CVE-2026-74391: tracing: Bound synthetic-field strings with seq_buf

In the Linux kernel, the following vulnerability has been resolved:

tracing: Bound synthetic-field strings with seqbuf

The synthetic field helpers build a prefixed synthetic variable name and a generated hist command in fixed MAXFILTERSTRVAL buffers. The current code appends those strings with raw strcat(), so long key lists, field names, or saved filters can run past the end of the staging buffers.

Build both strings with seqbuf and propagate -E2BIG if either the synthetic variable name or the generated command exceeds MAXFILTERSTRVAL. This keeps the existing tracing-side limit while using the helper intended for bounded command construction.

[ sdr: Moved struct seqbuf s for upside-down x-mas tree formatting ]