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Date: Mon, 12 Jun 2017 10:30:12 +0200
From: u-uy74@...ey.se
To: musl@...ts.openwall.com
Subject: Re: more fun with non-Linux Linux ABI
On Sun, Jun 11, 2017 at 05:50:20PM -0400, Rich Felker wrote:
> On Sun, Jun 11, 2017 at 08:01:58PM +0200, u-uy74@...ey.se wrote:
> > FWIIW: when running under Linux ABI on FreeBSD
> > in musl-linked programs the child processes segfault right after a
> > vfork(), before doing anything else:
>
> My first guess is that this is a FreeBSD bug...
I agree.
> > Remarkably this apparently does not affect glibc-based builds
> > (I have not tested right now but otherwise it would have been known).
> > Wonder what makes the difference.
>
> Is it possible that FreeBSD's Linux syscall emulation uses the
> userspace stack to store some state during syscalls? For example maybe
I am not sufficiently familiar with the FreeBSD internals to tell this.
The linux_fork() and linux_vfork() functions are about 30 lines each
and the only difference is
--- fork
+++ vfork
...
struct fork_req fr;
...
bzero(&fr, sizeof(fr));
- fr.fr_flags = RFFDG | RFPROC | RFSTOPPED;
+ fr.fr_flags = RFFDG | RFPROC | RFMEM | RFPPWAIT | RFSTOPPED;
fr.fr_procp = &p2;
if ((error = fork1(td, &fr)) != 0)
return (error);
...
I guess the musl vs glibc difference can happen if the latter implements
vfork() in terms of clone() (?)
linux_clone() has a much larger implementation in FreeBSD than
linux_vfork() which could explain why the one works and the other
does not.
> > Otherwise a simple workaround would be an option to make vfork()
> > a fork() synonym while building musl. (I do the this at applications
> > build time instead, which helps.)
> >
> > Such an option would most probably result in a pretty small performance
> > impact on modern (native) Linux.
>
> It's actually a pretty large impact; recent (4.x+ IIRC) versions of
> GNU makes are considerably slower because they dropped use of vfork
> and switched to fork rather than making it use posix_spawn like it
> should.
Thanks, good to know. Then I have to live with this impact on Linux
or convince the FreeBSD team to fix linux_vfork().
Rune
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