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Message-ID: <20200424141239.GE11469@brightrain.aerifal.cx>
Date: Fri, 24 Apr 2020 10:12:39 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Invalid pointer subtractions in __shlim and __shgetc
On Fri, Apr 24, 2020 at 09:40:15AM +0000, Pascal Cuoq wrote:
> Hello,
>
> Rich Felker <dalias@...c.org> wrote:
> > The attached should fix it, I think.
>
> The patch sets f.rpos and f.rend to buf+4, but it also leaves
> f.buf containing 0 from “FILE f = {0};”:
> --- a/src/stdlib/wcstol.c
> +++ b/src/stdlib/wcstol.c
> @@ -35,8 +35,7 @@ static unsigned long long wcstox(const wchar_t *s, wchar_t **p, int base, unsign
> unsigned char buf[64];
> FILE f = {0};
> f.flags = 0;
> - f.rpos = f.rend = 0;
> - f.buf = buf + 4;
> + f.rpos = f.rend = buf + 4;
> f.buf_size = sizeof buf - 4;
> f.lock = -1;
> f.read = do_read;
>
> Unfortunately, the function __shlim also subtracts f.rpos from f.buf, at this line:
>
> f->shcnt = f->buf - f->rpos;
Uhg, this was purely a mechanical error in the edit (selecting too
much text to delete) and I should have tested before sending. Should
be:
- f.rpos = f.rend = 0;
- f.buf = buf + 4;
+ f.rpos = f.rend = f.buf = buf + 4;
> (https://git.musl-libc.org/cgit/musl/tree/src/internal/shgetc.c?id=33338ebc853d37c80f0f236cc7a92cb0acc6aace#n11 )
>
> So that is now where the invalid subtraction happens.
>
> For what it's worth, we have tested the patch consisting in
> initializing all three of f.rpos, f.rend and f.buf to buf+4, and that
> does not cause UB in this test. But we can't tell if if provides the
> correct functional behavior for this test and for other inputs.
Yep, that's what I intended. Sorry for wasting your time with a bad
patch.
Rich
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