[PATCH bpf-next 2/3] bpf: Document usage of the new __bpf_kfunc macro

[David Vernet <void@manifault.com>]

Now that the __bpf_kfunc macro has been added to linux/btf.h, include a
blurb about it in the kfuncs.rst file. In order for the macro to
successfully render with .. kernel-doc, we'll also need to add it to the
c_id_attributes array.

Signed-off-by: David Vernet <void@manifault.com>
---
 Documentation/bpf/kfuncs.rst | 18 ++++++++++++++++++
 Documentation/conf.py        |  3 +++
 2 files changed, 21 insertions(+)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 9fd7fb539f85..900f6b96487c 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -37,6 +37,7 @@ An example is given below::
         __diag_ignore_all("-Wmissing-prototypes",
                           "Global kfuncs as their definitions will be in BTF");
 
+        __bpf_kfunc
         struct task_struct *bpf_find_get_task_by_vpid(pid_t nr)
         {
                 return find_get_task_by_vpid(nr);
@@ -62,6 +63,7 @@ kfunc with a __tag, where tag may be one of the supported annotations.
 This annotation is used to indicate a memory and size pair in the argument list.
 An example is given below::
 
+        __bpf_kfunc
         void bpf_memzero(void *mem, int mem__sz)
         {
         ...
@@ -82,6 +84,7 @@ safety of the program.
 
 An example is given below::
 
+        __bpf_kfunc
         void *bpf_obj_new(u32 local_type_id__k, ...)
         {
         ...
@@ -121,6 +124,21 @@ flags on a set of kfuncs as follows::
 This set encodes the BTF ID of each kfunc listed above, and encodes the flags
 along with it. Ofcourse, it is also allowed to specify no flags.
 
+kfunc definitions should also always be annotated with the ``__bpf_kfunc``
+macro. This prevents issues such as the compiler inlining the kfunc if it's a
+static kernel function, or the function being elided in an LTO build as it's
+not used in the rest of the kernel. Developers should not manually add
+annotations to their kfunc to prevent these issues. If an annotation is
+required to prevent such an issue with your kfunc, it is a bug and should be
+added to the definition of the macro so that other kfuncs are similarly
+protected. An example is given below::
+
+        __bpf_kfunc
+        struct task_struct *bpf_get_task_pid(s32 pid)
+        {
+        ...
+        }
+
 2.4.1 KF_ACQUIRE flag
 ---------------------
 
diff --git a/Documentation/conf.py b/Documentation/conf.py
index a5c45df0bd83..05ff3186b243 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -110,6 +110,9 @@ if major >= 3:
 
             # include/linux/linkage.h:
             "asmlinkage",
+
+            # include/linux/btf.h
+            "__bpf_kfunc",
         ]
 
 else:
-- 
2.39.0