Use of copy_to_user() and copy_from_user() functions
Hi All, To read/write data to user-space from kernel-space we use copy_from_user() and copy_to_user() functions. What is the use of these function? Why kernel can't directly access user address and read/write on to it? Can any one explain why kernel can't directly access the user-space address. Thanks
On Mon, Feb 11, 2013 at 4:51 PM, पारस <beparas@gmail.com> wrote:
Hi All,
To read/write data to user-space from kernel-space we use copy_from_user() and copy_to_user() functions.
What is the use of these function? Why kernel can't directly access user address and read/write on to it? what will happen if the user space process gets scheduled out while you were using it? Can any one explain why kernel can't directly access the user-space address.
Thanks
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On Mon, Feb 11, 2013 at 5:25 PM, anish singh <anish198519851985@gmail.com>wrote:
On Mon, Feb 11, 2013 at 4:51 PM, पारस <beparas@gmail.com> wrote:
Hi All,
To read/write data to user-space from kernel-space we use copy_from_user() and copy_to_user() functions.
What is the use of these function? Why kernel can't directly access user address and read/write on to it? what will happen if the user space process gets scheduled out while you were using it?
copy_to/from_user should always be called from process context. So even if
a process is scheduled out there is not impact, as when process schedule back in copying will resume to/from correct page.
Can any one explain why kernel can't directly access the user-space address.
Thanks
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On Tue, Feb 12, 2013 at 9:24 AM, Chetan Nanda <chetannanda@gmail.com> wrote:
On Mon, Feb 11, 2013 at 5:25 PM, anish singh <anish198519851985@gmail.com> wrote:
On Mon, Feb 11, 2013 at 4:51 PM, पारस <beparas@gmail.com> wrote:
Hi All,
To read/write data to user-space from kernel-space we use copy_from_user() and copy_to_user() functions.
What is the use of these function? Why kernel can't directly access user address and read/write on to it? what will happen if the user space process gets scheduled out while you were using it?
copy_to/from_user should always be called from process context. So even if a process is scheduled out there is >not impact, as when process schedule back in copying will resume to/from correct page.
because page fault will be triggered and the user space will be swapped in. That answers the original question.
Can any one explain why kernel can't directly access the user-space address.
Thanks
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On Mon, Feb 11, 2013 at 4:51 PM, पारस <beparas@gmail.com> wrote:
Hi All,
To read/write data to user-space from kernel-space we use copy_from_user() and copy_to_user() functions.
What is the use of these function? Why kernel can't directly access user address and read/write on to it? Can any one explain why kernel can't directly access the user-space address.
There are a lot resources out there which answer this question adequately in detail (just google for them). I'll give a short answer here: 'User-space' is essentially a particular process' address space, i.e the pages (of physical memory) that have been mapped to this process' virtual memory. Since pages of virtual memory _might_ be swapped out by the kernel, one cannot directly transfer data from the kernel memory to the user mapped memory. For this reason, copy_to/from_user() family of functions is there, which verify first that the user space address (pointed to by the user-space pointer) is valid and accessible and _then_ transfer data to/from it. If the page corresponding to the address pointed to by the user-space pointer has been swapped out, the kernel will first swap in that page and then do the transfer. CMIIW. HTH, -mandeep
Thanks
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anish singh -
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पारस