\
 	FN(get_branch_snapshot),	\
 	FN(trace_vprintk),		\
+	FN(kallsyms_lookup_name),	\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
-- 
2.33.0