[PATCH RFC bpf-next v1 07/32] bpf: Allow specifying volatile type modifier for kptrs

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

This is useful in particular to mark the pointer as volatile, so that
compiler treats each load and store to the field as a volatile access.
The alternative is having to define and use READ_ONCE and WRITE_ONCE in
the BPF program.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/btf.h | 5 +++++
 kernel/bpf/btf.c    | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index ad93c2d9cc1c..b3d47e9b9d5c 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -279,6 +279,11 @@ static inline bool btf_type_is_typedef(const struct btf_type *t)
 	return BTF_INFO_KIND(t->info) == BTF_KIND_TYPEDEF;
 }
 
+static inline bool btf_type_is_volatile(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_VOLATILE;
+}
+
 static inline bool btf_type_is_func(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_FUNC;
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 903719b89238..5e860f76595c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3225,6 +3225,9 @@ static int btf_find_kptr(const struct btf *btf, const struct btf_type *t,
 	enum bpf_kptr_type type;
 	u32 res_id;
 
+	/* Permit modifiers on the pointer itself */
+	if (btf_type_is_volatile(t))
+		t = btf_type_by_id(btf, t->type);
 	/* For PTR, sz is always == 8 */
 	if (!btf_type_is_ptr(t))
 		return BTF_FIELD_IGNORE;
-- 
2.34.1