Hi,
On Wed, Dec 11, 2024 at 7:40 AM Amal Raj T <tjarlama@xxxxxxxxx> wrote:
>
> From: Amal Raj T <amalrajt@xxxxxxxx>
>
> Add a new query `linux.vmcoreinfo` to kgdb that returns
> vmcoreinfo to the client using the mem2ebin encoding.
Can you add more documentation about `linux.vmcoreinfo`? Is there a
GDB patch that goes along with this that does something with it? I
assume that GDB patch would need the same magic escaping sequence
you've come up with?
How does one end up having a VM core for kgdb to serve up? Does it
automatically get generated before we enter kdb due to a crash now
that we select VMCORE_INFO, or is there some other mechanism?
> Maximum size of output buffer is set to 3X the maximum
> size of VMCOREINFO_BYTES (kgdb_mem2ebin() requires 1x
> for the temporary copy plus up to 2x for the escaped
> data).
Can you explain the 3x more? Specifically, it looks like the temporary
copy doesn't take up any extra space, unless I read your code wrong
(always possible).
Let's assume mem is '}}' and count is 2.
When the function starts you end up copying mem to the end of the
buffer. Thus, buffer will be '??}}'
The first time through the loop, you'll end filling in the escaped
first character and the buffer will be '}]}}'
The second time through the loop you'll have '}]}]'.
...so I think you need 2x the escaped data (or 2x +1 unless you remove
the '\0' termination from your earlier patch).
Oh, I guess you actually also need 1 extra byte for the 'Q' in your response?
> Signed-off-by: Amal Raj T <amalrajt@xxxxxxxx>
> ---
> kernel/debug/gdbstub.c | 10 +++++++++-
> lib/Kconfig.kgdb | 1 +
> 2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/debug/gdbstub.c b/kernel/debug/gdbstub.c
> index 6198d2eb49c4..5bec444fc6d3 100644
> --- a/kernel/debug/gdbstub.c
> +++ b/kernel/debug/gdbstub.c
> @@ -34,13 +34,14 @@
> #include <linux/uaccess.h>
> #include <asm/cacheflush.h>
> #include <linux/unaligned.h>
> +#include <linux/vmcore_info.h>
> #include "debug_core.h"
>
> #define KGDB_MAX_THREAD_QUERY 17
>
> /* Our I/O buffers. */
> static char remcom_in_buffer[BUFMAX];
> -static char remcom_out_buffer[BUFMAX];
> +static char remcom_out_buffer[MAX(VMCOREINFO_BYTES*3, BUFMAX)];
> static int gdbstub_use_prev_in_buf;
> static int gdbstub_prev_in_buf_pos;
>
> @@ -768,6 +769,13 @@ static void gdb_cmd_query(struct kgdb_state *ks)
> *(--ptr) = '\0';
> break;
>
> + case 'l':
> + if (memcmp(remcom_in_buffer + 1, "linux.vmcoreinfo", 16) == 0) {
Is there termination in the `remcom_in_buffer`? If so, I'd rather see
a string comparison function used. Otherwise
`linux.vmcoreinfoWithExtraStuffAtTheEnd` will also match.
