RE: [PATCH v2 1/4] tools/nolibc: sys.h: add __syscall() and __sysret() helpers

[David Laight <David.Laight@ACULAB.COM>]

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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