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bpf: Document usage of the new __bpf_kfunc macro

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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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230201173016.342758-3-void@manifault.com
This commit is contained in:
David Vernet 2023-02-01 11:30:14 -06:00 committed by Daniel Borkmann
parent 57e7c169cd
commit 98e6ab7a04
2 changed files with 20 additions and 3 deletions
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20
Documentation/bpf/kfuncs.rst
View File

@ -41,7 +41,7 @@ An example is given below::
__diag_ignore_all("-Wmissing-prototypes",
"Global kfuncs as their definitions will be in BTF");
struct task_struct *bpf_find_get_task_by_vpid(pid_t nr)
__bpf_kfunc struct task_struct *bpf_find_get_task_by_vpid(pid_t nr)
{
return find_get_task_by_vpid(nr);
}
@ -66,7 +66,7 @@ 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::
void bpf_memzero(void *mem, int mem__sz)
__bpf_kfunc void bpf_memzero(void *mem, int mem__sz)
{
...
}
@ -86,7 +86,7 @@ safety of the program.
An example is given below::
void *bpf_obj_new(u32 local_type_id__k, ...)
__bpf_kfunc void *bpf_obj_new(u32 local_type_id__k, ...)
{
...
}
@ -125,6 +125,20 @@ 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
---------------------

3
Documentation/conf.py
View File

@ -116,6 +116,9 @@ if major >= 3:
# include/linux/linkage.h:
"asmlinkage",
# include/linux/btf.h
"__bpf_kfunc",
]
else: