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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