[PATCH] linux: spawni.c: simplify error reporting to parent

[Rich Felker]

On Tue, Sep 20, 2016 at 11:01:00PM +0200, Rasmus Villemoes wrote:
> Using VFORK already ensures that the parent does not run until the
> child has either exec'ed succesfully or called _exit. Hence we don't
> need to read from a CLOEXEC pipe to ensure proper synchronization - we
> just make explicit use of the fact the the child and parent run in the
> same VM, so the child can write an error code to a field of the
> posix_spawn_args struct instead of sending it through a pipe.
> 
> To ensure that this mechanism really works, the parent initializes the
> field to -1 and the child writes 0 before execing.
> 
> This eliminates some annoying bookkeeping that is necessary to avoid
> the file actions from clobbering the write end of the pipe, and
> getting rid of the pipe creation in the first place means fewer system
> calls (four in the parent, usually one in the child) and fewer
> chanches for the spawn to fail (e.g. if we're close to EMFILE).

This is a bad idea for at least one reason: running under strace seems
to cause vfork _not_ to wait in the parent, leading to stack
clobbering and runaway wrong code execution. I have not tested lately
so I don't have a recipe to reproduce it, but I know this was one of
the motivations for musl's use of a pipe.

Rich