On Tue, Mar 12, 2024 at 11:30:02AM -0400, Steven Rostedt wrote:

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The WARN_ON() check in __assign_str() to catch where the source variable
to the macro doesn't match the source variable to __string() gives an
error in clang:

quoted

quoted

include/trace/events/sunrpc.h:703:4: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]

     670 |                         __assign_str(progname, "unknown");

That's because the __assign_str() macro has:

   WARN_ON_ONCE((src) != __data_offsets.dst##_ptr_);

Where "src" is a string literal. Clang warns when comparing a string
literal directly as it is undefined to what the value of the literal is.

Since this is still to make sure the same string that goes to __string()
is the same as __assign_str(), for string literals do a test for that and
then use strcmp() in those cases

Note that this depends on commit 51270d573a8d ("tracing/net_sched: Fix
tracepoints that save qdisc_dev() as a string") being applied, as this was
what found that bug.

Reported-by: kernel test robot <redacted>
Closes: https://lore.kernel.org/oe-kbuild-all/202402292111.KIdExylU-lkp@intel.com/ (local)
Fixes: 433e1d88a3be ("tracing: Add warning if string in __assign_str() does not match __string()")

Is this change destined for 6.9 or 6.10? I applied it to current
trace/core (eb1533d156d3) along with 51270d573a8d but the warning is
still present. I even tried

    __builtin_choose_expr(__is_constexpr((src)),
                         strcmp((src), __data_offsets.dst##_ptr_),
                         (src) != __data_offsets.dst##_ptr_));

but not even that silenced the warning. I think we will still need a
diag directive to fully silence this warning.