Re: [PATCH bpf-next v1 01/15] bpf: Factor out fd returning from bpf_btf_find_by_name_kind

On Tue, Feb 22, 2022 at 10:58:11AM IST, Alexei Starovoitov wrote:

On Sun, Feb 20, 2022 at 07:17:59PM +0530, Kumar Kartikeya Dwivedi wrote:

quoted

In next few patches, we need a helper that searches all kernel BTFs
(vmlinux and module BTFs), and finds the type denoted by 'name' and
'kind'. Turns out bpf_btf_find_by_name_kind already does the same thing,
but it instead returns a BTF ID and optionally fd (if module BTF). This
is used for relocating ksyms in BPF loader code (bpftool gen skel -L).

We extract the core code out into a new helper
btf_find_by_name_kind_all, which returns the BTF ID and BTF pointer in
an out parameter. The reference for the returned BTF pointer is only
bumped if it is a module BTF, this needs to be kept in mind when using
this helper.

Hence, the user must release the BTF reference iff btf_is_module is
true, otherwise transfer the ownership to e.g. an fd.

In case of the helper, the fd is only allocated for module BTFs, so no
extra handling for btf_vmlinux case is required.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/btf.c | 47 +++++++++++++++++++++++++++++++----------------
 1 file changed, 31 insertions(+), 16 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 2c4c5dbe2abe..3645d8c14a18 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -6545,16 +6545,10 @@ static struct btf *btf_get_module_btf(const struct module *module)
 	return btf;
 }

-BPF_CALL_4(bpf_btf_find_by_name_kind, char *, name, int, name_sz, u32, kind, int, flags)
+static s32 btf_find_by_name_kind_all(const char *name, u32 kind, struct btf **btfp)

The name is getting too long.
How about bpf_find_btf_id() ?

quoted

 {
 	struct btf *btf;
-	long ret;
-
-	if (flags)
-		return -EINVAL;
-
-	if (name_sz <= 1 || name[name_sz - 1])
-		return -EINVAL;
+	s32 ret;

 	btf = bpf_get_btf_vmlinux();
 	if (IS_ERR(btf))

@@ -6580,19 +6574,40 @@ BPF_CALL_4(bpf_btf_find_by_name_kind, char *, name, int, name_sz, u32, kind, int
 			spin_unlock_bh(&btf_idr_lock);
 			ret = btf_find_by_name_kind(mod_btf, name, kind);
 			if (ret > 0) {
-				int btf_obj_fd;
-
-				btf_obj_fd = __btf_new_fd(mod_btf);
-				if (btf_obj_fd < 0) {
-					btf_put(mod_btf);
-					return btf_obj_fd;
-				}
-				return ret | (((u64)btf_obj_fd) << 32);
+				*btfp = mod_btf;
+				return ret;
 			}
 			spin_lock_bh(&btf_idr_lock);
 			btf_put(mod_btf);
 		}
 		spin_unlock_bh(&btf_idr_lock);
+	} else {
+		*btfp = btf;
+	}

Since we're refactoring let's drop the indent.
How about
  if (ret > 0) {
    *btfp = btf;
    return ret;
  }
  idr_for_each_entry().

and move the func right after btf_find_by_name_kind(),
so that later patch doesn't need to do:
static s32 bpf_find_btf_id();
Eventually this helper might become global with this name.

Ok, will change.

Also may be do btf_get() for vmlinux_btf too?
In case it reduces 'if (btf_is_module())' checks.

Right, should also change this for btf_get_module_btf then, to make things
consistent.

--
Kartikeya