Yes, you are right. This code is accessing uninitialized data at the  
end of the item list.

I think the fix should maybe look something like this:

	register u_int		i, total;

...
total = result->sr_item.i_total_size - node_ref_size) / full_item_size;

...
if (i < total)
	xt_get_res_record_ref(bitem + ind->mi_key_size, result);


total can then be used where "result->sr_item.i_total_size -  
node_ref_size) / full_item_size" is used.

On Mar 16, 2010, at 11:57 AM, Kristian Nielsen wrote:

> Actually, there is an even simpler case where i points after the  
> array:
>
> 	else if (search_flags & XT_SEARCH_AFTER_LAST_FLAG)
> 		i = (result->sr_item.i_total_size - node_ref_size) / full_item_size;
>
> -- 
> Read past end of buffer in xt_scan_branch_single() and similar  
> functions
> https://bugs.launchpad.net/bugs/539480
> You received this bug notification because you are subscribed to PBXT.
>
> Status in Maria: New
> Status in PrimeBase XT: New
>
> Bug description:
> I found this from a Valgrind warning:
>
> ==3619== Invalid read of size 4
> ==3619==    at 0x9FF73F: xt_get_res_record_ref(unsigned char*,  
> XTIdxResult*) (index_xt.h:454)
> ==3619==    by 0x9F4E90: xt_scan_branch_single(XTTable*, XTIndex*,  
> XTIdxBranch*, XTIdxKeyValue*, XTIdxResult*) (index_xt.cc:501)
> ==3619==    by 0x9FEE6B: xt_idx_insert(XTOpenTable*, XTIndex*,  
> unsigned, unsigned, unsigned char*, unsigned char*, int)  
> (index_xt.cc:1877)
> ==3619==    by 0xA1924B: xt_tab_new_record(XTOpenTable*, unsigned  
> char*) (table_xt.cc:4392)
> ==3619==    by 0x9EECD9: ha_pbxt::write_row(unsigned char*)  
> (ha_pbxt.cc:2645)
> ==3619==    by 0x7C4CC1: handler::ha_write_row(unsigned char*)  
> (handler.cc:4642)
> ==3619==    by 0x7E3087: copy_data_between_tables(st_table*,  
> st_table*, List<Create_field>&, bool, unsigned, st_order*, unsigned  
> long long*, unsigned long long*, enum_enable_or_disable, bool)  
> (sql_table.cc:7779)
> ==3619==    by 0x7F05E2: mysql_alter_table(THD*, char*, char*,  
> st_ha_create_information*, TABLE_LIST*, Alter_info*, unsigned,  
> st_order*, bool) (sql_table.cc:7221)
> ==3619==    by 0x68BE71: mysql_execute_command(THD*) (sql_parse.cc: 
> 2959)
> ==3619==    by 0x692F7F: mysql_parse(THD*, char const*, unsigned,  
> char const**) (sql_parse.cc:6034)
> ==3619==    by 0x693D91: dispatch_command(enum_server_command, THD*,  
> char*, unsigned) (sql_parse.cc:1247)
> ==3619==    by 0x69528F: do_command(THD*) (sql_parse.cc:886)
> ==3619==    by 0x68066C: handle_one_connection (sql_connect.cc:1132)
> ==3619==    by 0x50463F6: start_thread (pthread_create.c:297)
> ==3619==    by 0x6026B4C: clone (in /usr/lib/debug/libc-2.7.so)
> ==3619==  Address 0xcf98030 is 0 bytes after a block of size  
> 33,701,888 alloc'd
> ==3619==    at 0x4C22FAB: malloc (vg_replace_malloc.c:207)
> ==3619==    by 0x9FF9A2: xt_malloc(XTThread*, unsigned long)  
> (memory_xt.cc:101)
> ==3619==    by 0xA439F2: xt_ind_init(XTThread*, unsigned long)  
> (cache_xt.cc:632)
> ==3619==    by 0x9EBDBC: pbxt_call_init(XTThread*) (ha_pbxt.cc:974)
> ==3619==    by 0x9EC109: pbxt_init(void*) (ha_pbxt.cc:1194)
> ==3619==    by 0x7C8E01: ha_initialize_handlerton(st_plugin_int*)  
> (handler.cc:429)
> ==3619==    by 0x88ADD6: plugin_initialize(st_plugin_int*)  
> (sql_plugin.cc:1033)
> ==3619==    by 0x88E979: plugin_init(int*, char**, int)  
> (sql_plugin.cc:1258)
> ==3619==    by 0x67A21C: init_server_components() (mysqld.cc:4069)
> ==3619==    by 0x67ACF5: main (mysqld.cc:4541)
>
> This is from test case pbxt.multi_update (which takes ~1h to run under
> Valgrind :-/).
>
> I think this is a real bug, although possibly minor. Not knowing the  
> code in
> detail, I need help in determining the proper fix.
>
> xt_scan_branch_single() basically does a binary search on a block of
> memory. There is a while () loop, which exits with i pointing at the  
> position
> being searched for.
>
> Note that (at least from a quick look at the code), if the value being
> searched for is bigger than all values in the buffer, then the loop  
> will exit
> with i pointing past the last element.
>
> After the loop, the code does this:
>
>        bitem = base + i * full_item_size;
>        xt_get_res_record_ref(bitem + ind->mi_key_size, result);
>
>> From the Valgrind trace, it seems to me the problem is that in this  
>> case i
> points past the last element as described above, so the
> xt_get_res_record_ref() reads 8 bytes past the end of the buffer.
>
> This should probably be fixed (in theory this could cause the  
> process to
> segfault if memory allocation is really unfortunate and these 8  
> bytes happen
> to be unmapped). Maybe just something like
>
>   if (result.sr_found)
>        xt_get_res_record_ref(bitem + ind->mi_key_size, result);
>
> in case the values read are only used in the "found" case?
>
> Hope you can sort out the rest from this explanation.
>
>



--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com