Re: negative seek offsets in VFS

[Jamie Lokier <jamie@shareable.org>]

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.

Example, from <asm-x86_64/unistd.h>:

	if ((unsigned long)(res) >= (unsigned long)(-127)) { \
		errno = -(res); \
		res = -1; \
	} \
	return (type) (res); \

>From <asm-x86_64/errno.h>:

	#include <asm-generic/errno.h>

>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	#define	EKEYREVOKED	128	/* Key has been revoked */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 :)

x86_64 tests against -127; SH64 against -125; etc. most of the ones I
looked at are wrong, they're different from each other (despite using
<asm-generic/errno.h>), and most of them have a comment preceding the
test which states yet another different wrong number.

Example, from <asm-s390/unistd.h>:

	/* user-visible error numbers are in the range -1 - -122:
	   see <asm-s390/errno.h> */

	#define __syscall_return(type, res)			     \
	do {							     \
		if ((unsigned long)(res) >= (unsigned long)(-125)) { \

-- Jamie