ot memory allocation question
Don Raikes
don.raikes at oracle.com
Fri Jul 19 05:53:23 EDT 2013
Sorry folks, but I still have a question about all this kernel programming stuff.
Is it possible from within a kernel module to allocate memory in userspace?
The reason I am asking is the function I am hooking into sys_write takes a buffer as input, but the buffer is defined as const __user *.
To me this means that if I modify the text that I was given in the buffer and want it to get written to the file, then I need to update the buffer.
However since the buffer is a const, I shuldn't write to it, plus in my case I am enlarging the amount of data, so if I were to cast the buffer to a writable __user *, then I am in danger of overwriting something I shouldn't in the user process (which in my case is the file pointer).
After completing the sys_write by passing the updated buffer to the original sys_write, I try to close the file and get an error.
My solution:
Allocate a new larger buffer inside of the userspace and copy_to_user into the new buffer and then when I pass control to the "real" sys_write function point it to the new buffer.
But the problem is how do I allocate this new buffer?
--
Best Regards, Donald
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