bpf_doc: Support 1st const parameter of bpf_d_path()

Since commit 1b8abbb ("bpf...d_path(): constify path argument"),
the first parameter of the bpf_d_path() has been changed to a const
constant. We need to modify the header file and bpf_doc.py.

The two error messages are as follows:

    linux/tools/testing/selftests/bpf$ make

      CLNG-BPF [test_progs] bpf_iter_task_vmas.bpf.o
    progs/bpf_iter_task_vmas.c:52:14: error: passing 'const struct path *'
    to parameter of type 'struct path *' discards qualifiers
    [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
       52 |                 bpf_d_path(&file->f_path, d_path_buf, D_PATH_BUF_SIZE);
          |                            ^~~~~~~~~~~~~
    1 error generated.
    ....
    progs/verifier_vfs_accept.c:80:7: error: assigning to 'struct path *'
    from 'const struct path *' discards qualifiers
    [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
       80 |         path = &file->f_path;
          |              ^ ~~~~~~~~~~~~~
    1 error generated.

Fixes: 1b8abbb ("bpf...d_path(): constify path argument")
Signed-off-by: Rong Tao <[email protected]>

Author: Rtoax

include/uapi/linux/bpf.h

@@ -4891,7 +4891,7 @@ union bpf_attr {
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
  *
- * long bpf_d_path(struct path *path, char *buf, u32 sz)
+ * long bpf_d_path(const struct path *path, char *buf, u32 sz)
  *	Description
  *		Return full path for given **struct path** object, which
  *		needs to be the kernel BTF *path* object. The path is

scripts/bpf_doc.py

@@ -788,6 +788,7 @@ def __init__(self, parser):
             'struct task_struct',
             'struct cgroup',
             'struct path',
+            'const struct path',
             'struct btf_ptr',
             'struct inode',
             'struct socket',

tools/include/uapi/linux/bpf.h

@@ -4891,7 +4891,7 @@ union bpf_attr {
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
  *
- * long bpf_d_path(struct path *path, char *buf, u32 sz)
+ * long bpf_d_path(const struct path *path, char *buf, u32 sz)
  *	Description
  *		Return full path for given **struct path** object, which
  *		needs to be the kernel BTF *path* object. The path is

tools/testing/selftests/bpf/progs/verifier_vfs_accept.c

@@ -70,7 +70,7 @@ __success
 int BPF_PROG(path_d_path_from_file_argument, struct file *file)
 {
 	int ret;
-	struct path *path;
+	const struct path *path;
 
 	/* The f_path member is a path which is embedded directly within a
 	 * file. Therefore, a pointer to such embedded members are still