Difference between inbuilt suid programs and user defined root suid programs under bash shell?

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# 1  
Old 09-20-2015
Difference between inbuilt suid programs and user defined root suid programs under bash shell?

Hey guys,

Suppose i run passwd via bash shell. It is a suid program, which temporarily runs as root(owner) and modifies the user entries.

However, when i write a C file and give 4755 permission and root ownership to the 'a.out' file , it doesn't run as root in bash shell. I verified this by comparing uid and euid within the program which turned out to be the same. However, in zsh shell, it runs with root privileges

My question is : Why does an inbuilt program like suid is able to run with root privileges in bash shell while a user defined suid root program is not ?
# 2  
Old 09-20-2015
What does "a.out" doing ?
It try to launch bash in setuid root ?
You shall know that bash not work by default in setuid root, you must use "-p" option.

In man:
If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not
supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH,
and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user
id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.
# 3  
Old 09-20-2015
I think in a C-program you need to
seteuid (0)

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