Re: negative seek offsets in VFS

[Andi Kleen <ak@muc.de>]

On Sat, May 28, 2005 at 01:37:31PM +0100, Jamie Lokier wrote:
> Andi Kleen wrote:
> > The linux system call interface on most (all?) architectures
> 
> Actually that's not true on many architectures, including most of the
> 64-bit ones.  See (e.g.) the Alpha, MIPS32, MIPS64, IA64, PPC32 and
> PPC64 versions of <asm/unistd.h>, which use a separate register to
> indicate an error return.
> 
> That's the reason for the `force_successful_syscall_return' macro,
> defined in <linux/ptrace.h>:
> 
> 	/*
> 	 * System call handlers that, upon successful completion, need
> 	 * to return a negative value should call
> 	 * force_successful_syscall_return() right before returning.
> 	 * On architectures where the syscall convention provides for
> 	 * a separate error flag (e.g., alpha, ia64, ppc{,64},
> 	 * sparc{,64}, possibly others), this macro can be used to
> 	 * ensure that the error flag will not get set.  On
> 	 * architectures which do not support a separate error flag,
> 	 * the macro is a no-op and the spurious error condition needs
> 	 * to be filtered out by some other means (e.g., in
> 	 * user-level, by passing an extra argument to the syscall
> 	 * handler, or something along those lines).
> 	 */
> 
> > reserves -1 to -4095 for error returns. When such a error is detected
> > it is converted to errno and -1. This applies to all system
> > calls.
> > 
> > Take a look at unistd.h of your favourite architecture if you
> > dont believe me.
> 
> Most unistd.h's are wrong by now, as they don't test against -4095.

You are right, but at least glibc tests against -4095.

I will fix x86-64.

> 
> >From <asm-generic/errno.h>:
> 
> 	#define	ECANCELED	125	/* Operation Canceled */
> 	#define	ENOKEY		126	/* Required key not available */
> 	#define	EKEYEXPIRED	127	/* Key has expired */
> 	#define	EKEYREVOKED	128	/* Key has been revoked */
> 	#define	EKEYREJECTED	129	/* Key was rejected by service */
> 
> Spot the inconsistency :)

Very nasty indeed.  But fortunately near nobody uses the unistd.h
macros anyways.

-Andi