linux-kernel - Re: [PATCH] genksyms: Duplicate function pointer type definitions segfault

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Date:	Mon, 20 Jul 2015 21:28:44 -0400
From:	Richard Yao <richard.yao@...sterhq.com>
To:	Richard Yao <ryao@...too.org>
Cc:	linux-kernel@...r.kernel.org,
	Madhuri Yechuri <madhuriyechuri@...sterhq.com>
Subject: Re: [PATCH] genksyms: Duplicate function pointer type definitions segfault

On 20 July 2015 at 19:52, Richard Yao <ryao@...too.org> wrote:
>
> From: Richard Yao <richard.yao@...sterhq.com>
>
> I noticed that genksyms will segfault when it sees duplicate function
> pointer type declaration when I placed the same function pointer
> definition in two separate headers in a local branch as an intermediate
> step of some refactoring. This can be reproduced by piping the following
> minimal test case into `genksyms -r /dev/null` or alternatively, putting
> it into a C file attempting a build:
>
> int (*f)();
> int (*f)();


There is a typo in the commit message. This should have been:

typedef int (*f)();
typedef int (*f)();
>
>
> Attaching gdb to genksyms to understand this failure is useless without
> changing CFLAGS to emit debuginfo. Once you have debuginfo, you will
> find that the failure is that `char *s` was NULL and the program
> executed `while(*s)`. At which point, further debugging requires
> familiarity with compiler front end / parser development.
>
> What happens is that flex identifies the first instance of the token "f"
> as IDENT and the yacc parser adds it to the symbol table. On the second
> instance, flex will identify "f" as TYPE, which triggers an error case
> in the yacc parser. Given that TYPE would have been IDENT had it not
> been in the symbol table, the the segmentaion fault could be avoided by
> treating TYPE as IDENT in the affected rule.
>
> Some might consider placing identical function pointer type declarations
> in different headers to be poor style might consider a failure to be
> beneficial. However, failing through a segmentation fault makes the
> cause non-obvious and can waste the time of anyone who encounters it.
>
> Signed-off-by: Richard Yao <richard.yao@...sterhq.com>
> Acked-by: Madhuri Yechuri <madhuriyechuri@...sterhq.com>
> ---
>  scripts/genksyms/parse.y | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/scripts/genksyms/parse.y b/scripts/genksyms/parse.y
> index b9f4cf2..723ab30 100644
> --- a/scripts/genksyms/parse.y
> +++ b/scripts/genksyms/parse.y
> @@ -303,6 +303,15 @@ direct_declarator:
>                     $$ = $1;
>                   }
>                 }
> +       | TYPE
> +               { if (current_name != NULL) {
> +                   error_with_pos("unexpected second declaration name");
> +                   YYERROR;
> +                 } else {
> +                   current_name = (*$1)->string;
> +                   $$ = $1;
> +                 }
> +               }
>         | direct_declarator '(' parameter_declaration_clause ')'
>                 { $$ = $4; }
>         | direct_declarator '(' error ')'
> --
> 2.3.6
>
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