[v2] Do not abort compilation when dump file is /dev/*

The `configure` scripts generated with autoconf often tests compiler
features by setting output to `/dev/null`, which then sets the dump
folder as being /dev/* and the compilation halts with an error because
GCC cannot create files in /dev/. This is a problem when configure is
testing for compiler features because it cannot tell if the failure was
due to unsupported features or any other problem, and disable it even
if it is working.

As an example, running configure overriding CFLAGS="-fdump-ipa-clones"
will result in several compiler-features as being disabled because of
gcc halting with an error creating files in /dev/*.

This commit fixes this issue by checking if the output file is
/dev/null or /dev/zero. In this case we use the current working
directory for dump output instead of the directory of the output
file because we cannot write to /dev/*.

gcc/ChangeLog
2021-11-16  Giuliano Belinassi  <gbelinassi@suse.de>

	* gcc.c (process_command): Skip dumpdir override on -o /dev/null
        or -o /dev/zero.

gcc/testsuite/ChangeLog
2021-11-16  Giuliano Belinassi  <gbelinassi@suse.de>

	* gcc.dg/devnull-dump.c: New.

Signed-off-by: Giuliano Belinassi <gbelinassi@suse.de>
---
 gcc/doc/invoke.texi                 |  4 +++-
 gcc/gcc.c                           | 19 +++++++++++++++----
 gcc/testsuite/gcc.dg/devnull-dump.c |  7 +++++++
 3 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/devnull-dump.c

On Thu, 18 Nov 2021, Giuliano Belinassi wrote:

> The `configure` scripts generated with autoconf often tests compiler
> features by setting output to `/dev/null`, which then sets the dump
> folder as being /dev/* and the compilation halts with an error because
> GCC cannot create files in /dev/. This is a problem when configure is
> testing for compiler features because it cannot tell if the failure was
> due to unsupported features or any other problem, and disable it even
> if it is working.
> 
> As an example, running configure overriding CFLAGS="-fdump-ipa-clones"
> will result in several compiler-features as being disabled because of
> gcc halting with an error creating files in /dev/*.
> 
> This commit fixes this issue by checking if the output file is
> /dev/null or /dev/zero. In this case we use the current working
> directory for dump output instead of the directory of the output
> file because we cannot write to /dev/*.

Comments below.

> gcc/ChangeLog
> 2021-11-16  Giuliano Belinassi  <gbelinassi@suse.de>
> 
> 	* gcc.c (process_command): Skip dumpdir override on -o /dev/null
>         or -o /dev/zero.
> 
> gcc/testsuite/ChangeLog
> 2021-11-16  Giuliano Belinassi  <gbelinassi@suse.de>
> 
> 	* gcc.dg/devnull-dump.c: New.
> 
> Signed-off-by: Giuliano Belinassi <gbelinassi@suse.de>
> ---
>  gcc/doc/invoke.texi                 |  4 +++-
>  gcc/gcc.c                           | 19 +++++++++++++++----
>  gcc/testsuite/gcc.dg/devnull-dump.c |  7 +++++++
>  3 files changed, 25 insertions(+), 5 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/devnull-dump.c
> 
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 6070288856c..4c056606e0c 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -1877,7 +1877,9 @@ named @file{dir/bar.*}, combining the given @var{dumppfx} with the
>  default @var{dumpbase} derived from the primary output name.  Dump
>  outputs also take the input name suffix: @file{dir/bar.c.*}.
>  
> -It defaults to the location of the output file; options
> +It defaults to the location of the output file, unless @option{-o /dev/null}
> +or @option{-o /dev/zero} is passed to the compiler: on those cases the @option{cwd}
> +will be used instead. Options

"It defaults to the location of the output file, unless the output
file is a special file like @code{/dev/null}."

>  @option{-save-temps=cwd} and @option{-save-temps=obj} override this
>  default, just like an explicit @option{-dumpdir} option.  In case
>  multiple such options are given, the last one prevails:
> diff --git a/gcc/gcc.c b/gcc/gcc.c
> index 506c2acc282..0173018ac04 100644
> --- a/gcc/gcc.c
> +++ b/gcc/gcc.c
> @@ -5109,10 +5109,21 @@ process_command (unsigned int decoded_options_count,
>      {
>        free (dumpdir);
>        dumpdir = NULL;
> -      temp = lbasename (output_file);
> -      if (temp != output_file)
> -	dumpdir = xstrndup (output_file,
> -			    strlen (output_file) - strlen (temp));
> +      if (strcmp(output_file, "/dev/null") == 0
> +	  || strcmp(output_file, "/dev/zero") == 0)

Please merge this into the condition of the block and update the comment

  /* If -save-temps=obj and -o name, create the prefix to use for %b.
     Otherwise just make -save-temps=obj the same as -save-temps=cwd.  */
  else if (save_temps_flag != SAVE_TEMPS_CWD && output_file != NULL)

please split out

          strcmp(output_file, "/dev/null") == 0
          || strcmp(output_file, "/dev/zero") == 0

into a bool special_output_location (const char *) function.  For
example on Windows the special output location would be NUL
(appearantly any path ending in NUL works like that - huh).

Note it might be possible to stat() the file and simply behave
this way whenever it exists and is not a regular file.  For start
just matching literal /dev/null is good enough IMHO (did you
run into cases with /dev/zero?).

Thanks,
Richard.

> +	{
> +	  /* If output is /dev/null or /dev/zero, we cannot set the dumpdir to
> +	     /dev/ because no files can be created on that directory.  In this
> +	     case, we do nothing.  */
> +	}
> +      else
> +	{
> +	  /* Set dumpdir as being the same directory where the output file is.  */
> +	  temp = lbasename (output_file);
> +	  if (temp != output_file)
> +	    dumpdir = xstrndup (output_file,
> +				strlen (output_file) - strlen (temp));
> +	}
>      }
>    else if (dumpdir)
>      {
> diff --git a/gcc/testsuite/gcc.dg/devnull-dump.c b/gcc/testsuite/gcc.dg/devnull-dump.c
> new file mode 100644
> index 00000000000..378e0901c28
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/devnull-dump.c
> @@ -0,0 +1,7 @@
> +/* { dg-do assemble } */
> +/* { dg-options "-fdump-ipa-clones -o /dev/null" } */
> +
> +int main()
> +{
> +  return 0;
> +}
>