linux-kernel - Re: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and _

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Date:   Thu, 8 Jun 2023 18:06:16 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Zhangjin Wu' <falcon@...ylab.org>, "w@....eu" <w@....eu>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and
 __sysret() helpers

Hi David,

On 2023-06-08 14:35:49+0000, David Laight wrote:
> From: Zhangjin Wu
> > Sent: 06 June 2023 09:10
> > 
> > most of the library routines share the same code model, let's add two
> > helpers to simplify the coding and shrink the code lines too.
> > 
> ...
> > +/* Syscall return helper, set errno as -ret when ret < 0 */
> > +static inline __attribute__((always_inline)) long __sysret(long ret)
> > +{
> > +	if (ret < 0) {
> > +		SET_ERRNO(-ret);
> > +		ret = -1;
> > +	}
> > +	return ret;
> > +}
> 
> If that right?
> I thought that that only the first few (1024?) negative values
> got used as errno values.
> 
> Do all Linux architectures even use negatives for error?
> I thought at least some used the carry flag.
> (It is the historic method of indicating a system call failure.)

I guess you are thinking about the architectures native systemcall ABI.

In nolibc these are abstracted away in the architecture-specific
assembly wrappers: my_syscall0 to my_syscall6.
(A good example would be arch-mips.h)

These normalize the architecture systemcall ABI to negative errornumbers
which then are returned from the sys_* wrapper functions.

The sys_* wrapper functions in turn are used by the libc function which
translate the negative error number to the libc-style
"return -1 and set errno" mechanism.
At this point the new __sysret function is used.

Returning negative error numbers in between has the advantage that it
can be used without having to set up a global/threadlocal errno
variable.

In hope this helped,
Thomas