exec: Check executable bit when searching path

exec: Check executable bit when searching path

Andrej Shadura <andrew.shadura@collabora.co.uk> wrote:
>
> Here's an old bug from 2017, but it was brought to my attention in some
> recent discussion about which "which" is which. There's also a patch in
> one of the follow-ups, but I'm afraid I don't know enough about that
> part of code to judge the consequences of it being applied:
>
> https://bugs.debian.org/874264
>
> -------- Forwarded Message --------
> Subject: dash: 'command -v' mistakenly returns a shell script whose
> executable is not set
> Date: Mon, 04 Sep 2017 10:45:48 -0400
> From: Norman Ramsey <nr@cs.tufts.edu>
> To: Debian Bug Tracking System <submit@bugs.debian.org>
>
> Package: dash
> Version: 0.5.8-2.4
> Severity: normal
>
> Dear Maintainer,
>
>
> I tracked a build bug in s-nail to a problem with dash.  Symptom:
> building s-nail tries to run /home/nr/bin/clang, a script whose
> executable bit is not set.  We tracked the problem to the result of
> running `command -v clang` with /bin/sh:
>
>   nr@homedog ~/n/s-nail> /bin/sh -c 'command -v clang'
>   /home/nr/bin/clang
>   nr@homedog ~/n/s-nail> ls -l /home/nr/bin/clang
>   -rw-rw-r-- 1 nr nr 1009 Aug 29  2011 /home/nr/bin/clang
>   nr@homedog ~/n/s-nail> ls -l /bin/sh
>   lrwxrwxrwx 1 root root 4 Jan 24  2017 /bin/sh -> dash
>   nr@homedog ~/n/s-nail> ksh -c 'command -v clang'
>   /usr/bin/clang
>   nr@homedog ~/n/s-nail> bash -c 'command -v clang'
>   /usr/bin/clang
>   nr@homedog ~/n/s-nail> sh -c 'command -v clang'
>   /home/nr/bin/clang
>   nr@homedog ~/n/s-nail> dash -c 'command -v clang'
>   /home/nr/bin/clang
>   nr@homedog ~/n/s-nail> fish -c 'command -v clang'
>   /usr/bin/clang
>
> When I run `command -v clang` I expect it to answer /usr/bin/clang.
>
> -- System Information:
> Debian Release: 9.1
>   APT prefers stable
>   APT policy: (990, 'stable'), (500, 'stable'), (1, 'experimental')
> Architecture: i386 (x86_64)
> Foreign Architectures: amd64
>
> Kernel: Linux 4.9.0-3-amd64 (SMP w/4 CPU cores)
> Locale: LANG=C, LC_CTYPE=C (charmap=UTF-8) (ignored: LC_ALL set to
> en_US.utf8), LANGUAGE=C (charmap=UTF-8) (ignored: LC_ALL set to en_US.utf8)
> Shell: /bin/sh linked to /bin/dash
> Init: systemd (via /run/systemd/system)
>
> Versions of packages dash depends on:
> ii  debianutils  4.8.1.1
> ii  dpkg         1.18.24
> ii  libc6        2.24-11+deb9u1
>
> dash recommends no packages.
>
> dash suggests no packages.
>
> -- debconf information:
> * dash/sh: true

This is inherited from NetBSD.  There is even a commented-out
block of code that tried to fix this.

Anyway, we now have faccessat so we can simply use it.

Reported-by: Norman Ramsey <nr@cs.tufts.edu>
Reported-by: Nicola Lamacchia <nicola.lamacchia@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>