bpf: Add comment to document BTF type PTR_TO_BTF_ID_OR_NULL

The meaning of PTR_TO_BTF_ID_OR_NULL differs slightly from other types
denoted with the *_OR_NULL type. For example the types PTR_TO_SOCKET
and PTR_TO_SOCKET_OR_NULL can be used for branch analysis because the
type PTR_TO_SOCKET is guaranteed to _not_ have a null value.

In contrast PTR_TO_BTF_ID and BTF_TO_BTF_ID_OR_NULL have slightly
different meanings. A PTR_TO_BTF_TO_ID may be a pointer to NULL value,
but it is safe to read this pointer in the program context because
the program context will handle any faults. The fallout is for
PTR_TO_BTF_ID the verifier can assume reads are safe, but can not
use the type in branch analysis. Additionally, authors need to be
extra careful when passing PTR_TO_BTF_ID into helpers. In general
helpers consuming type PTR_TO_BTF_ID will need to assume it may
be null.

Seeing the above is not obvious to readers without the back knowledge
lets add a comment in the type definition.

Editorial comment, as networking and tracing programs get closer
and more tightly merged we may need to consider a new type that we
can ensure is non-null for branch analysis and also passing into
helpers.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Lorenz Bauer <lmb@cloudflare.com>

include/linux/bpf.h

@@ -383,8 +383,22 @@ enum bpf_reg_type {
 	PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */
 	PTR_TO_TP_BUFFER,	 /* reg points to a writable raw tp's buffer */
 	PTR_TO_XDP_SOCK,	 /* reg points to struct xdp_sock */
-	PTR_TO_BTF_ID,		 /* reg points to kernel struct */
-	PTR_TO_BTF_ID_OR_NULL,	 /* reg points to kernel struct or NULL */
+	/* PTR_TO_BTF_ID points to a kernel struct that does not need
+	 * to be null checked by the BPF program. This does not imply the
+	 * pointer is _not_ null and in practice this can easily be a null
+	 * pointer when reading pointer chains. The assumption is program
+	 * context will handle null pointer dereference typically via fault
+	 * handling. The verifier must keep this in mind and can make no
+	 * assumptions about null or non-null when doing branch analysis.
+	 * Further, when passed into helpers the helpers can not, without
+	 * additional context, assume the value is non-null.
+	 */
+	PTR_TO_BTF_ID,
+	/* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not
+	 * been checked for null. Used primarily to inform the verifier
+	 * an explicit null check is required for this struct.
+	 */
+	PTR_TO_BTF_ID_OR_NULL,
 	PTR_TO_MEM,		 /* reg points to valid memory region */
 	PTR_TO_MEM_OR_NULL,	 /* reg points to valid memory region or NULL */
 	PTR_TO_RDONLY_BUF,	 /* reg points to a readonly buffer */