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

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/e3f5d42cdc2f167719564693675f1eead81378ea
• https://git.kernel.org/stable/c/37377b39ff86dacbc533275c1155210d4fd5dc91
• https://git.kernel.org/stable/c/0402a1d3ddec565132867337ed44514a09d84728
• https://git.kernel.org/stable/c/e91687643c440ca3997d67646e6f80b92edc6703
• https://git.kernel.org/stable/c/be4e99038c1603fa6b329d8ee3e364825e17c353
• https://git.kernel.org/stable/c/d6c8b3ebdcdb12b59ad4212acb137cc56cae453d
• https://git.kernel.org/stable/c/915c1254fe0788abddc31095b360e9dc98907a34
• https://git.kernel.org/stable/c/576ec047d20b368b43c4d5db98c4f2e0f3c101ec