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posix_spawn and script execution
- From: Daniel Drake <drake at endlessm dot com>
- To: libc-help at sourceware dot org
- Cc: shea at shealevy dot com, drepper at gmail dot com
- Date: Tue, 5 Jun 2018 13:05:19 -0600
- Subject: posix_spawn and script execution
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:
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:
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.