Re: [PATCH v5 3/5] x86: Split syscall_trace_enter into two phases

["H. Peter Anvin" <hpa@zytor.com>]

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