Rusty Russell wrote:
That can't happen, since 0xc100000 is not in the kernel address space. 0xc1000000 is though, perhaps that's what you meant?
Yes, it is. Though it doesn't actually make any material difference to my argument.
0x4bccef00 is positive. The correct number is 0xc1000000-0xc0431100 = 0xbcef00So, in this case the %gs base will be loaded with 0xc100000-0xc0431100 = 0x4bccef00A negative offset, exactly, which can't happen, as I said.
The %gs:per_cpu__foo addressing mode still calculates 0xbcef00+0xc0433800, which is still a subtraction. My essential point is that *all* kernel addresses (=kernel symbols) are negative, so using them as an offset from a segment base (any segment base) is a subtraction, which requires a 4G limit.
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