On Thu, Nov 07, 2024 at 05:08:58PM -0800, Doug Anderson wrote: > 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? That sounds reasonable. I was concerned whether this would affect anything else using the ->poll_put_char() tty operation, but kgdb seems to be the only user, does that sound right? Thanks for the quick response! Omar
