problems with the open() function in different file permissions - c

I am implementing the virtual disk system in C, which includes handling the file system as well. I just want to know, why the open function in C returns -1 when I try to open a file with some group permission or otherspermissions.
Lets say we have file mode that is 040 (Read permission bit for the group owner of the file):
int main(){
int filedes;
filedes = open(filename, O_RDWR, 040);
if(filedes < 0)
return -1;
printf("Open success\n");
}
This snippet return without printing open success. Where this code with file mode 0644 works perfectly fine
int main(){
int filedes;
filedes = open(filename, O_RDWR, 0644);
if(filedes < 0)
return -1;
printf("Open success\n");
}
I don't understand why is this happening?

040 specifically disallows the owner of said file to do anything with it. Even though your group can, you've explicitly defined that your own user can't use it. It may seem weird, but OS only does what you've told it to do.

Related

reopen a directory using openat

As it seems, it is possible to use openat() to reopen an already opened directory. For instance on my Linux system I can do the following:
#define _GNU_SOURCE
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int main(void) {
int fd1 = open(".", O_PATH);
if (fd1 == -1) {
perror("open");
return 1;
}
int fd2 = openat(fd1, ".", O_RDONLY);
if (fd2 == -1) {
perror("openat");
close(fd1);
return 1;
}
close(fd1);
// do fancy things with fd2, now opened
// with access mode read-only
return 0;
}
I could not find this documented anywhere and it feels a bit like an edge case. Also I didn't find other code doing this. Is this well-defined behavior?
EDIT: changed the title: file -> directory
This is just the same as calling open twice on the same file, which you are allowed to do:
int fd1 = open("filename", flags1);
int fd2 = open("filename", flags2);
where filename refers to an existing file (of any type) and flags1 and flags2 are any set of O_ flags that can be validly applied to that type of file and won't destroy its contents. (In particular, we assume that they do not include O_CREAT, O_TRUNC, or O_EXCL.)
fd1 and fd2 will refer to separate "open file descriptions", so for instance lseek on one will not affect the other, flock on one will block flock on the other, etc.
With openat(), the first argument, fd, should be the file descriptor for a directory — such as the one you obtained from opening "." — or the special value AT_FDCWD (which means open relative paths relative the current directory). Note that the O_PATH option you use is a Linux-only extension to openat().
So, because you're using a valid file descriptor for a directory, the call to openat() should succeed. You now have two file descriptors both pointing (independently — with separate open file descriptions) to the current directory. In general, it is possible to open the same file multiple times in a single process (or in multiple processes — ensuring access by a single process is actually very hard on Unix-like (POSIX) systems).
There isn't a lot else you can do with those descriptors other than use them in *at() system calls. Either of the file descriptors would have been sufficient; opening both was overkill.

Using stat() after fopen() to Avoid TOCTOU Problems?

Title says it all: can one use stat() after fopen() to avoid Time of Check to Time of Use (TOCTOU) race conditions?
Some details:
I am writing a C program that only reads files, but needs to error properly when asked to read a directory. As of right now, it uses open() (with O_RDWR) to generate an error and then checks errno for EISDIR, like so:
int fd = open(path, O_RDWR);
if (fd == -1) {
if (errno == EISDIR) return PATH_IS_DIR;
else return FILE_ERR;
}
The problem with the above solution is that this program only needs to read files, so by opening a file with O_RDWR, I might wrongly get a permissions error if the user has read permissions, but not write permissions.
Is it possible to do the following to avoid TOCTOU race conditions?
struct stat pstat;
FILE *f = fopen(path, "r");
if (!f) return FILE_ERR;
if (stat(path, &pstat) == -1) {
fclose(f);
return FILE_ERR;
}
if (S_ISDIR(pstat.st_mode)) {
fclose(f);
return PATH_IS_DIR;
}
If it is not possible, is there another solution to prevent TOCTOU bugs and also wrong permission errors?
No, the code presented in the question does not avoid a TOCTOU race.
Testing after use is prone to exactly the same errors as testing before use. In both cases, the name is resolved at two different times, with possibly different results. This is the cause of the race, and it can happen whichever access happens first.
The only way to avoid this is to open the file once, and use the file descriptor so obtained for any other checks you need. Modern OSes provide interfaces such as fstat() for exactly this purpose.
If you want to use C's buffered I/O, you can get the file descriptor from a FILE* using fileno() - or you can create a FILE* from a file descriptor using fdopen().
It requires a very small change to your code:
# Untested
struct stat pstat;
FILE *f = fopen(path, "r");
if (!f) return FILE_ERR;
if (fstat(fileno(f), &pstat) == -1) {
// ^^^^^^^^^^^^^^^ <-- CHANGED HERE
fclose(f);
return FILE_ERR;
}
if (S_ISDIR(pstat.st_mode)) {
fclose(f);
return PATH_IS_DIR;
}
EDIT (2018-10-25): Toby Speight's answer is better.
There is a solution: use open(), then
fstat().
An example:
struct stat pstat;
int fd = open(path, O_RDONLY);
if (fd == -1) return FILE_ERR;
if (fstat(fd, &pstat) == -1) {
close(fd);
return FILE_ERR;
}
if (S_ISDIR(pstat.st_mode)) {
close(fd);
return PATH_IS_DIR;
}
I found this while checking that I had covered all of my bases before asking this question.

C open() function fails. Are my parameters wrong?

This function fails to open the file. Are my parameters wrong or what could be causing this problem?
int CreateFile(const char *filename){
char filepath[strlen(filename) + 3];
sprintf(filepath, "./%s", filename);
int fd = open(filepath, O_CREAT, O_APPEND, S_IWGRP);
if(fd == -1) printf("file read failed\n");
return fd;
}
Xcode prints only "file read failed" to the console. I tried to run this via Terminal aswell but that didn't help either.
I fixed an issue pointed by NetMage:
int CreateFile(const char *filename){
char filepath[strlen(filename) + 3];
sprintf(filepath, "./%s", filename);
int fd = open(filepath, O_CREAT|O_APPEND, S_IWGRP);
if(fd == -1) printf("file read failed\n");
return fd;
}
Unfortunately that didn't fix the issue
Step 1 - Verify that filepath is being set correctly, either by printing it to the terminal or examining it in a debugger.
Step 2 - Verify that the file exists in that path, and that its permissions are set so that you can open it. If filepath is "./foo", then a file named foo had better exist in the current working directory (the directory from which you ran the program), and it needs to have at least read permission.
Step 3 - If the file does not exist, verify that you have permission to create new files in the current working directory.
Step 4 - If after doing all of that you still get an error, check errno. It will give you some additional information beyond "it didn't work."
#include <errno.h>
...
if(fd == -1)
{
switch( errno )
{
case EACCESS: // permission issues
handle_permission_issue();
break;
case EEXIST: // file already exists and you used O_CREAT and O_EXCL
handle_already_exists_issue();
break;
case EFAULT: // bad path
handle_bad_path_issue()
break;
...
}
printf("file read failed\n");
}
NetMage has pointed out one problem - your flags need to be bitwise-OR'd together, rather than listed as separate arguments. Surprised the compiler didn't yell at you over that.
The open function takes only one parameter for oflags, which must be bit-ored together:
#include <errno.h>
#include <string.h>
int fd = open(filepath, O_CREAT|O_APPEND, S_IWGRP);
if (fd == -1) printf("file read failed: %s\n", strerror(errno));
Per the POSIX documentation for open() (somewhat reformatted, and note the bolded text):
SYNOPSIS
#include <sys/stat.h> #include <fcntl.h>
int open(const char *path, int oflag, ...);
...
Values for oflag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in . Applications shall
specify exactly one of the first five values (file access modes)
below in the value of oflag:
O_EXEC
Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.
O_RDONLY
Open for reading only.
O_RDWR
Open for reading and writing. The result is undefined if this flag is applied to a FIFO.
O_SEARCH
Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.
O_WRONLY
Open for writing only.
...
You need to include at least one of those five flags, perhaps like:
int fd = open(filepath, O_WRONLY|O_CREAT|O_APPEND, S_IWGRP);
Note that other failures may still occur. As noted in the comments, you're prepending "./" to the file name, which may cause problems if, for example, you get passed "/tmp/filename" and the tmp directory doesn't exist in your current working directory, as open() will not create missing directories in any path.

C: cannot create or append to a file

I am trying to open a file for both read and write operations.
If the file is already there, it should append. (I want to be able to write to it, and maybe read from it later)
However, if the file is there, I cannot append to it (I get a permission denied: cannot create file)
int main()
{
int file;
file = open("redirect.txt", O_RDWR | O_APPEND | O_CREAT, 777);
if(!(file == -1)) //edited per comment
{
close(file);
}
else
perror("File could not be created\n");
return 0;
}
This only opens a new file if it does not exist, but does not append to an existing file if it does exist.
You're forgetting that the mode parameter to open() must be in octal. This will work:
file = open("redirect.txt", O_RDWR | O_APPEND | O_CREAT, 0777);
As zwol also mentioned, it's generally a good idea to create files with 0666 (since they don't need to be executable).

How to set output file mode permissions (rw-rw-rw-)

I've created a code that should be able to copy a file a user suggests. What I am wondering is this: how do I set the output file mode and how do I determine what the output file mode permissions will be in this code?
#include <stdio.h>
#include <stdlib.h>
int main()
{
char c;
char source_file, target_file;
FILE *in, *out;
printf("Enter name of file to copy\n");
gets(source_file);
printf("Enter name of file to copy to\n");
gets(target_file);
in = (source, O_RDONLY);
out = (target_file, O_CREAT|WRONLY, S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH);
/* error handing */
if( in == NULL )
{
printf("Error. \n");
exit(0);
}
printf("Enter the copied file name \n");
gets(target_file);
out = fopen(target_file, "w");
/*error handing*/
if( out == NULL )
{
fclose(in);
printf("File opening error.\n");
exit(0);
}
while(( c = fgetc(in) ) != EOF )
fputc(c,out);
fclose(in);
fclose(out);
return 0;
}
Controlling file permissions using standard I/O
One of the demerits of the standard I/O library is that you can't control the permissions on the files that are created, primarily because such permissions are rather platform-specific (more so than the C standard allows for, anyway). The POSIX open() function allows you to control the permissions on the file as it is created.
With a POSIX-like system, you can use the chmod() or fchmod() system calls. You need to know that your rw-rw-rw- pattern is octal 0666.
chmod(target_file, 0666);
fchmod(fileno(out), 0666);
The functions can fail; you should check that they don't.
You can also use the umask() function or (with care) the umask command to influence the default permissions. For example, setting umask 022 in the shell means that files will not be created that are writable by group or others.
Revising the modified code
You don't need to worry about the permissions on a file you open for reading (or, at least, you seldom need to do so).
Worrying about the permissions on the file you write to is more normal.
Your current code proposal is:
in = (source, O_RDONLY);
out = (target_file, O_CREAT|WRONLY, S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH);
This does not invoke open(), and assigns an integer value to the two FILE * variables, which should be generating compiler warnings. Note that the comma expressions evaluate the LHS and then the RHS of the expression, yielding the RHS as the overall value. O_RDONLY is classically 0; the combination of S_IRUSR etc terms is not zero.
If you're going to open the file with those options, then you need something like:
int fd_i = open(source_file, O_RDONLY);
if (fd_i < 0)
…report error opening source_file…
FILE *in = fdopen(fd_i, "r");
if (in == 0)
…report error creating file stream for source_file…
int fd_o = open(target_file, O_CREAT|WRONLY, S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH);
if (fd_o < 0)
…report error opening target_file…
FILE *out = fdopen(fd_o, "w");
if (out == 0)
…report error creating file stream for target_file…
However, I would probably not use fdopen() for the input file — I'd use fopen() directly as you did originally — but I might use it for the output file.

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