Re: Understanding of write file operation in char driver

[me storage <me.storage126@xxxxxxxxx>]

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

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....

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