[RFC PATCH 1/2] init.h: improve __setup and early_param documentation

[Randy Dunlap]

Igor noted in [1] that there are quite a few __setup() handling
functions that return incorrect values. Doing this can be harmless,
but it can also cause strings to be added to init's argument or
environment list, polluting them.

Since __setup() handling and return values are not documented,
first add documentation for that. Also add more documentation for
early_param() handling and return values.

For __setup() functions, returning 0 (not handled) has questionable
value if it is just a malformed option value, as in
  rodata=junk
since returning 0 would just cause "rodata=junk" to be added to init's
environment unnecessarily:
  Run /sbin/init as init process
    with arguments:
      /sbin/init
    with environment:
      HOME=/
      TERM=linux
      splash=native
      rodata=junk

Also, there are no recommendations on whether to print a warning when
an unknown parameter value is seen. I am not addressing that here.

[1] lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@xxxxxxxxxxxx

Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: Igor Zhbanov <i.zhbanov@xxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/init.h |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- linux-next-20220217.orig/include/linux/init.h
+++ linux-next-20220217/include/linux/init.h
@@ -320,12 +320,19 @@ struct obs_kernel_param {
 		__aligned(__alignof__(struct obs_kernel_param))		\
 		= { __setup_str_##unique_id, fn, early }
 
+/*
+ * NOTE: __setup functions return values:
+ * @fn returns 1 (or non-zero) if the option argument is "handled"
+ * and returns 0 if the option argument is "not handled".
+ */
 #define __setup(str, fn)						\
 	__setup_param(str, fn, fn, 0)
 
 /*
- * NOTE: fn is as per module_param, not __setup!
- * Emits warning if fn returns non-zero.
+ * NOTE: @fn is as per module_param, not __setup!
+ * I.e., @fn returns 0 for no error or non-zero for error
+ * (possibly @fn returns a -errno value, but it does not matter).
+ * Emits warning if @fn returns non-zero.
  */
 #define early_param(str, fn)						\
 	__setup_param(str, fn, fn, 1)