fake getpwuid home dir with LD_PRELOAD

fakehome.c

/*
 * Force the home directory for polluters that bypass $HOME with `getpwuid`.
 * To use, set your desired $HOME and LD_PRELOAD this library.
 *
 * Compile with:
 *
 *   gcc -O2 -Wall -Wextra -shared -fPIC -ldl -o libfakehome.so fakehome.c
 *
 */

#define _GNU_SOURCE 1
#include <dlfcn.h>
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>

struct passwd *getpwuid(uid_t uid) {
    static struct passwd fake;
    char *fake_home = getenv("HOME");
    struct passwd *(*real_getpwuid)(uid_t) = dlsym(RTLD_NEXT, "getpwuid");
    struct passwd *pwentry;
    pwentry = real_getpwuid(uid);
    if (uid != getuid()) {
        return pwentry;
    }
    if (pwentry == NULL || fake_home == NULL) {
        fprintf(stderr, "Failed to fake home directory.\n");
        return NULL;
    }
    fake = *pwentry;
    fake.pw_dir = fake_home;
    fprintf(stderr, "Faking home directory: %s\n", fake.pw_dir);
    return &fake;
}