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.
Signed-off-by: Xiaochen Wang <[email protected]>
---
scripts/kallsyms.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 60dd3eb..487ac6f 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -500,6 +500,8 @@ static void optimize_result(void)
/* find the token with the breates profit value */
best = find_best_token();
+ if (token_profit[best] == 0)
+ break;
/* place it in the "best" table */
best_table_len[i] = 2;
--
1.7.2.3
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 <[email protected]>
--
Paulo Marques - http://www.grupopie.com
"Conservation of angular momentum makes the world go around" - John Clark
On Sun, May 01, 2011 at 11:41:41AM +0800, 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.
>
>
> Signed-off-by: Xiaochen Wang <[email protected]>
> ---
> scripts/kallsyms.c | 2 ++
> 1 files changed, 2 insertions(+), 0 deletions(-)
Pushed to kbuild-2.6.git#kbuild, thanks.
Michal