Re: Alpine: useless-if-before-free: Exec format error

[Bruno Haible]

Eric Blake wrote:
> Conversely, POSIX requires that execution by 'sh' is the expected
> behavior when #! is missing, and that use of #! renders a script
> non-portable:
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_01

This paragraph specifies what happens when you run
  sh some-file

Here, maint.mk invokes xargs, which invokes 'useless-if-before-free'.
xargs is not a shell. Therefore the portions of POSIX that matter here are

1) The description of "Executable File" in section 3.154 of
   https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html
   "The internal format of an executable file is unspecified"

2) The description of the exec() system call:
   https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
   "Historically, there have been two ways that implementations can exec shell
    scripts.
    One common historical implementation is that the execl(), execv(), execle(),
    and execve() functions return an [ENOEXEC] error for any file not
    recognizable as executable, including a shell script. When the execlp() and
    execvp() functions encounter such a file, they assume the file to be a shell
    script and invoke a known command interpreter to interpret such files. This
    is now required by POSIX.1-2017. ..."

   Since xargs happens to use execvp(), the executable gets executed by "a
   known command interpreter". This command interpreter might be sh, python,
   emacs, or whatever.

So, the *omission* of #! renders a script non-portable, when that script is
invoked like an executable.

Bruno