[PATCH 0/5] posix_spawn fast path

[Corinna Vinschen]

On Jun 26 17:12, Jeremy Drake via Cygwin-patches wrote:
> On Thu, 26 Jun 2025, Jeremy Drake via Cygwin-patches wrote:
> 
> > I *thought* this was all finally good to go, but while working on
> > posix_spawn tests today I found that it does not properly handle the case
> > where an (f)chdir file action is present and the path/file argument is
> > relative.  In that case, it should be relative to the child's cwd, not the
> > parent's.  As a result, I have left out a patch to actually hook up the
> > (f)chdir actions in the fast path, so it will continue to fall back to
> > fork/exec in that case.
> >
> > I *think* it would need a find_exec overload that uses a specified cwd, as
> > well as perhaps_suffix.  I am not sure if av::setup needs to know about
> > this or not.  It takes the path_conv real_path that came out of
> > perhaps_suffix, and only uses the prog_arg in the case of a script
> > replacing argv[0].  By the time the interpreter is run, the startup code
> > would have already chdir'd, so that'd be fine.  I am not sure about the
> > use of find_exec in av::setup though, I don't think something like
> > #!./prog is legal but if it is that will need to know the child's cwd too.
> 
> Nope, I was wrong.  I tried it on Linux and this works:
> cp /bin/bash .
> echo '#!./bash' > test.sh
> echo 'readlink /proc/$$/exe' >> test.sh
> chmod +x test.sh
> ./test.sh

Yeah, the Linux man page says

  An interpreter script is a text file that has execute permission enabled
  and whose first line is of the form:

  #!interpreter [optional-arg]

  The interpreter must be a valid pathname for an executable file.

And that's all.  POSIX is even less forthcoming:

  Another way that some historical implementations handle shell
  scripts is by recognizing the first two bytes of the file as the
  character string "#!" and using the remainder of the first line of
  the file as the name of the command interpreter to execute.

There's no rule defining a requirement for an absolute path.  Linux,
but not every OS, even allows interpreter to be a script.


Corinna