linux - Giving S_IWUGO permission to module parameter results in compilation error (while S_IRUGO or S_IXUGO doesn't) - why? -


i wrote simple kernel module learn module_param feature of kernel module. however, if give s_iwugo, s_irwxugo or s_iallugo permissions perm field, follwing compilation error:

[root@localhost param]# make -c $kdir m=$pwd modules  make: entering directory `/usr/src/kernels/3.11.10-301.fc20.i686+pae'   cc [m]  /root/ldd/misc/param/param/hello.o /root/ldd/misc/param/param/hello.c:6:168: error: negative width in bit-field ‘<anonymous>’  module_param(a, int, s_iwugo);                                                                                                                                                                         ^ make[1]: *** [/root/ldd/misc/param/param/hello.o] error 1 make: *** [_module_/root/ldd/misc/param/param] error 2 make: leaving directory `/usr/src/kernels/3.11.10-301.fc20.i686+pae'

compilation successful s_irugo or s_ixugo (permission not containing write permssion). suppose must doing wrong because know, wrtie permission legal. doing wrong here?

the program:

#include<linux/module.h> #include<linux/stat.h>  int = 2;  module_param(a, int, s_ixugo);  int f1(void){          return 0; }  void f2(void){  }  module_init(f1); module_exit(f2);  module_author("lavya"); module_license("gpl v2"); module_description("experiment parameters");

linux not accept s_iwoth permission. if follow macro chain behind module_param, arrive __module_param_call includes:

build_bug_on_zero((perm) < 0 || (perm) > 0777 || ((perm) & 2))

s_iwoth == 2 test fails.

the negative width in bit-field error merely artefact of implementation of build_bug_on_zero

linux refuses make module parameters world-writable security reasons. should able use narrower permissions such s_iwusr | s_iwgrp.


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