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Subject: bug#73100: Regarding a bug in suspend-emacs
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Hello

I am running Linux with kernel 6.6.47 and am running emacs in xterm, using
the -nw command line argument.

I am new to emacs and was experimenting with the suspend-emacs command.
Following the example on
https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html
.

When following the example, I added the suspend-resume-hook to my
.emacs.d/init.el file. When I run M-: (suspend-emacs "pwd") it does not
show the current working directory. But when I do fg from the terminal that
I got into, I see the "Resumed!" message.

I asked in the libera chat about that, and also in the gnu-help-emacs list.
I have been talking to wasamasa on libera chat (#emacs-beginners) and we
pinpointed the problem to the stuff_char function (in
https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). We
came to this point from stuff_buffered_input (
https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11963),
and from  suspend-emacs function definition (in
https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11908).

The stuff_char function is using ioctl with TIOCSTI. TIOCSTI requires
CAP_SYS_ADMIN capability. You can set this capability using sysctl setting
dev.tty.legacy_tiocsti to 1.

Unless I had set "dev.tty.legacy_tiocsti" to 1 I could not run the
suspend-emacs command with an argument string.

Either emacs can check the return value of ioctl in stuff_char and if there
return value is EPERM, then handle this accordingly, with a message
regarding the problem.

Or the information relating to the kernel version and CAP_SYS_ADMIN can be
added to the infor page os suspend_emacs, along with the information on how
to set this capability using sysctl.

Kind Regards
Riza Dindir

--000000000000e2beaf062186ca62
Content-Type: text/html; charset="UTF-8"
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<div dir=3D"ltr"><div>Hello</div><div><br></div><div>I am running Linux wit=
h kernel 6.6.47 and am running emacs in xterm, using the -nw command line a=
rgument.<br></div><div><br></div><div>I am new to emacs and was experimenti=
ng with the suspend-emacs command. Following the example on <a href=3D"http=
s://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html=
" target=3D"_blank">https://www.gnu.org/software/emacs/manual/html_node/eli=
sp/Suspending-Emacs.html</a>.</div><div><br></div><div>When following the e=
xample, I added the suspend-resume-hook to my .emacs.d/init.el file. When I=
 run <span style=3D"font-weight:bold">M-: (suspend-emacs &quot;pwd&quot;)</=
span> it does not show the current working directory. But when I do fg from=
 the terminal that I got into, I see the &quot;Resumed!&quot; message.</div=
><div><br></div><div>I asked in the libera chat about that, and also in the=
 gnu-help-emacs list. I have been talking to wasamasa on libera chat (#emac=
s-beginners) and we pinpointed the problem to the stuff_char function (in <=
a href=3D"https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n40=
3" target=3D"_blank">https://git.savannah.gnu.org/cgit/emacs.git/tree/src/s=
ysdep.c#n403</a>). We came to this point from stuff_buffered_input (<a href=
=3D"https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11963"=
 target=3D"_blank">https://git.savannah.gnu.org/cgit/emacs.git/tree/src/key=
board.c#n11963</a>), and from=C2=A0 suspend-emacs function definition (in <=
a href=3D"https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n=
11908" target=3D"_blank">https://git.savannah.gnu.org/cgit/emacs.git/tree/s=
rc/keyboard.c#n11908</a>).</div><div><br></div><div>The stuff_char function=
 is using<span style=3D"font-family:arial,sans-serif"> ioctl </span>with TI=
OCSTI. TIOCSTI requires CAP_SYS_ADMIN capability. You can set this capabili=
ty using sysctl setting dev.tty.legacy_tiocsti to 1.</div><div><br></div><d=
iv>Unless I had set &quot;dev.tty.legacy_tiocsti&quot;  to 1 I could not ru=
n the suspend-emacs command with an argument string.</div><div><br></div><d=
iv>Either emacs can check the return value of ioctl in stuff_char and if th=
ere return value is EPERM, then handle this accordingly, with a message reg=
arding the problem.</div><div><br></div><div>Or the information relating to=
 the kernel version and CAP_SYS_ADMIN can be added to the infor page os sus=
pend_emacs, along with the information on how to set this capability using =
sysctl.</div><div><br></div><div>Kind Regards</div><div>Riza Dindir<br></di=
v></div>

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Riza Dindir <riza.dindir@HIDDEN>
Subject: bug#73100: Acknowledgement (Regarding a bug in suspend-emacs)
Message-ID: <handler.73100.B.17257202191807.ack <at> debbugs.gnu.org>
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Date: Sat, 07 Sep 2024 14:44:03 +0000

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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#73100: Regarding a bug in suspend-emacs
Resent-From: Eli Zaretskii <eliz@HIDDEN>
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 (message from Riza Dindir on Sat, 7 Sep 2024 15:37:17 +0300)
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> From: Riza Dindir <riza.dindir@HIDDEN>
> Date: Sat, 7 Sep 2024 15:37:17 +0300
> 
> I am running Linux with kernel 6.6.47 and am running emacs in xterm, using the -nw command line argument.
> 
> I am new to emacs and was experimenting with the suspend-emacs command. Following the example on
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html.
> 
> When following the example, I added the suspend-resume-hook to my .emacs.d/init.el file. When I run M-:
> (suspend-emacs "pwd") it does not show the current working directory. But when I do fg from the terminal that I
> got into, I see the "Resumed!" message.
> 
> I asked in the libera chat about that, and also in the gnu-help-emacs list. I have been talking to wasamasa on
> libera chat (#emacs-beginners) and we pinpointed the problem to the stuff_char function (in
> https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). We came to this point from
> stuff_buffered_input (https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11963), and from 
> suspend-emacs function definition (in https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11908).
> 
> The stuff_char function is using ioctl with TIOCSTI. TIOCSTI requires CAP_SYS_ADMIN capability. You can set
> this capability using sysctl setting dev.tty.legacy_tiocsti to 1.
> 
> Unless I had set "dev.tty.legacy_tiocsti" to 1 I could not run the suspend-emacs command with an argument
> string.
> 
> Either emacs can check the return value of ioctl in stuff_char and if there return value is EPERM, then handle
> this accordingly, with a message regarding the problem.

You mean, you want suspend-emacs signal an error if it is called with
STUFFSTRING argument, but fails to stuff the string into the
terminal's input buffer?

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 (message from Riza Dindir on Sat, 7 Sep 2024 16:34:45 +0300)
Subject: Re: bug#73101: Regarding a bug in suspend-emacs
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merge 73101 73100
thanks

> From: Riza Dindir <riza.dindir@HIDDEN>
> Date: Sat, 7 Sep 2024 16:34:45 +0300
> 
> Note: Sending this a second time, I forgot to confirm my request to register to the bug-gnu-emacs list.

That was a mistake, because by doing that you have created an
identical copy of the first bug report.

I'm now merging them.

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> From: Riza Dindir <riza.dindir@HIDDEN>
> Date: Sat, 7 Sep 2024 19:11:02 +0300
> Cc: 73100 <at> debbugs.gnu.org
> 
> I am new to the code base, and it was just a suggestion to check for the ioctl call for any failures and take
> precautions, maybe inform the user of the issue that the suspend-emacs command did not run correctly. Since
> the command was not printing out anything when called with 'M-: (suspend-emacs "pwd")' I was not sure what
> was happening.

I was asking for your user's expectations, not how to do that in the
code.

> For users that use the linux kernel 6.2+ the ioctl command does not work correctly. When the STUFFSTRING is
> passed to the suspend-emacs command, it does nothing. The problem with kernel 6.2+ is that it requires
> CAP_SYS_ADMIN capability (https://www.man7.org/linux/man-pages/man2/TIOCSTI.2const.html).
> 
> In the code at some point suspend-emacs command calls stuff_char, which uses ioctl (code snippet below, from
> https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). The ioctl call fails. But in the code there is
> no check for that, that might confuse people that use linux kernel 6.2+. I wanted to use suspend-emacs with
> "pwd" as the string (as shown in the example in the documentation in
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html). But the "pwd" command
> was not displaying anything. Because it was not being sent/stuffed to the superior process.

Does ioctl return -1 in that case?  (I don't have access to a system
where it fails, it works for me on GNU/Linux.)

> Adding an error check would be one of the solutions.

Assuming we can check that (see above), the question is what to do if
we do detect a failure.

> The other solution might be to add a piece of information to the documentation for suspend-emacs, (and
> maybe to the documentation string for suspend-emacs) regarding this problem. So that people that use linux
> kernel 6.2+ would be aware of this issue.

That is already done, except that the documentation cannot be too
specific regarding when and under what conditions it could happen.

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From: Riza Dindir <riza.dindir@HIDDEN>
Date: Sat, 7 Sep 2024 19:11:02 +0300
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--00000000000058905e062189c71f
Content-Type: text/plain; charset="UTF-8"

Hello Eli,

I am new to the code base, and it was just a suggestion to check for the
ioctl call for any failures and take precautions, maybe inform the user of
the issue that the suspend-emacs command did not run correctly. Since the
command was not printing out anything when called with 'M-: (suspend-emacs
"pwd")' I was not sure what was happening.

For users that use the linux kernel 6.2+ the ioctl command does not work
correctly. When the STUFFSTRING is passed to the suspend-emacs command, it
does nothing. The problem with kernel 6.2+ is that it requires CAP_SYS_ADMIN
capability (https://www.man7.org/linux/man-pages/man2/TIOCSTI.2const.html).

In the code at some point suspend-emacs command calls stuff_char, which
uses ioctl (code snippet below, from
https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). The
ioctl call fails. But in the code there is no check for that, that might
confuse people that use linux kernel 6.2+. I wanted to use suspend-emacs
with "pwd" as the string (as shown in the example in the documentation in
https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html).
But the "pwd" command was not displaying anything. Because it was not being
sent/stuffed to the superior process.

/* Should perhaps error if in batch mode */#ifdef TIOCSTI
  ioctl (fileno (CURTTY()->input), TIOCSTI, &c);#else /* no TIOCSTI */
  error ("Cannot stuff terminal input characters in this version of
Unix");#endif /* no TIOCSTI */


I have downloaded the code for emacs 29.4 and compiled it with a simple
error check and the ioctl call failed on my system silently (with kernel
6.6.47).

To add a check is of course for the people who develop emacs to decide. I
have seen the code today, and am not in a position to suggest anything at
this stage. Maybe there would be a better way of handling this error.

Adding an error check would be one of the solutions.

The other solution might be to add a piece of information to the
documentation for suspend-emacs, (and maybe to the documentation string for
suspend-emacs) regarding this problem. So that people that use linux kernel
6.2+ would be aware of this issue.

The issue is fixable by just setting the dev.tty.legacy_tiocsti sysctl
variable to 1. This variable was set to 0 on my system. Hence the ioctl
call for TIOCSTI was failing, and no indication of the error was given.
After I have read the man page for TIOCSTI, and set the sysctl
variable dev.tty.legacy_tiocsti to 1 was I able to call suspend-emacs with
a STUFFSTRING and it worked. It was displaying the output of the "pwd"
command.

Regards
Riza


On Sat, Sep 7, 2024, 18:20 Eli Zaretskii <eliz@HIDDEN> wrote:

> > From: Riza Dindir <riza.dindir@HIDDEN>
> > Date: Sat, 7 Sep 2024 15:37:17 +0300
> >
> > I am running Linux with kernel 6.6.47 and am running emacs in xterm,
> using the -nw command line argument.
> >
> > I am new to emacs and was experimenting with the suspend-emacs command.
> Following the example on
> >
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html
> .
> >
> > When following the example, I added the suspend-resume-hook to my
> .emacs.d/init.el file. When I run M-:
> > (suspend-emacs "pwd") it does not show the current working directory.
> But when I do fg from the terminal that I
> > got into, I see the "Resumed!" message.
> >
> > I asked in the libera chat about that, and also in the gnu-help-emacs
> list. I have been talking to wasamasa on
> > libera chat (#emacs-beginners) and we pinpointed the problem to the
> stuff_char function (in
> > https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403). We
> came to this point from
> > stuff_buffered_input (
> https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11963),
> and from
> > suspend-emacs function definition (in
> https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11908).
> >
> > The stuff_char function is using ioctl with TIOCSTI. TIOCSTI requires
> CAP_SYS_ADMIN capability. You can set
> > this capability using sysctl setting dev.tty.legacy_tiocsti to 1.
> >
> > Unless I had set "dev.tty.legacy_tiocsti" to 1 I could not run the
> suspend-emacs command with an argument
> > string.
> >
> > Either emacs can check the return value of ioctl in stuff_char and if
> there return value is EPERM, then handle
> > this accordingly, with a message regarding the problem.
>
> You mean, you want suspend-emacs signal an error if it is called with
> STUFFSTRING argument, but fails to stuff the string into the
> terminal's input buffer?
>

--00000000000058905e062189c71f
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr"><div dir=3D"auto"><div>Hello Eli,<br></div><div><br></div>=
<div>I am new to the code base, and it was just a suggestion to check for t=
he ioctl call for any failures and take precautions, maybe inform the user =
of the issue that the suspend-emacs command did not run correctly. Since th=
e command was not printing out anything when called with &#39;M-: (suspend-=
emacs &quot;pwd&quot;)&#39; I was not sure what was happening.<br></div><di=
v><br></div><div>For users that use the linux kernel 6.2+ the ioctl command=
 does not work correctly. When the STUFFSTRING is passed to the suspend-ema=
cs command, it does nothing. The problem with kernel 6.2+ is that it requir=
es <span class=3D"gmail-im">CAP_SYS_ADMIN capability (</span><a href=3D"htt=
ps://www.man7.org/linux/man-pages/man2/TIOCSTI.2const.html">https://www.man=
7.org/linux/man-pages/man2/TIOCSTI.2const.html</a>).</div><div><br></div><d=
iv>In the code at some point suspend-emacs command calls stuff_char, which =
uses ioctl (code snippet below, from <a href=3D"https://git.savannah.gnu.or=
g/cgit/emacs.git/tree/src/sysdep.c#n403">https://git.savannah.gnu.org/cgit/=
emacs.git/tree/src/sysdep.c#n403</a>). The ioctl call fails. But in the cod=
e there is no check for that, that might confuse people that use linux kern=
el 6.2+. I wanted to use suspend-emacs with &quot;pwd&quot; as the string (=
as shown in the example in the documentation in <a href=3D"https://www.gnu.=
org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html">https://ww=
w.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html</a>).=
 But the &quot;pwd&quot; command was not displaying anything. Because it wa=
s not being sent/stuffed to the superior process.<br></div><div><div><br></=
div><div><pre><code><span class=3D"gmail-hl gmail-com">/* Should perhaps er=
ror if in batch mode */</span>
<span class=3D"gmail-hl gmail-ppc">#ifdef TIOCSTI</span>
  <span class=3D"gmail-hl gmail-kwd">ioctl</span> <span class=3D"gmail-hl g=
mail-opt">(</span><span class=3D"gmail-hl gmail-kwd">fileno</span> <span cl=
ass=3D"gmail-hl gmail-opt">(</span><span class=3D"gmail-hl gmail-kwd">CURTT=
Y</span><span class=3D"gmail-hl gmail-opt">()-&gt;</span>input<span class=
=3D"gmail-hl gmail-opt">),</span> TIOCSTI<span class=3D"gmail-hl gmail-opt"=
>, &amp;</span>c<span class=3D"gmail-hl gmail-opt">);</span>
<span class=3D"gmail-hl gmail-ppc">#else</span> <span class=3D"gmail-hl gma=
il-com">/* no TIOCSTI */</span><span class=3D"gmail-hl gmail-ppc"></span>
  <span class=3D"gmail-hl gmail-kwd">error</span> <span class=3D"gmail-hl g=
mail-opt">(</span><span class=3D"gmail-hl gmail-str">&quot;Cannot stuff ter=
minal input characters in this version of Unix&quot;</span><span class=3D"g=
mail-hl gmail-opt">);</span>
<span class=3D"gmail-hl gmail-ppc">#endif</span> <span class=3D"gmail-hl gm=
ail-com">/* no TIOCSTI */</span></code></pre></div><div dir=3D"auto"><br></=
div></div>I have downloaded the code for emacs 29.4 and compiled it with a =
simple error check and the ioctl call failed on my system silently (with ke=
rnel 6.6.47).<br></div><div dir=3D"auto"><br></div><div>To add a check is o=
f course for the people who develop emacs to decide. I have seen the code t=
oday, and am not in a position to suggest anything at this stage. Maybe the=
re would be a better way of handling this error.</div><div><br></div><div>A=
dding an error check would be one of the solutions.<br></div><div><br></div=
><div>The other solution might be to add a piece of information to the docu=
mentation for suspend-emacs, (and maybe to the documentation string for sus=
pend-emacs) regarding this problem. So that people that use linux kernel 6.=
2+ would be aware of this issue.</div><div><br></div><div>The issue is fixa=
ble by just setting the dev.tty.legacy_tiocsti sysctl variable to 1. This v=
ariable was set to 0 on my system. Hence the ioctl call for TIOCSTI was fai=
ling, and no indication of the error was given. After I have read the man p=
age for TIOCSTI, and set the sysctl variable=C2=A0dev.tty.legacy_tiocsti to=
 1 was I able to call suspend-emacs with a STUFFSTRING and it worked. It wa=
s displaying the output of the &quot;pwd&quot; command.<br></div><div><br><=
/div><div>Regards</div><div>Riza<br></div><div dir=3D"auto"><br></div></div=
><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Sa=
t, Sep 7, 2024, 18:20 Eli Zaretskii &lt;<a href=3D"mailto:eliz@HIDDEN" tar=
get=3D"_blank">eliz@HIDDEN</a>&gt; wrote:<br></div><blockquote class=3D"gm=
ail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,=
204,204);padding-left:1ex">&gt; From: Riza Dindir &lt;<a href=3D"mailto:riz=
a.dindir@HIDDEN" rel=3D"noreferrer" target=3D"_blank">riza.dindir@gmail.=
com</a>&gt;<br>
&gt; Date: Sat, 7 Sep 2024 15:37:17 +0300<br>
&gt; <br>
&gt; I am running Linux with kernel 6.6.47 and am running emacs in xterm, u=
sing the -nw command line argument.<br>
&gt; <br>
&gt; I am new to emacs and was experimenting with the suspend-emacs command=
. Following the example on<br>
&gt; <a href=3D"https://www.gnu.org/software/emacs/manual/html_node/elisp/S=
uspending-Emacs.html" rel=3D"noreferrer noreferrer" target=3D"_blank">https=
://www.gnu.org/software/emacs/manual/html_node/elisp/Suspending-Emacs.html<=
/a>.<br>
&gt; <br>
&gt; When following the example, I added the suspend-resume-hook to my .ema=
cs.d/init.el file. When I run M-:<br>
&gt; (suspend-emacs &quot;pwd&quot;) it does not show the current working d=
irectory. But when I do fg from the terminal that I<br>
&gt; got into, I see the &quot;Resumed!&quot; message.<br>
&gt; <br>
&gt; I asked in the libera chat about that, and also in the gnu-help-emacs =
list. I have been talking to wasamasa on<br>
&gt; libera chat (#emacs-beginners) and we pinpointed the problem to the st=
uff_char function (in<br>
&gt; <a href=3D"https://git.savannah.gnu.org/cgit/emacs.git/tree/src/sysdep=
.c#n403" rel=3D"noreferrer noreferrer" target=3D"_blank">https://git.savann=
ah.gnu.org/cgit/emacs.git/tree/src/sysdep.c#n403</a>). We came to this poin=
t from<br>
&gt; stuff_buffered_input (<a href=3D"https://git.savannah.gnu.org/cgit/ema=
cs.git/tree/src/keyboard.c#n11963" rel=3D"noreferrer noreferrer" target=3D"=
_blank">https://git.savannah.gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11=
963</a>), and from <br>
&gt; suspend-emacs function definition (in <a href=3D"https://git.savannah.=
gnu.org/cgit/emacs.git/tree/src/keyboard.c#n11908" rel=3D"noreferrer norefe=
rrer" target=3D"_blank">https://git.savannah.gnu.org/cgit/emacs.git/tree/sr=
c/keyboard.c#n11908</a>).<br>
&gt; <br>
&gt; The stuff_char function is using ioctl with TIOCSTI. TIOCSTI requires =
CAP_SYS_ADMIN capability. You can set<br>
&gt; this capability using sysctl setting dev.tty.legacy_tiocsti to 1.<br>
&gt; <br>
&gt; Unless I had set &quot;dev.tty.legacy_tiocsti&quot; to 1 I could not r=
un the suspend-emacs command with an argument<br>
&gt; string.<br>
&gt; <br>
&gt; Either emacs can check the return value of ioctl in stuff_char and if =
there return value is EPERM, then handle<br>
&gt; this accordingly, with a message regarding the problem.<br>
<br>
You mean, you want suspend-emacs signal an error if it is called with<br>
STUFFSTRING argument, but fails to stuff the string into the<br>
terminal&#39;s input buffer?<br>
</blockquote></div>

--00000000000058905e062189c71f--

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Copyright (C) 1999 Darren O. Benham, 1997 nCipher Corporation Ltd, 1994-97 Ian Jackson.