linux-kernel - [PATCH v2] tracing: Use address-of operator on section symbols

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Date:   Wed, 19 Feb 2020 22:10:12 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v2] tracing: Use address-of operator on section symbols

Clang warns:

../kernel/trace/trace.c:9335:33: warning: array comparison always
evaluates to true [-Wtautological-compare]
        if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
                                       ^
1 warning generated.

These are not true arrays, they are linker defined symbols, which are
just addresses. Using the address of operator silences the warning and
does not change the runtime result of the check (tested with some print
statements compiled in with clang + ld.lld and gcc + ld.bfd in QEMU).

Link: https://github.com/ClangBuiltLinux/linux/issues/893
Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

v1 -> v2: https://lore.kernel.org/lkml/20200219045423.54190-4-natechancellor@gmail.com/

* No longer a series because there is no prerequisite patch.
* Use address-of operator instead of casting to unsigned long.

NOTE: The code generation does seem to change, unlike every other call
site that I did this change to but the result of the check remains the
same as noted in the commit message and I cannot really understand what
has changed in the assembly. Please let me know if there is something
catastrophically wrong.

 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c797a15a1fc7..78727dd9a6f5 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9332,7 +9332,7 @@ __init static int tracer_alloc_buffers(void)
 		goto out_free_buffer_mask;

 	/* Only allocate trace_printk buffers if a trace_printk exists */
-	if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
+	if (&__stop___trace_bprintk_fmt != &__start___trace_bprintk_fmt)
 		/* Must be called before global_trace.buffer is allocated */
 		trace_printk_init_buffers();

-- 
2.25.1