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Date: Sat, 15 Aug 2020 21:27:00 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Restrictions on child context after multithreaded fork
On Sat, Aug 15, 2020 at 12:25:47PM -0400, Rich Felker wrote:
> On Sat, Aug 15, 2020 at 01:51:00PM +0200, Natanael Copa wrote:
> > xfce4-terminal was more or less completely useless so I had to add a workaround for it in two patches:
> >
> > First uncover a useless setenv. Even the comment in the code says that it has no effect:
> > https://git.alpinelinux.org/aports/commit/community/vte3?id=ad687b01b2a5fa9d53fcd9d0ee3743882f3542b4
> >
> > In the second patch I use some of the hunks in the upstream and replace malloc with alloca:
> > https://git.alpinelinux.org/aports/commit/community/vte3?id=161434fcb87807dae40dffdd332db1624b747bc7
> >
> > After that xfce4-terminal becomes useable again.
>
> I'm not sure whether the alloca is safe. The parent could just do this
> allocation *before fork* rather than waiting til the last minute to do
> it. The data does not change between fork and exec. Note that the glib
> fix cited above did this right.
>
> The hardcoded fallback for fd limit seems like a bad idea, and I don't
> think I understand your fallback sequence. It looks like RLIM_INFINITY
> gets reinterpreted as 4096. I'm not sure how it should be interpreted;
> in principle, probably as INT_MAX+1U. This is again exposing how the
> whole "close-all" idiom is horribly wrong and these libraries should
> just be documenting that you must use O_CLOEXEC correctly if you don't
> want them to leak file descriptors.
Fortunately it looks like Linux doesn't let you set RLIM_INFINITY for
RLIM_NOFILE. The value of the rlimit is limited by sysctl fs.nr_open,
and fs.nr_open is limited between sysctl_nr_open_min (==BITS_PER_LONG)
and sysctl_nr_open_max. The latter is INT_MAX/8*8 on 64-bit archs, and
SIZE_MAX/16*4 on 32-bit archs. But in any case, on Linux, you can rely
on sysctl(_SC_OPEN_MAX) to give an actual meaningful number that fits
in the range of long, not -1/unlimited, and thus the invalid fallback
case is never hit.
Rich
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