[PATCH] Add nil check for decoding buffer

[PATCH] Add nil check for decoding buffer

[Lukas Fürmetz]

Hello,

This patch prevents a emacs crash under Windows for me. Sometimes the
buffer is split between \r\n and the \n is carried over, but the
decoding_buf is NIL, so "SCHARS (p->decoding_buf)" leads to a SEGFAULT.

Would be nice, if this is backported to emacs-26..

Best regards,
Lukas

---
 src/process.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/process.c b/src/process.c
index c357a8bdc3..ab2e6ce4da 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5996,7 +5996,7 @@ read_and_dispose_of_process_output (struct Lisp_Process *p, char *chars,

   if (coding->carryover_bytes > 0)
     {
-      if (SCHARS (p->decoding_buf) < coding->carryover_bytes)
+      if (NILP (p->decoding_buf) || (SCHARS (p->decoding_buf) < coding->carryover_bytes))
 	pset_decoding_buf (p, make_uninit_string (coding->carryover_bytes));
       memcpy (SDATA (p->decoding_buf), coding->carryover,
 	      coding->carryover_bytes);
--
2.17.0

Re: [PATCH] Add nil check for decoding buffer

[Eli Zaretskii]

> From: Lukas Fürmetz <fuermetz@mailbox.org>
> Date: Tue, 10 Apr 2018 23:44:22 +0200
> 
> This patch prevents a emacs crash under Windows for me. Sometimes the
> buffer is split between \r\n and the \n is carried over, but the
> decoding_buf is NIL, so "SCHARS (p->decoding_buf)" leads to a SEGFAULT.
> 
> Would be nice, if this is backported to emacs-26..
> 
> Best regards,
> Lukas
> 
> ---
>  src/process.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/process.c b/src/process.c
> index c357a8bdc3..ab2e6ce4da 100644
> --- a/src/process.c
> +++ b/src/process.c
> @@ -5996,7 +5996,7 @@ read_and_dispose_of_process_output (struct Lisp_Process *p, char *chars,
> 
>    if (coding->carryover_bytes > 0)
>      {
> -      if (SCHARS (p->decoding_buf) < coding->carryover_bytes)
> +      if (NILP (p->decoding_buf) || (SCHARS (p->decoding_buf) < coding->carryover_bytes))
>  	pset_decoding_buf (p, make_uninit_string (coding->carryover_bytes));
>        memcpy (SDATA (p->decoding_buf), coding->carryover,
>  	      coding->carryover_bytes);

Thanks, but I don't think this is the right fix for the problem.
Every process object must have an associated decoding_buf, so if some
process doesn't, it's a bug in that process's creation code.  My
reading of the code is that all the process types we support have
decoding_buf defined during creation -- except one: make-pipe-process
fails to do that.  So the fix should be in make-pipe-process.

Is the problem reproducible, or do you remember what you did when it
happened?  I'd like to make sure that indeed some pipe process was
involved in the crash, because otherwise I don't see how it could
happen.

Re: [PATCH] Add nil check for decoding buffer

[Lukas Fürmetz]

It was definitly a pipe process (cquery (github.com/cquery-project/cquery) via lsp-mode).
I have a reproducable workspace, but I cannot share it. The crash happens, when stdout is
split exactly between a windows newline, so between \r and \n.

Maybe I have time to debug it again tomorrow. Otherwise, I'll try to reproduce the problem
in another way..

> Eli Zaretskii <eliz@gnu.org> hat am 11. April 2018 um 19:00 geschrieben:
> 
> 
> > From: Lukas Fürmetz <fuermetz@mailbox.org>
> > Date: Tue, 10 Apr 2018 23:44:22 +0200
> > 
> > This patch prevents a emacs crash under Windows for me. Sometimes the
> > buffer is split between \r\n and the \n is carried over, but the
> > decoding_buf is NIL, so "SCHARS (p->decoding_buf)" leads to a SEGFAULT.
> > 
> > Would be nice, if this is backported to emacs-26..
> > 
> > Best regards,
> > Lukas
> > 
> > ---
> >  src/process.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/process.c b/src/process.c
> > index c357a8bdc3..ab2e6ce4da 100644
> > --- a/src/process.c
> > +++ b/src/process.c
> > @@ -5996,7 +5996,7 @@ read_and_dispose_of_process_output (struct Lisp_Process *p, char *chars,
> > 
> >    if (coding->carryover_bytes > 0)
> >      {
> > -      if (SCHARS (p->decoding_buf) < coding->carryover_bytes)
> > +      if (NILP (p->decoding_buf) || (SCHARS (p->decoding_buf) < coding->carryover_bytes))
> >  	pset_decoding_buf (p, make_uninit_string (coding->carryover_bytes));
> >        memcpy (SDATA (p->decoding_buf), coding->carryover,
> >  	      coding->carryover_bytes);
> 
> Thanks, but I don't think this is the right fix for the problem.
> Every process object must have an associated decoding_buf, so if some
> process doesn't, it's a bug in that process's creation code.  My
> reading of the code is that all the process types we support have
> decoding_buf defined during creation -- except one: make-pipe-process
> fails to do that.  So the fix should be in make-pipe-process.
> 
> Is the problem reproducible, or do you remember what you did when it
> happened?  I'd like to make sure that indeed some pipe process was
> involved in the crash, because otherwise I don't see how it could
> happen.

Re: [PATCH] Add nil check for decoding buffer

[Eli Zaretskii]

> Date: Thu, 12 Apr 2018 17:24:22 +0200 (CEST)
> From: Lukas Fürmetz <fuermetz@mailbox.org>
> Cc: emacs-devel@gnu.org
> 
> It was definitly a pipe process (cquery (github.com/cquery-project/cquery) via lsp-mode).
> I have a reproducable workspace, but I cannot share it. The crash happens, when stdout is
> split exactly between a windows newline, so between \r and \n.

Good, thanks.

Can you build your own Emacs?  If so, I'd like you to test an
alternative patch.

Re: [PATCH] Add nil check for decoding buffer

[Lukas Fürmetz]

Yes I can do that, just send me the patch and I'll give you feedback.

> Eli Zaretskii <eliz@gnu.org> hat am 12. April 2018 um 17:29 geschrieben:
> 
> 
> > Date: Thu, 12 Apr 2018 17:24:22 +0200 (CEST)
> > From: Lukas Fürmetz <fuermetz@mailbox.org>
> > Cc: emacs-devel@gnu.org
> > 
> > It was definitly a pipe process (cquery (github.com/cquery-project/cquery) via lsp-mode).
> > I have a reproducable workspace, but I cannot share it. The crash happens, when stdout is
> > split exactly between a windows newline, so between \r and \n.
> 
> Good, thanks.
> 
> Can you build your own Emacs?  If so, I'd like you to test an
> alternative patch.

Re: [PATCH] Add nil check for decoding buffer

[Eli Zaretskii]

> Date: Thu, 12 Apr 2018 17:34:45 +0200 (CEST)
> From: Lukas Fürmetz <fuermetz@mailbox.org>
> Cc: emacs-devel@gnu.org
> 
> Yes I can do that, just send me the patch and I'll give you feedback.

Below.

diff --git a/src/process.c b/src/process.c
index b201e9b..45ab1fd 100644
--- a/src/process.c
+++ b/src/process.c
@@ -2461,6 +2461,10 @@ usage:  (make-pipe-process &rest ARGS)  */)
   /* This may signal an error.  */
   setup_process_coding_systems (proc);
 
+  pset_decoding_buf (p, empty_unibyte_string);
+  eassert (p->decoding_carryover == 0);
+  pset_encoding_buf (p, empty_unibyte_string);
+
   specpdl_ptr = specpdl + specpdl_count;
 
   return proc;

Re: [PATCH] Add nil check for decoding buffer

[Lukas Fürmetz]

The patch works perfectly, thanks!

> Eli Zaretskii <eliz@gnu.org> hat am 12. April 2018 um 18:24 geschrieben:
> 
> 
> > Date: Thu, 12 Apr 2018 17:34:45 +0200 (CEST)
> > From: Lukas Fürmetz <fuermetz@mailbox.org>
> > Cc: emacs-devel@gnu.org
> > 
> > Yes I can do that, just send me the patch and I'll give you feedback.
> 
> Below.
> 
> diff --git a/src/process.c b/src/process.c
> index b201e9b..45ab1fd 100644
> --- a/src/process.c
> +++ b/src/process.c
> @@ -2461,6 +2461,10 @@ usage:  (make-pipe-process &rest ARGS)  */)
>    /* This may signal an error.  */
>    setup_process_coding_systems (proc);
>  
> +  pset_decoding_buf (p, empty_unibyte_string);
> +  eassert (p->decoding_carryover == 0);
> +  pset_encoding_buf (p, empty_unibyte_string);
> +
>    specpdl_ptr = specpdl + specpdl_count;
>  
>    return proc;

Re: [PATCH] Add nil check for decoding buffer

[Eli Zaretskii]

> Date: Fri, 13 Apr 2018 10:47:55 +0200 (CEST)
> From: Lukas Fürmetz <fuermetz@mailbox.org>
> Cc: emacs-devel@gnu.org
> 
> The patch works perfectly, thanks!

Thanks for testing, I pushed the patch to the release branch.