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From: Denys Vlasenko <vda.linux@googlemail.com>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: DASH shell mailing list <dash@vger.kernel.org>
Subject: Re: [PATCH] parser: Fix VSLENGTH parsing with trailing garbage
Date: Tue, 22 Jun 2021 10:34:49 +0200	[thread overview]
Message-ID: <CAK1hOcOZv1Ze3YAv15oF6J9=i5BDhaADRbdWgabrho7OyS==2Q@mail.gmail.com> (raw)
In-Reply-To: <20210622001910.GA11745@gondor.apana.org.au>

On Tue, Jun 22, 2021 at 2:19 AM Herbert Xu <herbert@gondor.apana.org.au> wrote:
> On Mon, Jun 21, 2021 at 04:21:40PM +0200, Denys Vlasenko wrote:
> > > -                       } while (!subtype && is_digit(c));
> > > +                       } while ((subtype <= 0 || subtype >= VSLENGTH) &&
> > > +                                is_digit(c));
> >
> > ... you use (subtype == 0 || subtype == VSLENGTH) here.
> > Also, (subtype == 0 || subtype == VSLENGTH) is less confusing:
> > it says "loop if ${VAR} or ${#VAR} syntax", whereas <= >=
> > are a bit misleading.
>
> Yes it looks a bit confusing, but it turns into a single branch
> instead of two.

Yes, I know that. Compiler turns it into "(unsigned)(x-1) >= VSLENGTH-1"
expression.

But is it worth the obfuscation? Especially that it also has another
downside (it requires an additional free CPU register to hold (x-1)
result, which can force compiler to spill other values to stack).

      reply	other threads:[~2021-06-22  8:35 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-19 12:44 SEGV parsing of ${#1\x82} and ${#1\x84} Denys Vlasenko
2021-06-21  9:57 ` [PATCH] parser: Fix VSLENGTH parsing with trailing garbage Herbert Xu
2021-06-21 14:21   ` Denys Vlasenko
2021-06-22  0:19     ` Herbert Xu
2021-06-22  8:34       ` Denys Vlasenko [this message]

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