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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Zhangjin Wu' <falcon@tinylab.org>,
        "thomas@t-8ch.de" <thomas@t-8ch.de>
CC:     "arnd@arndb.de" <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
        "w@1wt.eu" <w@1wt.eu>
Subject: RE: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and
 __sysret() helpers
Thread-Topic: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and
 __sysret() helpers
Thread-Index: AQHZmozMSUsfyUxi2UiZnouHjWtcta+CLgRA
Date:   Fri, 9 Jun 2023 09:15:05 +0000
Message-ID: <13d8756927914b578b076babc8b7f7fb@AcuMS.aculab.com>
References: <9edb16b4-0d3f-4355-a7b1-684a28f9b4cb@t-8ch.de>
 <20230609044230.165494-1-falcon@tinylab.org>
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From: Zhangjin Wu
> Sent: 09 June 2023 05:43
> 
> Hi, Thomas, David, Willy
> 
> > 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.
> > >
> 
> Thanks David, this question did inspire me to think about the syscalls
> who returns pointers, we didn't touch them yet:

I'm also not sure whether lseek() is expected to return values
that would be negative.

(I do remember having to patch out some checks (not Linux) in order to use:
    echo -n xxxx | dd of=/dev/kmem oseek=nnn
in order to patch a live kernel!)

Technically read() and write() can do longer transfers, but
Linux limits them to MAXINT.
IIRC both BSD and SYSV allow drivers return all values (except -1)
form ioctl().

The check for -4095UL is probably reasonable.

	David

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