On Tue, 15 Mar 2005, Robert Hancock wrote:
linux-os wrote:
The attached file shows that the kernel thinks it's doing something helpful by checking the length of the input buffer for a read(). It will return "Bad Address" until the length is 1632 bytes. Apparently the kernel thinks 1632 is a good length!
Likely because only 1632 bytes of memory is accessible after the start of the buf buffer, and trying to read in more than that results in copy_to_user failing to write some data.
There was NO DATA read or written! The read() call returns immediately without reading anything. Look at the code, assume nothing. This is a blocking read from standard-input.
Did anybody consider the overhead necessary to do this and the fact that the kernel has no way of knowing if the pointer to the buffer is valid until it actually does the write. What was wrong with copy_to_user()? Why is there the additional bogus check?
What additional check?
Somebody added some very dumb check of the input value of a read() length that occurs before anything is actually read.
Previously, a read(), which is a kernel write to user-data space, would seg-fault if the read exceeded the data-space that had been mapped. It is done by the CPU, it generates a trap. The performance cost, when done by the CPU, if the data doesn't exceed bounds, is zero. Now, there is a beginning check (a wrong check BTW), in software, before the data is even obtained from the device.
-- Robert Hancock Saskatoon, SK, Canada To email, remove "nospam" from [EMAIL PROTECTED] Home Page: http://www.roberthancock.com/
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