4 ms·

In case anyone's curious, in absence of a shebang (#!), Linux [1] returns an ENOEXEC to the execve syscall, after which the invoking program (the shell) handles

• abbeyj 4y ago
  This can lead to an interesting situation where a program will work if launched by a shell (or by another program that uses `execlp`), but will fail if it is launched with a different variant of `exec`. For example: $ touch empty $ chmod a+x empty $ ./empty # Works $ valgrind -q ./empty # Works $ timeout 10s ./empty # Works $ /usr/bin/time ./empty # Works $ perl -e 'exec("./empty") or die' # Works $ python -c 'import subprocess; subprocess.check_call("./empty", shell=True)' # Works $ python -c 'import subprocess; subprocess.check_call("./empty")' # Fails ... OSError: [Errno 8] Exec format error $ ruby -e 'exec "./empty"' # Fails -e:1:in `exec': Exec format error - ./empty (Errno::ENOEXEC) from -e:1 $ strace ./empty # Fails execve("./empty", ["./empty"], 0x7fff6639f3a0 /* 84 vars */) = -1 ENOEXEC (Exec format error) strace: exec: Exec format error +++ exited with 1 +++ It can be quite fun to track down why a program executes successfully and then later fails to execute, with no changes made to the program in between.
  • jjnoakes 4y ago
    I knew this about the programming languages which wrap execution in a shell, but I never knew that execlp/execvp/execvpe handled ENOEXEC internally and wrapped the command in a shell too. TIL!
• donio 4y ago
  A fun consequence of this is that running a python script that doesn't have the shebang will mysteriously hang with a crosshair cursor until you click the mouse. When the shell encounters the first "import foo" line it tries to run ImageMagick's "import" utility which tries to take a screenshot of the selected window.
• pyuser583 4y ago
  Interesting!
• adren123 4y ago
  What is also interesting is that while strace seems to report much less overhead in "./empty" compared to /bin/true $ strace -c ./empty strace: exec: Erreur de format pour exec() % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 0,00 0,000000 0 1 1 execve ------ ----------- ----------- --------- --------- ---------------- 100,00 0,000000 0 1 1 total $ strace -c /bin/true % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 54,05 0,000060 15 4 mprotect 24,32 0,000027 27 1 set_tid_address 10,81 0,000012 12 1 set_robust_list 8,11 0,000009 9 1 munmap 2,70 0,000003 3 1 prlimit64 0,00 0,000000 0 1 read 0,00 0,000000 0 2 close 0,00 0,000000 0 8 mmap 0,00 0,000000 0 1 brk 0,00 0,000000 0 4 pread64 0,00 0,000000 0 1 1 access 0,00 0,000000 0 1 execve 0,00 0,000000 0 2 1 arch_prctl 0,00 0,000000 0 2 openat 0,00 0,000000 0 2 newfstatat ------ ----------- ----------- --------- --------- ---------------- 100,00 0,000111 3 32 2 total the multiple execution seems to be much slower with the empty executable than with /bin/true using the multitime tool https://tratt.net/laurie/src/multitime/ https://tratt.net/laurie/src/multitime/ $ multitime -n 10 ./empty ===> multitime results 1: ./empty Mean Std.Dev. Min Median Max real 0.006 0.000 0.005 0.006 0.006 user 0.001 0.001 0.000 0.002 0.002 sys 0.000 0.001 0.000 0.000 0.002 $ multitime -n 10 /bin/true ===> multitime results 1: /bin/true Mean Std.Dev. Min Median Max real 0.002 0.000 0.001 0.002 0.003 user 0.001 0.000 0.001 0.001 0.001 sys 0.000 0.000 0.000 0.000 0.000
  • faho 4y ago
    > What is also interesting is that while strace seems to report much less overhead in "./empty" compared to /bin/true That's because it reports an error and does not actually execute the file. >strace: exec: Erreur de format pour exec() `strace` uses one of the exec()-style functions that don't automatically invoke a shell here. And if it did, it would invoke an entire shell (/bin/sh, typically), which might have more overhead. ---- You need to be very careful when measuring this, because what happens is that a shell executes the shebangless empty file. If you try launching it from e.g. a C program using execlp(), it will have to first start that shell. If you try launching it from e.g. bash, it might directly run that in-process or at least after a fork() or similar, skipping some of the shell setup. So the overhead might depend on context.
• nerdponx 4y ago
  Was this behavior inherited from Unixes past? It seems like it'd be better if shells just returned the error back to the user.
  • deleted 4y ago
    [deleted]
  • fuzzybear3965 4y ago
    Shells return exit codes, not typed error values. What kind of output would you expect from the shell, exactly?
    • nerdponx 4y ago
      Status 129 or something else outside the usual range, plus a message that the file was not a valid ELF and also has no shebang, therefore cannot be executed.
      • fuzzybear3965 4y ago
        I always thought that 128+ are for (shell-spawned) processes and 0-127 are for shell. But, apparently POSIX requires this behavior for empty files - we need to amend the standard, first (or fail to comply, muahahaha).
    • deathanatos 4y ago
      I'd expect the same as any other failure from execv(2). Something like, zsh: Exec format error: empty-file (last command returned 127.) (127 is the exit status for other failures, so I've used it here. The second line is intended to be part of my PS1, the first zsh's normal reporting. The string used here is what I get from perror() for that code.)
      • fuzzybear3965 4y ago
        Okay, yeah. That could be useful. Seems POSIX forces the behavior described in the blog post. Time to change the standard!