Hey, Stewart, On Wednesday, February 07 2018, Stewart C. Russell via talk wrote:
Anyone know how to get in touch with the maintainers of 'file'? Seems the links in the man pages and Ian Darwin's site - http://www.darwinsys.com/file/ - don't work. The file magic database needs an update to correctly recognize PIE (Position Independent Executable) x86 ELF binaries as application/x-executable.
Unfortunately it seems that "file" is not "properly maintained" in the sense that the project doesn't have a trivial way to receive contributions. In this scenario, what I recommend is to file a bug downstream (against Debian's "file", for example), and ask the maintainer to forward the fix upstream.
This might seem an incredibly trivial thing, but it effectively stops graphical file managers from executing binaries, as they use magic (5) to identify files. Debian switched to making PIE a default for gcc for security reasons, but probably didn't expect it to break graphical UIs.
Here's what I'm seeing:
Expected behaviour:
$ echo "int main() { return 0; }" > foo.c $ gcc -o foo foo.c $ file foo foo: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=6e7749f995a89a53f74ec29d3c16fcf3f56be90f, not stripped $ file --mime-type foo foo: application/x-executable
Actual behaviour:
$ echo "int main() { return 0; }" > foo.c $ gcc -o foo foo.c $ file foo foo: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=6e7749f995a89a53f74ec29d3c16fcf3f56be90f, not stripped $ file --mime-type foo foo: application/x-sharedlib
This is expected, as you noted. The problem here is not only the magic number, but the fact that the binary has its ELF e_type marked as ET_DYN, and not ET_EXEC. This is proposital; GCC does that because it is possible to create shared libraries that are also executables when you use -pie. It seems to me that perhaps the graphical UI could rely not only on the MIME type of a file, but also if it is marked as executable or not. Debian explicitly advises (in the form of a lintian error) against having the executable bit set for libraries, so only executable files will have +x.
Workaround:
$ echo "int main() { return 0; }" > foo.c $ gcc -o foo-nopie foo.c -no-pie $ file foo-nopie foo-nopie: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=3eb8c581f43c19997e3c828f5a9730dbdc794470, not stripped $ file --mime-type foo-nopie foo-nopie: application/x-executable
I'm a bit worried that the workaround allows less safe binaries to be double-clicked and run. I'm not sure how much of a security issue PIE vs non-PIE is, though.
Yeah, this is not really a workaround per se, because it disables PIE when compiling the binary. Having PIE enabled is a nice security feature, so I would recommend against doing that. -- Sergio GPG key ID: 237A 54B1 0287 28BF 00EF 31F4 D0EB 7628 65FC 5E36 Please send encrypted e-mail if possible http://sergiodj.net/