linux-kernel - RE: [PATCH v5] tools/nolibc: fix up size inflate regression

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Date:   Mon, 14 Aug 2023 11:15:51 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhangjin Wu' <falcon@...ylab.org>, "w@....eu" <w@....eu>
CC:     "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>,
        "thomas@...ch.de" <thomas@...ch.de>
Subject: RE: [PATCH v5] tools/nolibc: fix up size inflate regression

From: Zhangjin Wu
> Sent: 14 August 2023 11:42
...
> [...]
> > > > Sure it's not pretty, and I'd rather just go back to SET_ERRNO() to be
> > > > honest, because we're there just because of the temptation to remove
> > > > lines that were not causing any difficulties :-/
> > > >
> > > > I think we can do something in-between and deal only with signed returns,
> > > > and explicitly place the test for MAX_ERRNO on the two unsigned ones
> > > > (brk and mmap). It should look approximately like this:
> > > >
> > > >  #define __sysret(arg)                                                \
> > > >  ({                                                                   \
> > > >  	__typeof__(arg) __sysret_arg = (arg);                           \
> > > >  	(__sysret_arg < 0) ? ({           /* error ? */                 \
> > > >  		SET_ERRNO(-__sysret_arg); /* yes: errno != -ret */      \
> > > >  		((__typeof__(arg)) -1);   /*      return -1 */          \

I'm pretty sure you don't need the explicit cast.
(It would be needed for a pointer type.)
Can you use __arg < ? SET_ERRNO(-__arg), -1 : __arg

Thinking, maybe it should be:

#define __sysret(syscall_fn_args)
({
	__typeof__(syscall_fn_args) __rval = syscall_fn_args;
	__rval >= 0 ? __rval : SET_ERRNO(-__rval), -1;
})

Since, IIRC, the usage is return __sysret(sycall_fn(args));

I'm not sure how public SET_ERRO() is.
But it could include the negate have the value of -1 cast to its argument type?
I think:
	error = -(int)(long)(arg + 0u);
will avoid any sign extension - the (int) might not even be needed.

	David

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