[PATCH bpf-next v2 02/25] bpf: Allow specifying volatile type modifier for kptrs

Kumar Kartikeya Dwivedi
2022-10-13T06:23:26Z

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 f9aababc5d78..86aad9b2ce02 100644
--- a/include/linux/btf.h
+++ b/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;
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index eba603cec2c5..ad301e78f7ee 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;
--
2.38.0