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. 0xfffffffffffffffe). Arnd <>< _______________________________________________ Linuxppc-embedded mailing list Linuxppc-embedded@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-embedded