The errno "variable" is required to be thread-specific.
It gets defined to a macro which dereferences a pointer
returned by a kernel function.
In user mode, we cannot simply read/write the thread struct.
We do not have thread-local storage mechanism, so for now
use the lowest address of the thread stack to store this
value, since this is guaranteed to be read/writable by
a user thread.
The downside of this approach is potential stack corruption
if the stack pointer goes down this far but does not exceed
the location, since a fault won't be generated in this case.
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
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