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kernel: fix errno access for user mode

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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>
This commit is contained in:
Andrew Boie 2018-07-19 11:09:33 -07:00 • committed by Andrew Boie
commit 7f4d006959
6 changed files with 69 additions and 5 deletions
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8
include/kernel.h
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@ -538,9 +538,17 @@ struct k_thread {
#endif
#ifdef CONFIG_ERRNO
#ifdef CONFIG_USERSPACE
/* Set to the lowest area in the thread stack since this needs to
* be directly read/writable by user mode. Not ideal, but best we
* can do until we have thread-local storage
*/
int *errno_location;
#else
/** per-thread errno variable */
int errno_var;
#endif
#endif
#if defined(CONFIG_THREAD_STACK_INFO)
/** Stack Info */