On 11/20/22 11:10 AM, Alexei Starovoitov wrote:
On Sun, Nov 20, 2022 at 10:55:54AM -0800, Yonghong Song wrote:
On 11/20/22 10:33 AM, Alexei Starovoitov wrote:
On Sun, Nov 20, 2022 at 08:15:22AM -0800, Yonghong Song wrote:
Implement bpf_cast_to_kern_ctx() kfunc which does a type cast
of a uapi ctx object to the corresponding kernel ctx. Previously
if users want to access some data available in kctx but not
in uapi ctx, bpf_probe_read_kernel() helper is needed.
The introduction of bpf_cast_to_kern_ctx() allows direct
memory access which makes code simpler and easier to understand.
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
include/linux/btf.h | 5 +++++
kernel/bpf/btf.c | 25 +++++++++++++++++++++++++
kernel/bpf/helpers.c | 6 ++++++
kernel/bpf/verifier.c | 21 +++++++++++++++++++++
4 files changed, 57 insertions(+)
diff --git a/include/linux/btf.h b/include/linux/btf.h
index d5b26380a60f..4b5d799f5d02 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -470,6 +470,7 @@ const struct btf_member *
btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
const struct btf_type *t, enum bpf_prog_type prog_type,
int arg);
+int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type);
bool btf_types_are_same(const struct btf *btf1, u32 id1,
const struct btf *btf2, u32 id2);
#else
@@ -514,6 +515,10 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
{
return NULL;
}
+static inline int get_kern_ctx_btf_id(struct bpf_verifier_log *log,
+ enum bpf_prog_type prog_type) {
+ return -EINVAL;
+}
static inline bool btf_types_are_same(const struct btf *btf1, u32 id1,
const struct btf *btf2, u32 id2)
{
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 0a3abbe56c5d..bef1b6cfe6b8 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5603,6 +5603,31 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
return kern_ctx_type->type;
}
+int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type)
+{
+ const struct btf_member *kctx_member;
+ const struct btf_type *conv_struct;
+ const struct btf_type *kctx_type;
+ u32 kctx_type_id;
+
+ conv_struct = bpf_ctx_convert.t;
+ if (!conv_struct) {
+ bpf_log(log, "btf_vmlinux is malformed\n");
+ return -EINVAL;
+ }
If we get to this point this internal pointer would be already checked.
No need to check it again. Just use it.
This is probably not true.
Currently, conv_struct is tested in function btf_get_prog_ctx_type() which
is called by get_kfunc_ptr_arg_type().
const struct btf_member *
btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
const struct btf_type *t, enum bpf_prog_type
prog_type,
int arg)
{
const struct btf_type *conv_struct;
const struct btf_type *ctx_struct;
const struct btf_member *ctx_type;
const char *tname, *ctx_tname;
conv_struct = bpf_ctx_convert.t;
if (!conv_struct) {
bpf_log(log, "btf_vmlinux is malformed\n");
return NULL;
}
...
}
In get_kfunc_ptr_arg_type(),
...
/* In this function, we verify the kfunc's BTF as per the argument
type,
* leaving the rest of the verification with respect to the register
* type to our caller. When a set of conditions hold in the BTF type
of
* arguments, we resolve it to a known kfunc_ptr_arg_type.
*/
if (btf_get_prog_ctx_type(&env->log, meta->btf, t,
resolve_prog_type(env->prog), argno))
return KF_ARG_PTR_TO_CTX;
Note that if bpf_ctx_convert.t is NULL, btf_get_prog_ctx_type() simply
returns NULL and the logic simply follows through.
Right. It will return NULL and the code further won't see KF_ARG_PTR_TO_CTX
and will not call get_kern_ctx_btf_id().
So it still looks to me that the check can be dropped.
Should we actually add a NULL checking for bpf_ctx_convert.t in
bpf_parse_vmlinux?
Ideally yes, but right now CONFIG_DEBUG_INFO_BTF can be enabled
independently and I'm afraid btf_get_prog_ctx_type() can be called
via btf_translate_to_vmlinux() even when btf_vmlinux == NULL.
So bpf_ctx_convert.t == NULL at that point
because btf_parse_vmlinux wasn't called.
Okay, I see. btf_get_prog_ctx_type() could be called even when vmlinux
btf is not parsed yet. But get_kfunc_ptr_arg_type() should already
have vmlinux parsed properly. So for bpf_cast_to_kern_ctx handling,
bpf_ctx_convert.t should not be NULL.
Will drop the error checking in the next revision.
...
err = btf_check_type_tags(env, btf, 1);
if (err)
goto errout;
/* btf_parse_vmlinux() runs under bpf_verifier_lock */
bpf_ctx_convert.t = btf_type_by_id(btf, bpf_ctx_convert_btf_id[0]);
bpf_struct_ops_init(btf, log);
...
