Hi, Dinesh Gandhewar wrote: > Hello, > Can you explain this code segment? > > oldfs = get_fs(); set_fs(KERNEL_DS); > err = file.f_op->read(&file, buf, *count, &file.f_pos); > set_fs(oldfs); > > What is the functionality of get_fs() and set_fs(). > > Thanking you, > Dinesh See http://www.linux.it/kerneldocs/ksys/ksys.html, section 'doing it in kernel space', which gives a similar example, and explains the code (especially the use of get_fs() and set_fs()). Summarizing, what happens is that set_fs(KERNEL_DS) sets some 'memory access information' such, that the validation of the addresses in the subsequent file.f_op->read(&file, buf, *count, &file.f_pos) succeeds, even though (some of) the arguments refer to kernel space addresses. get_fs() saves the old 'memory access information' in oldfs, in order to restore the memory access information to the old value after invoking file.f_op->read(&file, buf, *count, &file.f_pos. regards Martin -- Supercomputing System AG email: maletinsky@scs.ch Martin Maletinsky phone: +41 (0)1 445 16 05 Technoparkstrasse 1 fax: +41 (0)1 445 16 10 CH-8005 Zurich - Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
