2012-04-21
Bash's superintelligent errors about exec failures
Let's take a closer look at bash's error message from yesterday, because if you pay close attention something really interesting is going on. Here's the error message again:
bash: /tmp/exmpl: /bin/shx: bad interpreter: No such file or directory
On first blush, if you don't think about it too much, this looks
perfectly sensible: bash is reporting that when it tried to exec()
the /tmp/exmpl, the kernel told it that there was a problem with the
script's interpreter.
But, wait. When the exec() fails, all that kernel can tell bash is the
errno number. In this case the kernel returns ENOENT, which creates
the 'No such file or directory' portion of the error message. So how
does bash know that the reason that trying to run /tmp/exmpl failed is
because /bin/shx doesn't exist?
Here, have another example error message from bash:
bash: /tmp/a.out: /lib64/ld-ZZZZZ-x86-64.so.2: bad ELF interpreter: No such file or directory
(I did some binary editing to create that failure.)
That's right. When an exec() fails, bash opens the executable and
parses it to try to identify what went wrong. It recognizes shell
scripts, which is easy, but it also parses ELF binaries to find things
like the name of the ELF interpreter, so it can check that. Let me say
that again: bash knows how to parse ELF binaries so that it can give
you good error messages. I must applaud bash's attempt to be almost
as user friendly as possible, but at the same time I think it went more
than a little bit overboard.
(If you trace the system calls it's using, you can clearly see it selectively reading several bits of the ELF binary.)
If you try these same things in other, simpler shells, they will simply
report something like '/tmp/exmpl: No such file or directory', ie
they are simply doing a straightforward translation of what the kernel
told them (even if it is a rather puzzling message).
(zsh will report the interpreter problem with a shell script but not with a binary, which strikes me as a reasonable amount for a complex shell to do. Checking for this error with shell scripts is not that difficult and it does happen periodically.)