Could ltrace impact geteuid within a C program with sticky bits

Question

I have a simple C program that does this

int
main(void)
{
    printf("getuid: %d\n", getuid());
    printf("geteuid: %d\n", geteuid();
    return 0;
}

The owner of the executable of my program is the user "A", this executable have special permissions on user and group fields (sticky bits), and can be executed by anyone.

Here is the output with my user "A".

getuid: 1000 geteuid: 1000

Which corresponds to the id's of A.

If I run it with another user "B" I have this as output:

getuid: 1001 geteuid: 1000

So far so good since a file with SUID always runs as the user owning the file, regardless of which user issues the command.

However if I run the file with the ltrace program, I get:

getuid: 1001 geteuid: 1001

Can someone explain this behavior to me, is this normal ?

Answer 1

Yes, this is normal and expected behaviour. The reason for this discrepancy is due to the way the ltrace utility operates.

When you run a program in the usual way, the operating system loads the executable into memory and starts executing it directly. If the program has the SUID bit set, the OS will run the program with the privileges of the file owner.

On the other hand, ltrace doesn't directly execute the target program. Instead, they use the ptrace system call to control the execution of the target program. When a process is being ptraced, the operating system ignores the SUID bit. The reason for this is security: if the SUID bit wasn't ignored, then any user could use ltrace to gain elevated privileges by tracing a SUID program.

That's why when you run your program under ltrace, both the real UID (from getuid()) and the effective UID (from geteuid()) are the ID of the user running the ltracecommand, not the owner of the file.

This feature can be used as a safety mechanism. For instance, if you're investigating a program and you're not sure if it's safe, you can use ltrace to run it without worrying about the program doing anything harmful with elevated privileges.