linux-kernel - Re: [PATCH] scripts/kallsyms.c: fix potential segfault

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Date:	Tue, 03 May 2011 12:56:00 +0100
From:	Paulo Marques <pmarques@...popie.com>
To:	Xiaochen Wang <wangxiaochen0@...il.com>
CC:	Jean Sacren <sakiwit@...il.com>, Michal Marek <mmarek@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scripts/kallsyms.c: fix potential segfault

Xiaochen Wang wrote:
> Description:
> This bug hardly appears during real kernel compiling,
>  because the vmlinux symbols table is huge.
> 
> But we can still catch it under strict condition , as follows.
>    $ echo "c101b97b T do_fork" | ./scripts/kallsyms --all-symbols
>    #include <asm/types.h>
>    ......
>    ......
>    .globl kallsyms_token_table
>            ALGN
>    kallsyms_token_table:
>    Segmentation fault (core dumped)
>    $
> 
> If symbols table is small, all entries in token_profit[0x10000] may
> decrease to 0 after several calls of compress_symbols() in optimize_result().
> In that case, find_best_token() always return 0 and
> best_table[i] is set to "\0\0" and best_table_len[i] is set to 2.
> 
> As a result, expand_symbol(best_table[0]="\0\0", best_table_len[0]=2, buf)
> in write_src() will run in infinite recursion until stack overflows,
> causing segfault.
> 
> This patch checks the find_best_token() return value. If all entries in
> token_profit[0x10000] become 0 according to return value, it breaks the loop
> in optimize_result().
> And expand_symbol() works well when best_table_len[i] is 0.

For this to happen on a real kernel, the list of symbols needs to have
less than 256 single characters + two letter sequences, which is highly
unlikely. From a quick test, a symbol table of a mere 30 symbols was
able to overcome those constraints.

Having said that, the fix is very small, it is not on a hot-path and it
is the right thing to do from a correctness stand point, so you have my ack.

Acked-by: Paulo Marques <pmarques@...popie.com>

-- 
Paulo Marques - www.grupopie.com

"Conservation of angular momentum makes the world go around" - John Clark
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