tracing/histogram: Fix check for missing operands in an expression

JIRA: https://issues.redhat.com/browse/RHEL-67679

commit 1cab6bc
Author: Kalesh Singh <[email protected]>
Date:   Fri Nov 12 11:13:24 2021 -0800

    tracing/histogram: Fix check for missing operands in an expression

    If a binary operation is detected while parsing an expression string,
    the operand strings are deduced by splitting the experssion string at
    the position of the detected binary operator. Both operand strings are
    sub-strings (can be empty string) of the expression string but will
    never be NULL.

    Currently a NULL check is used for missing operands, fix this by
    checking for empty strings instead.

    Link: https://lkml.kernel.org/r/[email protected]

    Signed-off-by: Kalesh Singh <[email protected]>
    Fixes: 9710b2f ("tracing: Fix operator precedence for hist triggers expression")
    Reported-by: Dan Carpenter <[email protected]>
    Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Signed-off-by: Tomas Glozar <[email protected]>

Author: lenticularis39

kernel/trace/trace_events_hist.c

@@ -2489,7 +2489,8 @@ static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
 	operand1_str = str;
 	str = sep+1;
 
-	if (!operand1_str || !str)
+	/* Binary operator requires both operands */
+	if (*operand1_str == '\0' || *str == '\0')
 		goto free;
 
 	operand_flags = 0;