How does __user works?

[Grzegorz Dwornicki]

How does __user macro works? I know it is defined in
include/linux/compiler.h as:
# define __user.        __attribute__((noderef, address_space(1)))
I could write thesse macros defs too but my real problem is: what does this
stuff do? Some functions use this macro and other does not. For example
compat_do_execve (from include/linux/comtap.h) use normal pointer named
filename and second pointer named argv as __user pointer. Why does argv
needs __user but filename not?

Gregory
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