bpf: Allow specifying volatile type modifier for kptrs

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>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221103191013.1236066-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: kkdwvd

include/linux/btf.h

@@ -288,6 +288,11 @@ 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;

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;