[PATCH net-next v2 0/4] netconsole: Allow userdata buffer to grow dynamically

[Gustavo Luiz Duarte <gustavold@gmail.com>]

The current netconsole implementation allocates a static buffer for
extradata (userdata + sysdata) with a fixed size of
MAX_EXTRADATA_ENTRY_LEN * MAX_EXTRADATA_ITEMS bytes for every target,
regardless of whether userspace actually uses this feature. This forces
us to keep MAX_EXTRADATA_ITEMS small (16), which is restrictive for
users who need to attach more metadata to their log messages.

This patch series enables dynamic allocation of the userdata buffer,
allowing it to grow on-demand based on actual usage. The series:

1. Refactors send_fragmented_body() to simplify handling of separated
   userdata and sysdata (patch 1/4)
2. Splits userdata and sysdata into separate buffers (patch 2/4)
3. Implements dynamic allocation for the userdata buffer (patch 3/4)
4. Increases MAX_USERDATA_ITEMS from 16 to 256 now that we can do so
   without memory waste (patch 4/4)

Benefits:
- No memory waste when userdata is not used
- Targets that use userdata only consume what they need
- Users can attach significantly more metadata without impacting systems
  that don't use this feature

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
Changes in v2:
- Added null pointer checks for userdata and sysdata buffers
- Added MAX_SYSDATA_ITEMS to enum sysdata_feature
- Moved code out of ifdef in send_msg_no_fragmentation()
- Renamed variables in send_fragmented_body() to make it easier to
  reason about the code
- Link to v1: https://lore.kernel.org/r/20251105-netconsole_dynamic_extradata-v1-0-142890bf4936@meta.com

---
Gustavo Luiz Duarte (4):
      netconsole: Simplify send_fragmented_body()
      netconsole: Split userdata and sysdata
      netconsole: Dynamic allocation of userdata buffer
      netconsole: Increase MAX_USERDATA_ITEMS

 drivers/net/netconsole.c                           | 370 ++++++++++-----------
 .../selftests/drivers/net/netcons_overflow.sh      |   2 +-
 2 files changed, 179 insertions(+), 193 deletions(-)
---
base-commit: 68fa5b092efab37a4f08a47b22bb8ca98f7f6223
change-id: 20251007-netconsole_dynamic_extradata-21bd9d726568

Best regards,
-- 
Gustavo Duarte <gustavold@meta.com>