Date: Sun, 16 Aug 2020 14:48:54 +0200 From: Natanael Copa <ncopa@...inelinux.org> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: Restrictions on child context after multithreaded fork On Sat, 15 Aug 2020 21:27:00 -0400 Rich Felker <dalias@...c.org> wrote: > 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. That's why I called it a workaround and not a fix. They did refactor the code but didn't want to backport it. > > > > 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 I think you have good points. Would be good to forward those upstream: https://gitlab.gnome.org/GNOME/vte/-/commit/ed78b9e2ec47675a9dccf045caca4d7a0b6c9fe8 -nc
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