Re: Are there any changes to the access control to /proc/<PID>/fd/1 in cygwin 3?

["Till Riedel [TM]" <till dot riedel at kit dot edu>]

Hi Brian,

you actually hinted me at sth. : powershell.exe Start-Process bash  
\"-c\",\"\'echo hello world  \>`readlink /proc/$$/fd/1`\;sleep 10\'\" 
works as well.

I verified this:

>echo hello >/proc/3235/fd/0
-bash: echo: write error: Bad file descriptor

>echo hello >`readlink /proc/3235/fd/0`
works!?

Am I missing sth about symlinks?

BR

Till

Till Riedel [TM] @ 20.06.2019 (21:49) :
> Hi Brian,
>
> thanks for the reply.
>
> But proc/$$/fd/1 references the callers  stdout and not the callees as 
> /dev/stdout or /proc/self/fd/2 if called from eg. from bash, because 
> it is expanded before the call.
>
> What I was able to do is to inject messages into the callers stdout 
> and read from the callers stdin, which is when executing under 
> elevated priviledges (the behaviour you would want for a sudo).
>
> As said echo >/proc/$$/fd/1 hello world is not the problem and works 
> fine as you said as well. (I probably should not have said strangely 
> in the first place, because it is a whole different deal...)
>
> BR
>
> Till
>
>
>
> Brian Inglis @ 16.06.2019 (17:42) :
> > On 2019-06-15 12:27, Till Riedel wrote:
> > > I have been using a very handy sudo hack, that broke lately (I 
> > > think) when
> > > updating cygwin: https://github.com/imachug/win-sudo
> > > The trick is to spawn an elevated process using powershell and hook 
> > > up the
> > > calling file descripters. I now get "write error: Bad file descriptor"
> > > To reproduce call:
> > > powershell.exe Start-Process bash  \"-c\",\"\'echo \>\>/proc/$$/fd/1 
> > > hello
> > > world\;sleep 10\'\"
> > > IMHO this used to work in former versions (print out hello world on 
> > > the calling
> > > shell). Strangely
> > > bash -c "echo >/proc/$$/fd/1 hello world"
> > > works. So my initial guess is that is has to do with the decoupling 
> > > of Cygwin
> > > PIDs from Windows PIDs, but there were also changes in the proc file 
> > > system...
> > > Thanks a lot in advance for any help/thoughts!
> > Given:
> > $ bash -c 'ls -dglo /dev/std* /proc/self /proc/$$ /proc/self/fd/[012]'
> > lrwxrwxrwx 1 15 May 14  2013 /dev/stderr -> /proc/self/fd/2
> > lrwxrwxrwx 1 15 May 14  2013 /dev/stdin -> /proc/self/fd/0
> > lrwxrwxrwx 1 15 May 14  2013 /dev/stdout -> /proc/self/fd/1
> > dr-xr-xr-x 3  0 Jun 16 01:37 /proc/23846
> > lrwxrwxrwx 1  0 Jun 16 01:37 /proc/self -> 23846
> > lrwxrwxrwx 1  0 Jun 16 01:37 /proc/self/fd/0 -> /dev/pty0
> > lrwxrwxrwx 1  0 Jun 16 01:37 /proc/self/fd/1 -> /dev/pty0
> > lrwxrwxrwx 1  0 Jun 16 01:37 /proc/self/fd/2 -> /dev/pty0
> >
> > the following seems to work as expected:
> > $ powershell Start-Process bash  \"-c\",\"\'echo \>\>/dev/stdout hello
> > world\;sleep 10\'\"
--
KIT- Research University in the Helmholtz Association

Dr.-Ing. Till Riedel
Lab Leader TECO, SDSC-BW/SDIL (AR)

Karlsruhe Institute of Technology (KIT)
Institute of Telematics (TM)
Pervasive Computing Systems - Prof. Beigl


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+49 (0)721 608 41706
www.teco.kit.edu/~riedel


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