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From: Richard Sandiford <richard.sandiford@arm.com>
To: Alex Coplan via Gcc-patches <gcc-patches@gcc.gnu.org>
Mail-Followup-To: Alex Coplan via Gcc-patches <gcc-patches@gcc.gnu.org>,Alex Coplan <alex.coplan@arm.com>, richard.sandiford@arm.com
Cc: Alex Coplan <alex.coplan@arm.com>
Subject: Re: [PATCH] varasm: Fix type confusion bug
References: <Y4jJSkO6Ccew5OjL@arm.com>
Date: Thu, 01 Dec 2022 16:12:09 +0000
In-Reply-To: <Y4jJSkO6Ccew5OjL@arm.com> (Alex Coplan via Gcc-patches's message
	of "Thu, 1 Dec 2022 15:33:30 +0000")
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Alex Coplan via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hi,
>
> This patch fixes a type confusion bug in varasm.cc:assemble_variable.
> The problem is that the current code calls:
>
>   sect = get_variable_section (decl, false);
>
> and then accesses sect->named.name without checking whether the section
> is in fact a named section. In the surrounding else clause, we only know
> that SECTION_STYLE (sect) != SECTION_NOSWITCH, so it is possible that
> the section is an unnamed section.
>
> In practice, this means that we end up doing a wild string compare
> between a function pointer and the string literal ".vtable_map_vars".
> This is because sect->named.name aliases sect->unnamed.callback in the
> section union.
>
> This can be seen in GDB with a simple testcase such as "int x;".
>
> This patch fixes the issue by checking the SECTION_STYLE of the section
> is in fact SECTION_NAMED before trying to do the string comparison.
>
> We drop the existing check of whether sect->named.name is non-NULL
> because this should presumably always be the case for a named section.
>
> Bootstrapped/regtested on aarch64-none-linux-gnu, OK for trunk?

OK, thanks.  I think it's OK for backports too if you like,
since it's a regression from around 2013.

Richard

>
> Thanks,
> Alex
>
> gcc/ChangeLog:
>
> 	* varasm.cc (assemble_variable): Fix type confusion bug when
> 	checking for ".vtable_map_vars" section.
>
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index 9dfbebbb915..6851201b6a2 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -2400,7 +2400,7 @@ assemble_variable (tree decl, int top_level ATTRIBUTE_UNUSED,
>    else
>      {
>        /* Special-case handling of vtv comdat sections.  */
> -      if (sect->named.name
> +      if (SECTION_STYLE (sect) == SECTION_NAMED
>  	  && (strcmp (sect->named.name, ".vtable_map_vars") == 0))
>  	handle_vtv_comdat_section (sect, decl);
>        else