.gitignore

preload-statfs.so
test-preload-statfs

Makefile

CFLAGS := -Wall -Wextra -O2 -pipe

BINS := preload-statfs.so test-preload-statfs

default: all

all: $(BINS)

preload-statfs.so: preload-statfs.c
	$(CC) -o $@ $(CFLAGS) -shared -fPIC -ldl $<

test-preload-statfs: test-preload-statfs.c
	$(CC) -o $@ $(CFLAGS) $<

clean:
	rm -f preload-statfs.so test-preload-statfs

.PHONY: clean

check: $(BINS)
	./test-preload-statfs /
	./test-preload-statfs /tmp
	if ./test-preload-statfs /thisshouldnotexist; then false; else true; fi
	if LD_PRELOAD=./preload-statfs.so ./test-preload-statfs /; then false; else true; fi
	LD_PRELOAD=./preload-statfs.so ./test-preload-statfs /tmp
	if LD_PRELOAD=./preload-statfs.so ./test-preload-statfs /thisshouldnotexist; then false; else true; fi

.PHONY: check

preload-statfs.c

#define _GNU_SOURCE

#include <stdlib.h>
#include <errno.h>
#include <dlfcn.h>
#include <string.h>
#include <sys/vfs.h>

typedef int (*statfs_proto)(const char *path, struct statfs *buf);

static statfs_proto real_statfs = NULL;

int statfs(const char *path, struct statfs *buf) {
        if(real_statfs == NULL) {
                real_statfs = (statfs_proto)dlsym(RTLD_NEXT, "statfs");
        }

        if(strlen(path) == 1 && path[0] == '/') {
                /* Emulate error: "The filesystem does not support this call." */
                errno = ENOSYS;
                return -1;
        }
        else {
                return real_statfs(path, buf);
        }
}

test-preload-statfs.c

#include <stdio.h>
#include <sys/vfs.h>

int main(int argc, char **argv) {
        struct statfs buf = { 0 };
        int res = 0;

        if(argc != 2) {
                fprintf(stderr, "Required argument\n");
                return 1;
        }

        res = statfs(argv[1], &buf);
        if(res < 0) {
                perror("statfs");
                return 1;
        }

        printf("Success!\n");
        return 0;
}