Change read/write permissions on a file descriptor

Question

I'm working on a linux C project and I'm having trouble working with file descriptors.

I have an orphan file descriptor (the file was open()'d then unlink()'d but the fd is still good) that has write-only permission. The original backing file had full permissions (created with S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH), but alas the file was opened with O_WRONLY. Is it possible to duplicate the file descriptor and change the copy to O_RDWR?

psudo-code:

//open orphan file
int fd = open(fname, O_WRONLY, ...)
unlink(fname)
//fd is still good, but I can't read from it

//...

//I want to be able to read from orphan file
int fd2 = dup(fd)
//----change fd2 to read/write???----

Thanks in advance! -Andrew

Schade

fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | O_RDWR)) seems like it would be the thing, except the man page specifically says that won't work. I guess there's some reason the kernel "needs" this to be impossible? – Jennee

so why do you open it in wronly mode if you plan to read it? – Lorrianelorrie

Or just open it again using the correct flags. – Heteronomy

I guess I like to make things difficult for myself. My code doesn't create the orphan file, but reimplementing to wheel is fun, so maybe I'll have to – Schade

I would assume if a file has been unlinked and the only references to it are write-only, the kernel would be perfectly justified in deleting it and replacing it with the equivalent of /dev/null, i.e. discarding all further data written and just keeping a dummy file position. – Roulette

@aschepler: You need to remove the O_WRONLY flag before adding O_RDWR. O_WRONLY|O_RDWR != O_RDWR. – Roulette

@R..: Linux doesn't allow changing fd access mode. The fcntl call may not fail, but any attempt to read that fd will return an error, effectively setting errno to EBADF. – Heteronomy

Odd, how is freopen implemented then? Does it just fail? (It's allowed to fail.) – Roulette

@R..: under Linux the very same rules apply. It isn't allowed to change access modes, and also doesn't return an error (in this case a NULL stream). Any attempt to read/write will cause EBADF too. If you want a reason for not allowing it, imagine changing stdin to allow writes, and stdout to allow reads - nonsense – Heteronomy

In general, it seems to me that linux allows some nonsense things to happen for the sake of flexibility. If there was such a function (which I'm hoping there is) and you did change access modes of stdin, that wouldn't buy you much. However, if there isn't such a function, it would make it inconvenient in situations like this. – Schade

Yes, on Linux you can do this by opening /proc/self/fd/n. See this stackoverflow answer for source code. – Hosbein

no need @Hosbein see my answer. – Argufy

Answer 1

No, there is no POSIX function to change the open mode. You will need to open it in read / write mode. Since you are created a temporary file, though, I strongly recommend that you use mkstemp. That function properly opens the file in read/write mode and unlinks it. Most importantly, it avoids a race condition in naming and creating the file, thereby avoiding a vulnerability in the creation of temporary files.

Siddon

mkstemp doesn't unlink AFAIK? – Lorrianelorrie

what MK said. I can always add O_EXCL to open() and retry on fail – Schade

Answer 2

int fd = open(fname, O_WRONLY, ...)
int fd_ro = open(fname, O_RDONLY, ...)
unlink(fname)
{ write to fd }
close (fd);
read or execute(!) fd_ro

Argufy

add some details please – Ordain