posix_spawn and script execution

[Daniel Drake <drake at endlessm dot com>]

Hi,

The posix_spawn man page says:

>The  only  difference  between  posix_spawn() and posix_spawnp() is the
>manner in which they specify the file  to  be  executed  by  the  child
>process.   With  posix_spawn(),  the  executable file is specified as a
>pathname (which can be absolute or relative).  With posix_spawnp(), the
>executable  file is specified as a simple filename; the system searches
>for this file in the list of directories specified by PATH (in the same
>way  as for execvp(3)).  For the remainder of this page, the discussion
>is phrased in terms  of  posix_spawn(),  with  the  understanding  that
>posix_spawnp() differs only on the point just described.

That seems rather definitive in communicating that there are no other
differences other than the path-searching behavioural aspect.

However, I have found another difference:

posix_spawnp() can execute scripts, by that I mean a text file that
has executable permissions that does not have a shebang. When used in
this way, it will use the shell to execute the script.

You can try this by taking the sample program in the posix_spawn man
page and switching it between posix_spawn/posix_spawnp and launching a
script created with:

  echo "/bin/echo hello" > test.sh
  chmod a+x test.sh

posix_spawn fails to execute it, but it runs fine with posix_spawnp.

Is this an omission in the man page that should be corrected, to state
that a second difference between posix_spawn and posix_spawnp is that
the spawnp variant can execute scripts, in the same way that exec(3)
documents the exact same behavioural exception for execlp/execvp?

Or is the presence of this behavioural difference a bug in glibc?

Looking at the history, posix_spawn() used to be able to launch
scripts too, but this behaviour was changed here:
https://sourceware.org/bugzilla/show_bug.cgi?id=13134

The resulting commit looks like it tries to make the change both for
posix_spawn and posix_spawnp, in that it creates compat versions of
both functions that set SPAWN_XFLAGS_TRY_SHELL while also omitting
that flag from the "fixed" functions:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=d96de9634a334af16c0ac711074c15ac1762b23c

however ultimately posix_spawnp script execution is still possible
today because the spawnp variant uses __execvpe. Check the source code
for __execvpe and you can clearly see the script exec ENOEXEC
fallback, and I believe that's why posix_spawnp can run scripts.

Clarifications appreciated!

Thanks
Daniel