Regarding select() on PPC

Arnd Bergmann arnd at
Wed Sep 24 16:45:51 EST 2008

On Wednesday 24 September 2008, Benjamin Herrenschmidt wrote:
> Why do we want it ?
> It's an int, it's zero extended, but that should still be a valid signed
> 32 bits int in compat_sys_select() or do I miss something ?

IIRC, the calling conventions on 64 bit ppc assume that a signed int
argument is sign-extended into a 64 bit register. The compat syscall
entry point does not know which registers are signed or unsigned, so
it will always to zero-extend, making the register contain an undefined
bit pattern (e.g. 0x00000000fffffffe), which may be interpreted as being
a positive number. An explicit cast as it is done in ppc32_select turns
this into a well-defined 32-bit number stored in a 64-bit register (e.g.

	Arnd <><

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