[ECOS] RedBoot xyModem bug
Gary Thomas
gary@mlbassoc.com
Tue Jan 22 23:47:00 GMT 2008
Rutger Hofman wrote:
> Is *really* *nobody* interested in bugs in the RedBoot upload
> implementation?
>
> Rutger
>
> Rutger Hofman wrote on Jan 11, 2008:
>> Hello list,
>>
>> I ran into a bug in RedBoot xyModem.c. It is in lines 420..434 in the
>> current version (cvs 1.21):
>>
>> while (!xyz.at_eof && (size > 0)) {
>> ... code for each block in the xyzModem protocols: ...
>>
>> #if defined(xyzModem_zmodem) || defined(USE_YMODEM_LENGTH)
>> if (xyz.mode == xyzModem_xmodem ||
>> xyz.file_length == 0) {
>> #else
>> if (1) {
>> #endif
>> // Data blocks can be padded with ^Z (EOF)
>> characters
>> // This code tries to detect and remove them
>> if ((xyz.bufp[xyz.len-1] == EOF) &&
>> (xyz.bufp[xyz.len-2] == EOF) &&
>> (xyz.bufp[xyz.len-3] == EOF)) {
>> while (xyz.len && (xyz.bufp[xyz.len-1]
>> == EOF)) {
>> xyz.len--;
>> }
>> }
>> }
>>
>> What this does, is inspect each uploaded block for trailing ^Z = 0x1a
>> bytes, and strip them. Rationale: xmodem or ymodem may signal
>> end-of-file with ^Z a.k.a. 'CP/M EOF', and may optionally pad the
>> block with more ^Z chars. This is live behaviour: sx and sy (0.12.20)
>> under Linux do ^Z padding in the last block. The xyModem.c code above
>> is wrong. ^Z chars are stripped at the end of *each block* in stead of
>> at the end of the file.
>>
>> This problem surfaced when I enabled zlib in eCos. In one of its
>> tables, there is a sequence of bytes 26 = 0x1a = ^Z (in
>> packages/services/compress/zlib/current/src/trees.h, lines 90, 91,
>> 112-114), which may trigger the test above. Depending on the offset in
>> the file, these 0x1a's hit a block boundary or not, and the file is
>> telescope-truncated or not. (I noticed that my executable crashed or
>> not depending on code size, even depending on a number of 'nop'
>> assembly instructions that I inserted anywhere in the code; it was a
>> beast of a bug to pin down...)
>>
>> The fix is nonobvious to me. For ymodem, if #define USE_YMODEM_LENGTH
>> is on and a file length is specified, any trailing characters are
>> removed elsewhere: this works correctly. However, for xmodem or ymodem
>> without file length, any trailing ^Z at the end-of-file must still be
>> removed. The code above is inherently unaware whether this is the last
>> block or not: xmodem sends end-of-file control info, but that is
>> consumed only in the *next* block read. The solution might be to
>> maintain one more read-ahead block buffer to see if the current block
>> is the last normal block.
>>
>> Some random remarks:
>>
>> As a workaround (I dare not flash the RedBoot in my device, because we
>> bought it ready-made; I use the code base for my eCos application), I
>> tried disabling the table in zlib. That lead to another problem, which
>> I will report separately.
Why not test using a RAM RedBoot configuration? This should be
totally safe and won't "brick" your device.
>> I am in the dark what must be done if a file to be xmodem'd actually
>> contains trailing ^Z characters. There is nothing that forbids that
>> outside CP/M or MS-DOS, I'd say.
>>
>> I also wondered how this bug can have lived for so long in RedBoot. Or
>> doesn't anyone use xyModem nowadays?
Indeed, it does get used, but this problem has only been
seen/reported by you. If you want to see things happen, send patches.
--
------------------------------------------------------------
Gary Thomas | Consulting for the
MLB Associates | Embedded world
------------------------------------------------------------
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