[FFmpeg-devel] [PATCHv2] ffmpeg: modify tty state when stderr is redirected

[Ganesh Ajjanagadde]

On Mon, Oct 12, 2015 at 9:15 AM, wm4 <nfxjfg at googlemail.com> wrote:
> On Mon, 12 Oct 2015 09:12:02 -0400
> Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>
>> On Mon, Oct 12, 2015 at 8:56 AM, wm4 <nfxjfg at googlemail.com> wrote:
>> > On Mon, 12 Oct 2015 08:39:53 -0400
>> > Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> >
>> >> On Mon, Oct 12, 2015 at 8:24 AM, wm4 <nfxjfg at googlemail.com> wrote:
>> >> > On Mon, 12 Oct 2015 08:14:14 -0400
>> >> > Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> >> >
>> >> >> On Mon, Oct 12, 2015 at 8:03 AM, Michael Niedermayer
>> >> >> <michael at niedermayer.cc> wrote:
>> >> >> > On Mon, Oct 12, 2015 at 01:46:19PM +0200, Michael Niedermayer wrote:
>> >> >> >> On Fri, Jul 31, 2015 at 01:48:18PM +0200, Nicolas George wrote:
>> >> >> >> [...]
>> >> >> >> >    -> "ttyctl -f" in ~/.zshrc, "bash_tty_mode=$(stty -g);
>> >> >> >> >    PROMPT_COMMAND='stty $bash_tty_mode'" in ~/.bashrc; that can be added in
>> >> >> >> >    the FAQ or the wiki.
>> >> >> >>
>> >> >> >>
>> >> >> >> the patch was applied and this does not work
>> >> >> >>
>> >> >> >> the first segfault i see breaks the terminal
>> >> >> >>
>> >> >> >> a testcase is a script like:
>> >> >> >> #!/bin/sh
>> >> >> >> valgrind ./ffmpeg_g -i matrixbench_mpeg2.mpg -encryption_key 0123456789abcdef0011223344556677  -encryption_iv aabbccddeeff11335577999876542222  crypto:file-crypt.nut
>> >> >> >>
>> >> >> >> (with the aesni patches or anything else that causes a segfault)
>> >> >> >>
>> >> >> >> ls -alF /bin/sh
>> >> >> >> lrwxrwxrwx 1 root root 4 Mar 29  2012 /bin/sh -> dash*
>> >> >> >> (this is default on ubuntu)
>> >> >> >
>> >> >> > i think i got it working as long as bash is the users shell with
>> >> >> > the suggested stty commands
>> >> >>
>> >> >> So it does not work with dash?
>> >> >
>> >> > dash isn't meant to be used interactively anyway?
>> >>
>> >> Good point, just now read up on this weird shell I have never heard
>> >> of: https://en.wikipedia.org/wiki/Debian_Almquist_shell. However,
>> >> Michael does bring up a point in that /bin/sh by default symlinks to
>> >> dash on a rather large platform, namely Ubuntu. We need to find out a
>> >> workaround for it.  I am of no use here; hopefully Michael finds
>> >> something and posts.
>> >>
>> >> I will then collect them all and add to the wiki.
>> >
>> > No, for interactive shell, Debian and Ubuntu still use bash by default.
>> > So even if /bin/sh is used to run a script, the /bin/sh itself will be
>> > started from a bash. So I do not see the problem here.
>>
>> There is still a problem (albeit a minor use case): suppose a user is
>> running /bin/sh as his interactive shell. Maybe Michael for the sake
>> of testing only POSIX compliant stuff prefers to work within a /bin/sh
>> shell, and thus as soon as he opens his terminal (running bash as you
>> said), he invokes a /bin/sh subshell on Debian or Ubuntu (other
>> places, /bin/sh is usually a symlink to bash, with no issues).
>
> What if a user runs "rm -rf /"? You can't care about _everything_,
> nor should you - especially if the user decides to intentionally shoots
> himself into the foot.

true :), though this is at a slightly different level: as I explained,
there is a legitimate reason for Michael (who runs by far the most
obscure configurations for FATE) to use /bin/sh for interaction, while
there is no legitimate reason for rm -rf / - for secure erase, one
should use either low level ATA commands, write with /dev/urandom
(possibly multiple times), or shred and the like (which do roughly the
/urandom trick), and for unsecure rm -rf /, one should simply format
the partition (likely faster as well).

Nevertheless, by no means do I consider this sufficient grounds for
reverting the commit: far more users get hit by Ticket 2964 than run
/bin/sh for interaction.
I would like to think that Michael agrees, but am not sure.

>
>> In such a situation, the user is screwed: unfortunately, AFAIK, there
>> is no way to do a "pre-prompt" command in /bin/sh, all one has is
>> $PS1. Furthermore, I can't really ask Michael to change from /bin/sh
>> symlinking to dash to a symlink to bash since he wants to test /bin/sh
>> as implemented on a user's machine.
>
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