file position after error from read()

file position after error from read()

Posted Nov 11, 2004 0:39 UTC (Thu) by jreiser (subscriber, #11027)
In reply to: Some Linux kernel security vulnerabilities by jwb
Parent article: Some Linux kernel security vulnerabilities

And, if read returns -1, there's absolutely nothing you can do about it without closing the fd and starting from scratch (because the file position becomes undefined).

In most cases (EAGAIN, EISDIR, EBADF, EINVAL, EFAULT, and non-POSIX EINTR) you can interpret errno and resume. Only for EIO or for POSIX EINTR is there the possibility of undefined position, and some of that is due to POSIX allowing the kernel a choice of what to do with EINTR. As long as the fd is seekable and the error condition is transient, then the program can recover by seeking to any previous known-good position [the program must track such positions] and resuming. Also, if the fd is seekable then the current position can be determined using lseek(fd, (off_t)0, SEEK_CUR). All in all, that is a long way from being forced to close the fd and start from scratch. In practice, read() on a disk file is very well behaved, especially for reads of 1 sector. [Reading from a socket is different.]

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