compiler_types: Define __rcu as __attribute__((btf_type_tag("rcu")))

Currently, without rcu attribute info in BTF, the verifier treats
rcu tagged pointer as a normal pointer. This might be a problem
for sleepable program where rcu_read_lock()/unlock() is not available.
For example, for a sleepable fentry program, if rcu protected memory
access is interleaved with a sleepable helper/kfunc, it is possible
the memory access after the sleepable helper/kfunc might be invalid
since the object might have been freed then. To prevent such cases,
introducing rcu tagging for memory accesses in verifier can help
to reject such programs.

To enable rcu tagging in BTF, during kernel compilation,
define __rcu as attribute btf_type_tag("rcu") so __rcu information can
be preserved in dwarf and btf, and later can be used for bpf prog verification.

Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221124053206.2373141-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index eb0466236661f51cb11d9c06416fc11eca35a950..7c1afe0f4129c5a5f76de071a68d41c41e3e1353 100644 (file)
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -49,7 +49,8 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 # endif
 # define __iomem
 # define __percpu      BTF_TYPE_TAG(percpu)
-# define __rcu
+# define __rcu         BTF_TYPE_TAG(rcu)
+
 # define __chk_user_ptr(x)     (void)0
 # define __chk_io_ptr(x)       (void)0
 /* context/locking */