readme.md

Purpose

Demonstrate an odd effect that holding onto a file handle can still allow you to edit the contents, of that file even after its permissions have been changed, despite even invoking .Sync() or *flush()

Tested on:

• Linux 4.15.0-147-generic #151-Ubuntu SMP
• Darwin 19.6.0 root:xnu-6153.141.2~1/RELEASE_X86_64 x86_64

Results

C

$ gcc -o demo demo.c && ./demo 
Unfortunately mismatched content
Got:  abcdefghijkl
Want: abcdef

Go

$ go run demo.go 
Unfortunately mismatched content
Got:  abcdefghijkl
Want: abcdef
exit status 1

Python

$ python3 demo.py 
Unfortunately mismatched content
Got:  abcdefghijkl
Want: abcdef

demo.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/errno.h>
#include <sys/stat.h>
#include <sys/types.h>

int main() {
    char *name = "the_file_c.txt";
    FILE *wf = fopen(name, "w");
    if (wf == NULL) {
        printf("\033[31mFailed to open file: %s: %s\033[00m", name, strerror(errno));
        exit(1);
    }

    int n = fprintf(wf, "abcdef");
    if (n != 6) {
        fclose(wf);
        printf("\033[31mShort write: got %d want 6\033[00m", n);
        exit(1);
    }

    // Flush the stream
    fflush(wf);

    // Change the permission to be ONLY "-r--r--r--"
    if ((n = chmod(name, 0444)) != 0) {
        fclose(wf);
        printf("\033[31mFailed to chmod the file: %s\033[00m", strerror(n));
        exit(1);
    }

    // Write more content to the file.
    fprintf(wf, "ghijkl");
    fflush(wf);

    // Revert the permission change.
    if ((n = chmod(name, 0755)) != 0) {
        fclose(wf);
        printf("\033[31mFailed to revert permissions, got: %s\033[00m", strerror(n));
        exit(1);
    }
    fclose(wf);

    struct stat *st;
    st = (struct stat*)malloc(sizeof(*st));
    if (stat(name, st) != 0) {
        free(st);
        printf("\033[31mFailed to stat the file: %s\033[00m", strerror(errno));
        exit(1);
    }

    size_t file_size = st->st_size;
    free(st);

    char *got = (char *)malloc(file_size);
    if (got == NULL) {
        printf("\033[31mMalloc failed: %s\033[00m", strerror(errno));
        exit(1);
    }

    // Reopen it for purely reading.
    FILE *rf = fopen(name, "r");

    if (fread(got, sizeof(*got), st->st_size, rf) < 0) {
        free(got);
        fclose(rf);
        printf("\033[31mFailed to read file: %s\033[00m", strerror(errno));
        exit(1);
    }

    fclose(rf);

    const char *ideal_want = "abcdef";
    if (strcmp(got, ideal_want) != 0) {
        printf("\033[31mUnfortunately mismatched content\nGot:  %s\nWant: %s\n\033[00m",
                got, ideal_want);
        free(got);
        exit(1);
    }

    free(got);

    return 0;
}

demo.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
)

func main() {
	name := "the_file_go.txt"
	f, err := os.OpenFile(name, os.O_CREATE|os.O_WRONLY, 0755)
	if err != nil {
		panic(err)
	}
	defer f.Close()

	if _, err := f.Write([]byte("abcdef")); err != nil {
		panic(err)
	}
	if err := f.Sync(); err != nil {
		panic(err)
	}

	// Change the permission to be ONLY "-r--r--r--"
	if err := os.Chmod(f.Name(), 0444); err != nil {
		panic(err)
	}

	// Write more content to the file.
	if _, err := f.Write([]byte("ghijkl")); err != nil {
		panic(err)
	}
	if err := f.Sync(); err != nil {
		panic(err)
	}

	// Revert the permission change.
	if err := os.Chmod(f.Name(), 0755); err != nil {
		panic(err)
	}

	idealWant := "abcdef"
	got, err := ioutil.ReadFile(f.Name())
	if err != nil {
		panic(err)
	}
	if string(got) != idealWant {
		fmt.Printf("\033[31mUnfortunately mismatched content\nGot:  %s\nWant: %s\033[00m\n", got, idealWant)
                os.Exit(1);
	}
}

demo.py

#!/usr/bin/env python3

import os
import sys

def main():
    name = 'the_file_py.txt'
    with open(name, mode='w') as wf:
        wf.write('abcdef')
        wf.flush()
        # Change the permissions to be ONLY "-r--r--r--"
        os.chmod(wf.name, 0o444)
        wf.write('ghijkl')
        wf.flush()

    # Revert the permission change
    os.chmod(name, 0o755)

    # Next let's try to read all the content.
    ideal_want =  'abcdef'

    with open(name, mode='r') as rf:
        got =  rf.readlines()[0].strip('\n')
        if got != ideal_want:
            print('\033[31mUnfortunately mismatched content\nGot:  %s\nWant: %s\033[00m\n' % (got, ideal_want))
            sys.exit(1)

if __name__ == '__main__':
    main()