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SubjectRe: [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@grupopie.com>

--
Paulo Marques - www.grupopie.com

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


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