Hi Prasad, On Sat, Jan 3, 2015 at 9:23 PM, me storage <me.storage126@xxxxxxxxx> wrote: > Hi, > Thanks Valdis for your nice explanation. After practising i got another > doubt > if read from my driver as below > > cat /dev/myDev > > for this how many times read function will call because some times it is > executing only once but some times it is executing infinite times. > Like for write also > echo "hello" > /dev/myDev > for that particular call it is executing 1 or 2 times. > So can one please clarify my doubts Make sure you return the correct amount to be read based on the amount asked and the amount of data available from current value of fpos. If there's nothing to read then return 0 based on the current value of fpos. Doing strace would help. You would see that cat stops if the amount of data read is lesser than it asks for which is usually a page. Try using generic read/write routines and instead override the readpage(s)/writepage(s) of aops. You won't need to handle those checks then. > Thanks & Regards > Prasad > > On 2 January 2015 at 21:32, <Valdis.Kletnieks@xxxxxx> wrote: >> >> On Fri, 02 Jan 2015 07:50:55 +0530, me storage said: >> >> > Can any one please tell me difference between kernal space & user space >> > in >> > code perspective >> >> Two biggies: >> >> 1) Kernel space pages are usually nailed down and not paging in and out, >> this is *not* true for userspace pages (so special tap-dancing in >> copy_(to/from)_user() is needed to make sure no page faults happen). >> >> 2) Data inside the kernel can usually be trusted from a security >> standpoint. >> Data in userspace *MUST NOT* be trusted. Also, beware of TOCTOU (time of >> check / time of use) bugs - that's why we should copy the user-supplied >> data >> to an internal buffer *first*, and then validity-check the buffer - if we >> check >> the value in userspace and then later copy it, there's a race condition >> where >> the userspace value can be changed after the check but before the copy. >> >> Also, keep in mind that a userspace pointer needs to be translated before >> using it to dereference data from kernel space.... > > > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > -- ---P.K.S _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
