How should posix_spawn() behave when given a file with an unrecognized executable magic number?

The glibc implementation of posix_spawn tries sending the file to /bin/sh if execve() fails with ENOEXEC:

__execve (file, argv, envp); 
if (errn开发者_JAVA技巧o == ENOEXEC)
  script_execute (file, argv, envp)

The POSIX sample implementation simply fails if execve() fails in any way:

execve(path, argv, envp);
exit(127);              /* exec failed */

The POSIX specification for posix_spawn() does not specify anything about how to handle files that would result in ENOEXEC if sent to execve(). As a possible contrast, the POSIX specification for the exec* family of functions specifically states that execlp() and execvp() should invoke sh in the case of an unrecognized executable type:

There are two distinct ways in which the contents of the process image file may cause the execution to fail, distinguished by the setting of errno to either [ENOEXEC] or [EINVAL] (see the ERRORS section). In the cases where the other members of the exec family of functions would fail and set errno to [ENOEXEC], the execlp() and execvp() functions shall execute a command interpreter and the environment of the executed command shall be as if the process invoked the sh utility using execl() as follows:

execl([shell path], arg0, file, arg1, ..., (char *)0);

where [shell path] is an unspecified pathname for the sh utility, file is the process image file, and for execvp(), where arg0, arg1, and so on correspond to the values passed to execvp() in argv[0], argv[1], and so on.

So, is the glibc implementation non-conforming? Or is the POSIX specification just a bit vague here? Isn't there a security risk in the glibc implementation?

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This behavior might be acceptable (by analogy) for posix_spawnp, even if not officially sanctioned, but I think you're right that it's non-conformant, and moreover that it's dangerously non-conformant for plain posix_spawn. The shell invocation code should simply be removed.