[Bug 335666] Re: libc6 integration gives way wrong error message to libc5 binaries.

[Geoffrey Thomas]

So I was looking into two workarounds for this.

One is a stub ld-linux.so.5 that gets mad at you and exits. If you just
compile a short program that write()s an error message with gcc -static
-fPIC, it "works" as an interpreter.

The other option, which I think is a little more suitable, is to address
this at the kernel level. The current code path in fs/binfmt_elf.c
attempts to find the interpreter specified in the file, and if this
returns an error, that errno returned is returned directly by exec. We
can translate ENOENT (and perhaps other errors?) to ENOEXEC before
returning. The patch attached has been tested on Intrepid, and should
actually apply to kernel 2.6.20 onwards.

unpatched$ sed "s/ld-linux.so.2/ld-linux.so.1/" < /bin/ls > ls && chmod +x ls && ./ls
bash: ./ls: No such file or directory

patched$ sed "s/ld-linux.so.2/ld-linux.so.1/" < /bin/ls > ls && chmod +x ls && ./ls
-bash: ./ls: cannot execute binary file

According to the Open Group's copy of POSIX at
http://opengroup.org/onlinepubs/009695399/functions/exec.html
ENOENT is only to be returned from execve if "path" or "file" doesn't exist, and ENOEXEC indicates the binary has an "unrecognized format".

So this seems like a reasonable (and very tiny) change to make. Could we
get it in Ubuntu's kernel? I also plan to submit this to the LKML.

** Attachment added: "Return ENOEXEC instead of ENOENT if an ELF binary's interpreter doesn't exist."
   http://launchpadlibrarian.net/23403740/kernel-libc5-enoent.patch

-- 
libc6 integration gives way wrong error message to libc5 binaries.
https://bugs.launchpad.net/bugs/335666
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