This is similar to commit 7472d5a642c9 ("compiler_types: define __user as
__attribute__((btf_type_tag("user")))"), where a type tag "user" was
introduced to identify the pointers that point to user memory. With that
change, the newest compile toolchain can encode __user information into
vmlinux BTF, which can be used by the BPF verifier to enforce safe
program behaviors.
Similarly, we have __percpu attribute, which is mainly used to indicate
memory is allocated in percpu region. The __percpu pointers in kernel
are supposed to be used together with functions like per_cpu_ptr() and
this_cpu_ptr(), which perform necessary calculation on the pointer's
base address. Without the btf_type_tag introduced in this patch,
__percpu pointers will be treated as regular memory pointers in vmlinux
BTF and BPF programs are allowed to directly dereference them, generating
incorrect behaviors. Now with "percpu" btf_type_tag, the BPF verifier is
able to differentiate __percpu pointers from regular pointers and forbids
unexpected behaviors like direct load.
The following is an example similar to the one given in commit
7472d5a642c9:
[$ ~] cat test.c
#define __percpu __attribute__((btf_type_tag("percpu")))
int foo(int __percpu *arg) {
return *arg;
}
[$ ~] clang -O2 -g -c test.c
[$ ~] pahole -JV test.o
...
File test.o:
[1] INT int size=4 nr_bits=32 encoding=SIGNED
[2] TYPE_TAG percpu type_id=1
[3] PTR (anon) type_id=2
[4] FUNC_PROTO (anon) return=1 args=(3 arg)
[5] FUNC foo type_id=4
[$ ~]
for the function argument "int __percpu *arg", its type is described as
PTR -> TYPE_TAG(percpu) -> INT
The kernel can use this information for bpf verification or other
use cases.
Like commit 7472d5a642c9, this feature requires clang (>= clang14) and
pahole (>= 1.23).
Cc: Yonghong Song <yhs@xxxxxx>
Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
---
include/linux/compiler_types.h | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 3f31ff400432..223abf43679a 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -38,7 +38,12 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
# define __user
# endif
# define __iomem
-# define __percpu
+# if defined(CONFIG_DEBUG_INFO_BTF) && defined(CONFIG_PAHOLE_HAS_BTF_TAG) && \
+ __has_attribute(btf_type_tag)
+# define __percpu __attribute__((btf_type_tag("percpu")))
+# else
+# define __percpu
+# endif
# define __rcu
# define __chk_user_ptr(x) (void)0
# define __chk_io_ptr(x) (void)0
--
2.35.1.616.g0bdcbb4464-goog
