Questions about setuid

Question

I have a problem with running the following C program:

#include<unistd.h>

void main()
{
    if (access("/root/main.c",R_OK)==0)
    {
        printf("/root/main.c can be read\n");
    }
    else
    {
        printf("can't be read\n");
    }
}

When I run the executable as root, I can get the output:

/root/main.c can be read

But when I set the executable's permissions to 4755, and run it as normal user, I only get the output:

can't be read

Is there any error in my protocol?

Answer 1

access ignores setuid/setgid bits. This is by design.

Quoting from man 2 access on Linux:

The check is done using the calling process's real UID and GID, rather than the effective IDs as is done when actually attempting an operation (e.g., open(2)) on the file. Similarly, for the root user, the check uses the set of permitted capabilities rather than the set of effective capabilities; and for non-root users, the check uses an empty set of capabilities.

This allows set-user-ID programs and capability-endowed programs to easily determine the invoking user's authority. In other words, access() does not answer the "can I read/write/execute this file?" question. It answers a slightly different question: "(assuming I'm a setuid binary) can the user who invoked me read/write/execute this file?", which gives set-user-ID programs the possibility to prevent malicious users from causing them to read files which users shouldn't be able to read.

If you want to know whether your process can actually open a file for reading, just open() it and handle the error, if any. (This also avoids a race condition.)