Re: [PATCH v2] tracing/histograms: Reject non-stacktrace fields for synthetic stacktrace fields

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 13 Oct 2026 11:24:48 +0000, Lee Jones <lee@kernel.org> wrote:
> In check_synth_field(), when a synthetic event field is a stacktrace
> (field->is_stack) and the histogram field is not a "long[" stacktrace
> array, validation falls through to the generic size and signedness
> comparison.
>
> Because parse_synth_field() sets a stacktrace field's type to
> "__data_loc unsigned long[]", its size is sizeof(u64) and
> synth_field_signed() marks it as signed.  Consequently, any 8-byte
> signed scalar histogram field passes the fallback check, and its scalar
> value is subsequently dereferenced as a kernel stacktrace pointer in
> get_field_size() and trace_stack().
>
> Furthermore, checking strstr(hist_field->type, "long[") is insufficient
> because tracepoint fields defined with __bitmask() or __rel_bitmask()
> also have type "__data_loc unsigned long[]" or "__rel_loc unsigned
> long[]", whilst create_hist_field() treats them as 4-byte scalar values
> without HIST_FIELD_FL_STACKTRACE.
>
> Propagate HIST_FIELD_FL_STACKTRACE to variable references and field
> variables (whilst excluding variable aliases from per-element
> stacktrace capture), and reject any mismatch between
> HIST_FIELD_FL_STACKTRACE and field->is_stack in check_synth_field().
>
> Fixes: 00cf3d672a9d ("tracing: Allow synthetic events to pass around stacktraces")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lee Jones <lee@kernel.org>

Good catch! Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>