Re: [PATCH] common/filter: add _filter_bash()

[Eric Biggers <ebiggers@kernel.org>]

On Sat, Dec 18, 2021 at 12:10:20PM -0500, Theodore Ts'o wrote:
> On Wed, Dec 15, 2021 at 04:54:19PM -0800, Eric Biggers wrote:
> > 
> > Doesn't this happen with any shell command passed to -c, not just commands that
> > run an executable?  In the commands below which are causing the problem, it is
> > actually the 'echo' built-in being used, not an executable.
> 
> Well.... it happens with all executables and *some* built-in commands
> which returns an error.  (See the postscript for a case where the line
> number is printed.)  In the case of an error opening the redirected
> standard output, such as permission denied, the difference does occur:
> 
> Compare:
> 
> % schroot -c buster-amd64 -- bash -c "echo foo >> /bin/bash"
> bash: /bin/bash: Permission denied
> % schroot -c buster-amd64 -- bash --version
> GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
> Copyright (C) 2019 Free Software Foundation, Inc.
> ...
> 
> with
> 
> % schroot -c bullseye-amd64 -- bash -c "echo foo >> /bin/bash"
> bash: line 1: /bin/bash: Permission denied
> % schroot -c bullseye-amd64 -- bash --version
> GNU bash, version 5.1.4(1)-release (x86_64-pc-linux-gnu)
> ...
> 
> I just used the example of "bash -c /etc/passwd" because it was
> simpler, and because it would be easy to verify regardless of whether
> the command was executed as root or not.  For example:
> 
> % schroot -c bullseye-amd64 -u root -- bash -c "echo foo >> /bin/bash"
> 
> would have succeeded (and corrupted /bin/bash in my chroot :-).

A good example that doesn't involve an executable would be

	bash -c "echo foo > /"

... since that command redirection will always fail.

> I suppose I could have used the "schroot -c ..." example in the commit
> description, but that's a debian-specific command, and I chose to err
> on the side of something simpler and easier to replicate.
> 
> If Eryu would prefer, I can resend with a modified commit description,
> or he can feel free to edit the commit description with the above
> example if he thinks it's clearer.

I'm more concerned about the misleading comment above _filter_bash(), not the
commit message which is less important.  Anyway, it's not too important, but I
thought it was worth pointing out.

- Eric