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

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Date:   Mon, 14 Aug 2023 12:27:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>
CC:     'Zhangjin Wu' <falcon@...ylab.org>,
        "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: Willy Tarreau
> Sent: 14 August 2023 13:10
> 
> Hi David,
> 
> On Mon, Aug 14, 2023 at 11:15:51AM +0000, David Laight wrote:
> > 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;
> > })
> 
> Yeah almost, since arg is necessarily signed in this version, it's
> just that I manually edited the previous macro in the mail and limited
> the amount of changes to what was necessary. It's just that SET_ERRNO
> only is an instruction, not an expression:
> 
>    #define SET_ERRNO(v) do { errno = (v); } while (0)
> 
> Thus the return value doesn't even pass through it. That's why it was
> so much simpler before. The rationale behind this was to bring the
> ability to completely drop errno for programs where you didn't care
> about it. It's particularly interesting when you don't need any other
> data either as the program gets strunk from a complete section.

Actually something like:

#define SET_ERRNO(v) (errno = -(long)(v), __typeof__(v)-1)

seems to work and allows the errno assignment be removed.
Also works for pointer types (after a different compare).

A quick check with godbolt doesn't show any sign extensions happening.

	David

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