On 02/06/2015 11:23 AM, Kees Cook wrote:

Strictly speaking (ISO C, "man 3 errno"), errno is supposed to be a
full int, though digging around I find this in include/linux/err.h:

That doesn't mean the kernel has to support them.

/*
 * Kernel pointers have redundant information, so we can use a
 * scheme where we can return either an error code or a normal
 * pointer with the same return value.
 *
 * This should be a per-architecture thing, to allow different
 * error and pointer decisions.
 */
#define MAX_ERRNO       4095

#ifndef __ASSEMBLY__

#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)

But no architecture overrides this.

We used to have a much lower value, that was per-architecture, in order
to optimize the resulting assembly (e.g. 8-bit immediates on x86).  This
didn't work as the number of errnos increased.  The other motivation was
probably binary compatibility with other Unices, which was an idea for a
while.

	-hpa