Online checksum calculator 16b how to#

LetвЂ™s check kcbhxoro:Д x00007fa2e1154b1a : callq 0x7fa2df2be5f8 ДЄs you can see, calling sxorchk is conditional and it can be skipped by highlighted instructions вЂ“ and it happens in our case. But sxorchk never fires when we count a checksum from BBED. IвЂ™ve consulted Frits and he told me that this kcbhxoro launches sxorchk, which is doing the work. Type commands for breakpoint(s) 1, one per line.ДЄfter executing a sum command in a BBED IвЂ™ll see a set of functions that returned with a specified value вЂ“ of course, IвЂ™m interested only in the first one:Д x00007f3c5857eb6b in kcbhxoro () from /u01/app/oracle/product/12.1.0/dbhome_1/lib/libclntsh.so.12.1 In most of the cases on Intel and AMD it will be RAX: The CPU documentation will tell which register to observe.

In assembler, a result from a function has to be set on one of the registers.

Online checksum calculator 16b how to#

Now we know what the checksum should be and how to trigger BBED to count it вЂ“ itвЂ™s debugging time!!!ДЇirstly IвЂ™ll attach to my BBED process with a GDB and IвЂ™ll set a watchpoint. '/u01/app/oracle/oradata/SKIPER/datafile/o1_mf_tbs_dupa_f5rzk9yl_.dbf'Д«BED will also show us that a checksum in a block is different than it shout be: ORA-01578: ORACLE data block corrupted (file # 7, block # 139) Now we can see that BBED is showing me the changes correctly, but when IвЂ™ll try to read a block from Oracle: Warning: contents of previous BIFILE will be lost. LetвЂ™s check what will happen if IвЂ™ll change the value in a database block at a low level without putting the appropriate checksum.

Since IвЂ™m doing this on Intel, the actual hex value in a datafile is written as 'E040вЂі.Д«BED can calculate a checksum with command 'sum' and apply it to a block with a command 'sum apply'. Please note that BBED shows the hex values format in a human-readable manner like in a Big Endian system. LetвЂ™s check it with BBED:ДЄs you can see the value is 0x40e0 so 13710. This checksum in a database block is stored on 16th byte. The database can use the checksum value to check consistency.

If the DB_BLOCK_CHECKSUM initialization parameter is enabled, then the database calculates the checksum for every data file or online redo log block and stores it in the block header when writing to disk. Meanwhile, I want to share with you my investigation on calculating a block checksum.ДЄ number calculated by the database from all the bytes stored in a data or redo block. Maybe as an option for ODBV or a separate tool? I havenвЂ™t decided yet but IвЂ™ll cover it with a blog post for sure! When IвЂ™ll finish the tool, it will be available on my GitHub. Recently IвЂ™ve started to write my own clone of BBED to have something handy and useful in extreme cases when you have to go deep and fix stuff on low level (I have only like 2 such cases a year but each time it is really fun and a nice money рџ‰ )