[nft PATCH] parser_bison: Fix for bison < 3.6

[nft PATCH] parser_bison: Fix for bison < 3.6

[Phil Sutter]

Support for 'custom' parse.error value was added in bison-3.6. Fall back
to previous value for earlier versions.

This is harder to get right than it seems: On one hand, preprocessor
macros can't be used in parser_bison.y's declaration section and
automake forbids conditional changes to AM_YFLAGS on the other.

Suggested-by: Jan Kończak <jan.konczak@cs.put.poznan.pl>
Fixes: 67b822f2b2624 ("parser_bison: on syntax errors, output expected tokens")
Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 Makefile.am        |  6 ++++++
 configure.ac       | 12 ++++++++++++
 src/parser_bison.y |  4 ++--
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/Makefile.am b/Makefile.am
index fa71e06eefee5..ddf145a87c810 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -164,6 +164,12 @@ AM_CFLAGS = \
 	$(NULL)
 
 AM_YFLAGS = -d -Wno-yacc
+if BISON_CUSTOM_ERROR
+YACC += -D parse.error=custom -D parse.lac=full
+AM_CFLAGS += -DBISON_CUSTOM_ERROR
+else
+YACC += -D parse.error=verbose
+endif
 
 if BUILD_PROFILING
 AM_CFLAGS += --coverage
diff --git a/configure.ac b/configure.ac
index 0d3ee2ac89f69..b6cad3117a51b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -45,6 +45,18 @@ then
         exit 1
 fi
 
+AC_ARG_ENABLE([extended_parser_errors],
+	      AS_HELP_STRING([--disable-extended-parser-errors],
+			     [Disable use of parse.error=custom and LAC in Bison]),
+	      [], [
+			enable_extended_parser_errors=no
+			AC_SUBST([BISON], [$ac_cv_prog_YACC])
+			AX_PROG_BISON_VERSION([3.6],
+					      [enable_extended_parser_errors=yes])
+	      ])
+AM_CONDITIONAL([BISON_CUSTOM_ERROR],
+	       [test "x$enable_extended_parser_errors" != xno])
+
 AM_PROG_AR
 LT_INIT([disable-static])
 AC_EXEEXT
diff --git a/src/parser_bison.y b/src/parser_bison.y
index 5a334bf0c4997..fc95597d898c0 100644
--- a/src/parser_bison.y
+++ b/src/parser_bison.y
@@ -221,8 +221,6 @@ int nft_lex(void *, void *, void *);
 %parse-param		{ void *scanner }
 %parse-param		{ struct parser_state *state }
 %lex-param		{ scanner }
-%define parse.error custom
-%define parse.lac full
 %locations
 
 %initial-action {
@@ -6537,6 +6535,7 @@ exthdr_key		:	HBH	close_scope_hbh	{ $$ = IPPROTO_HOPOPTS; }
 
 %%
 
+#ifdef BISON_CUSTOM_ERROR
 static int
 yyreport_syntax_error(const yypcontext_t *yyctx, struct nft_ctx *nft,
                       void *scanner, struct parser_state *state)
@@ -6592,3 +6591,4 @@ yyreport_syntax_error(const yypcontext_t *yyctx, struct nft_ctx *nft,
 	free(msg);
 	return 0;
 }
+#endif /* BISON_CUSTOM_ERROR */
-- 
2.54.0

Re: [nft PATCH] parser_bison: Fix for bison < 3.6

[Pablo Neira Ayuso]

Hi Phil,

On Wed, Jun 10, 2026 at 01:57:09PM +0200, Phil Sutter wrote:
> Support for 'custom' parse.error value was added in bison-3.6. Fall back
> to previous value for earlier versions.
> 
> This is harder to get right than it seems: On one hand, preprocessor
> macros can't be used in parser_bison.y's declaration section and
> automake forbids conditional changes to AM_YFLAGS on the other.
> 
> Suggested-by: Jan Kończak <jan.konczak@cs.put.poznan.pl>
> Fixes: 67b822f2b2624 ("parser_bison: on syntax errors, output expected tokens")
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
>  Makefile.am        |  6 ++++++
>  configure.ac       | 12 ++++++++++++
>  src/parser_bison.y |  4 ++--
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile.am b/Makefile.am
> index fa71e06eefee5..ddf145a87c810 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -164,6 +164,12 @@ AM_CFLAGS = \
>  	$(NULL)
>  
>  AM_YFLAGS = -d -Wno-yacc
> +if BISON_CUSTOM_ERROR
> +YACC += -D parse.error=custom -D parse.lac=full
> +AM_CFLAGS += -DBISON_CUSTOM_ERROR
> +else
> +YACC += -D parse.error=verbose
> +endif
>  
>  if BUILD_PROFILING
>  AM_CFLAGS += --coverage
> diff --git a/configure.ac b/configure.ac
> index 0d3ee2ac89f69..b6cad3117a51b 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -45,6 +45,18 @@ then
>          exit 1
>  fi
>  
> +AC_ARG_ENABLE([extended_parser_errors],
> +	      AS_HELP_STRING([--disable-extended-parser-errors],
> +			     [Disable use of parse.error=custom and LAC in Bison]),
> +	      [], [
> +			enable_extended_parser_errors=no
> +			AC_SUBST([BISON], [$ac_cv_prog_YACC])
> +			AX_PROG_BISON_VERSION([3.6],
> +					      [enable_extended_parser_errors=yes])
> +	      ])
> +AM_CONDITIONAL([BISON_CUSTOM_ERROR],
> +	       [test "x$enable_extended_parser_errors" != xno])

Can this be made transparent? ie. if bison >= 3.6, then enable it
always. Otherwise, disable it.

Then, include this information here in configure.ac:

echo "
nft configuration:
  cli support:                  ${with_cli}
  enable debugging symbols:     ${enable_debug}
  use mini-gmp:                 ${with_mini_gmp}
  enable man page:              ${enable_man_doc}
  libxtables support:           ${with_xtables}
  json output support:          ${with_json}
  collect profiling data:       ${enable_profiling}"

and here with -V:

# nft -V
nftables v1.1.6 (Commodore Bullmoose #7)
  cli:          editline
  json:         yes
  minigmp:      no
  libxtables:   yes

Maybe add:

  bison >= 3.6: yes

or similar?

Thanks

>  AM_PROG_AR
>  LT_INIT([disable-static])
>  AC_EXEEXT
> diff --git a/src/parser_bison.y b/src/parser_bison.y
> index 5a334bf0c4997..fc95597d898c0 100644
> --- a/src/parser_bison.y
> +++ b/src/parser_bison.y
> @@ -221,8 +221,6 @@ int nft_lex(void *, void *, void *);
>  %parse-param		{ void *scanner }
>  %parse-param		{ struct parser_state *state }
>  %lex-param		{ scanner }
> -%define parse.error custom
> -%define parse.lac full
>  %locations
>  
>  %initial-action {
> @@ -6537,6 +6535,7 @@ exthdr_key		:	HBH	close_scope_hbh	{ $$ = IPPROTO_HOPOPTS; }
>  
>  %%
>  
> +#ifdef BISON_CUSTOM_ERROR
>  static int
>  yyreport_syntax_error(const yypcontext_t *yyctx, struct nft_ctx *nft,
>                        void *scanner, struct parser_state *state)
> @@ -6592,3 +6591,4 @@ yyreport_syntax_error(const yypcontext_t *yyctx, struct nft_ctx *nft,
>  	free(msg);
>  	return 0;
>  }
> +#endif /* BISON_CUSTOM_ERROR */
> -- 
> 2.54.0
> 

Re: [nft PATCH] parser_bison: Fix for bison < 3.6

[Phil Sutter]

Hi Pablo,

On Wed, Jun 10, 2026 at 05:16:39PM +0200, Pablo Neira Ayuso wrote:
> On Wed, Jun 10, 2026 at 01:57:09PM +0200, Phil Sutter wrote:
> > Support for 'custom' parse.error value was added in bison-3.6. Fall back
> > to previous value for earlier versions.
> > 
> > This is harder to get right than it seems: On one hand, preprocessor
> > macros can't be used in parser_bison.y's declaration section and
> > automake forbids conditional changes to AM_YFLAGS on the other.
> > 
> > Suggested-by: Jan Kończak <jan.konczak@cs.put.poznan.pl>
> > Fixes: 67b822f2b2624 ("parser_bison: on syntax errors, output expected tokens")
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> >  Makefile.am        |  6 ++++++
> >  configure.ac       | 12 ++++++++++++
> >  src/parser_bison.y |  4 ++--
> >  3 files changed, 20 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Makefile.am b/Makefile.am
> > index fa71e06eefee5..ddf145a87c810 100644
> > --- a/Makefile.am
> > +++ b/Makefile.am
> > @@ -164,6 +164,12 @@ AM_CFLAGS = \
> >  	$(NULL)
> >  
> >  AM_YFLAGS = -d -Wno-yacc
> > +if BISON_CUSTOM_ERROR
> > +YACC += -D parse.error=custom -D parse.lac=full
> > +AM_CFLAGS += -DBISON_CUSTOM_ERROR
> > +else
> > +YACC += -D parse.error=verbose
> > +endif
> >  
> >  if BUILD_PROFILING
> >  AM_CFLAGS += --coverage
> > diff --git a/configure.ac b/configure.ac
> > index 0d3ee2ac89f69..b6cad3117a51b 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -45,6 +45,18 @@ then
> >          exit 1
> >  fi
> >  
> > +AC_ARG_ENABLE([extended_parser_errors],
> > +	      AS_HELP_STRING([--disable-extended-parser-errors],
> > +			     [Disable use of parse.error=custom and LAC in Bison]),
> > +	      [], [
> > +			enable_extended_parser_errors=no
> > +			AC_SUBST([BISON], [$ac_cv_prog_YACC])
> > +			AX_PROG_BISON_VERSION([3.6],
> > +					      [enable_extended_parser_errors=yes])
> > +	      ])
> > +AM_CONDITIONAL([BISON_CUSTOM_ERROR],
> > +	       [test "x$enable_extended_parser_errors" != xno])
> 
> Can this be made transparent? ie. if bison >= 3.6, then enable it
> always. Otherwise, disable it.

This is the default behaviour (which is the "else" case of that huge
AC_ARG_ENABLE). Users may choose to override the version-based setting
via --{en,dis}able-extended-parser-errors. Fine with you or am I missing
your point?

> Then, include this information here in configure.ac:
> 
> echo "
> nft configuration:
>   cli support:                  ${with_cli}
>   enable debugging symbols:     ${enable_debug}
>   use mini-gmp:                 ${with_mini_gmp}
>   enable man page:              ${enable_man_doc}
>   libxtables support:           ${with_xtables}
>   json output support:          ${with_json}
>   collect profiling data:       ${enable_profiling}"
> 
> and here with -V:
> 
> # nft -V
> nftables v1.1.6 (Commodore Bullmoose #7)
>   cli:          editline
>   json:         yes
>   minigmp:      no
>   libxtables:   yes

Oh right, I keep forgetting about the summary (which is really useful as
reading config.log needs training).

> Maybe add:
> 
>   bison >= 3.6: yes
> 
> or similar?

I'd go with "extended parser errors", to remain consistent with the
config option and also because it calls out what actually changed.

There is one other oddity still which I should check: The configure
script always checks bison version, but 'make dist' tarballs ship with
parser_bison.c and thus don't need bison at all. So this patch might
disable the extended error messages for tarball users which don't have
bison installed.

Cheers, Phil