CVE-2026-64028 — tracing: Avoid NULL return from hist_field_name() on truncation

Affected: Linux

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

tracing: Avoid NULL return from hist_field_name() on truncation

hist_field_name() returns "" everywhere except the fully-qualified VAR_REF/EXPR case, where snprintf() truncation returns NULL early and bypasses the bottom NULL->"" guard. Callers don't expect NULL: strcat(expr, hist_field_name(field, 0)) at trace_events_hist.c:1758 and the strcmp() in the sort-key match loop at :4804 both deref it.

system and event_name are bounded by MAX_EVENT_NAME_LEN, but the field name on a VAR_REF is kstrdup'd from a histogram variable name parsed out of the trigger string and has no length cap, so a long enough var name in a fully qualified reference can reach the truncation path.

Keep the length check but leave field_name as "" on overflow.

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CVE-2026-64028 — tracing: Avoid NULL return from hist_field_name() on truncation