Hi, On Thu, Nov 7, 2024 at 2:23 PM Omar Sandoval <osandov@xxxxxxxxxxx> wrote: > > Hi everyone, > > Amal is working on adding a custom query packet to kgdb for getting the > kernel's vmcoreinfo. The rationale and details are available here: > https://github.com/osandov/drgn/wiki/GDB-Remote-Protocol-proposal:-linux.vmcoreinfo-query-packet > > vmcoreinfo is about 3kB, so we were hoping to avoid hex-encoding the > response and doubling the time it takes to transmit over a slow serial > connection. Instead, we were hoping to use the escaped binary format, > which escapes the characters #$}* and leaves other bytes untouched. > > We ran into a problem, though: vmcoreinfo contains newline characters, > which the serial core replaces with CRLF; see commit c7d44a02ac60 > ("serial_core: Commonalize crlf when working w/ a non open console > port"). FWIW, the problem predates that commit, but that commit at least moved it to be someplace common. Before that some serial drivers were hardcoding it... ;-) > This effectively corrupts the data and causes a checksum > mismatch. > > We'd love some input on how to work around this, especially from the > kgdb maintainers. Here are a few options, in descending order of my > preference: > > 1. Disable the LF -> CRLF replacement while sending binary data. > 2. Escape the newlines using some other custom scheme. > 3. Give up and hex-encode the response. I haven't tried prototyping it, but what about moving the LR -> CRLF code to kdb_msg_write(). It would be really easy to do this in the case where we're doing "dbg_io_ops->write_char()" since we're already processing character at a time. It would be harder to do this when also sending the output to the various console, but may not _too_ hard? You could loop searching for "\n" and send all the characters before the "\n", then send a "\r", then send the "\n" and all the characters up to the next "\n". If you did this then you'd lose the "\n" to "\r\n" combination in the gdb stub, but _probably_ that doesn't matter? -Doug
