From: "Steven Rostedt (VMware)" rostedt@goodmis.org

When working on my user space applications, I found a bug in the synthetic event code where the automated synthetic event field was not matching the event field calculation it was attached to. Looking deeper into it, it was because the calculation hist_field was not given a size.

The synthetic event fields are matched to their hist_fields either by having the field have an identical string type, or if that does not match, then the size and signed values are used to match the fields.

The problem arose when I tried to match a calculation where the fields were "unsigned int". My tool created a synthetic event of type "u32". But it failed to match. The string was:

diff=field1-field2:onmatch(event).trace(synth,$diff)

Adding debugging into the kernel, I found that the size of "diff" was 0. And since it was given "unsigned int" as a type, the histogram fallback code used size and signed. The signed matched, but the size of u32 (4) did not match zero, and the event failed to be created.

This can be worse if the field you want to match is not one of the acceptable fields for a synthetic event. As event fields can have any type that is supported in Linux, this can cause an issue. For example, if a type is an enum. Then there's no way to use that with any calculations.

Have the calculation field simply take on the size of what it is calculating.

Link: https://lkml.kernel.org/r/20210730171951.59c7743f@oasis.local.home

Cc: Tom Zanussi zanussi@kernel.org Cc: Masami Hiramatsu mhiramat@kernel.org Cc: Namhyung Kim namhyung@kernel.org Cc: Ingo Molnar mingo@kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: stable@vger.kernel.org Fixes: 100719dcef447 ("tracing: Add simple expression support to hist triggers") Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/trace_events_hist.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c index 34325f41ebc0..362db9b81b8d 100644 --- a/kernel/trace/trace_events_hist.c +++ b/kernel/trace/trace_events_hist.c @@ -2287,6 +2287,10 @@ static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,

expr->operands[0] = operand1; expr->operands[1] = operand2; + + /* The operand sizes should be the same, so just pick one */ + expr->size = operand1->size; + expr->operator = field_op; expr->name = expr_str(expr, 0); expr->type = kstrdup(operand1->type, GFP_KERNEL);