a problem about "su" command

when I type "su" in the console ,there is no "need password" prompted,
but only the cursor flashing. If some words is inputted, they arenot
show in the hidden mode, and the curor flashing the last characters.
"su --help" is also just as the "su". this is some infomation of "su".

-rwsr-xr-x 1 root root 102537 Mar 14 06:06 /bin/su

I never miss those problem,and is puzzled now .
Thanks for your attention.

siqi

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