On Wed, Feb 19, 2020 at 09:44:31AM -0800, Nick Desaulniers wrote: > On Wed, Feb 19, 2020 at 6:34 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > On Tue, 18 Feb 2020 21:54:20 -0700 > > Nathan Chancellor <natechancellor@xxxxxxxxx> wrote: > > > > > 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 so there is not a real issue here. Use the > > > COMPARE_SECTIONS macro to silence this warning by casting the linker > > > defined symbols to unsigned long, which keeps the logic the same. > > > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/765 > > > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > > > 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..e1f3b16e457b 100644 > > > +++ 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 (COMPARE_SECTIONS(__stop___trace_bprintk_fmt, !=, __start___trace_bprintk_fmt)) > > > > Sorry, but this is really ugly and unreadable. Please find some other > > solution to fix this. > > > > NAK-by: Steven Rostedt > > > > Hey Nathan, > Thanks for the series; enabling the warning will help us find more > bugs. Revisiting what the warning is about, I checked on this > "referring to symbols defined in linker scripts from C" pattern. This > document [0] (by no means definitive, but I think it has a good idea) > says: > > Linker symbols that represent a data address: In C code, declare the > variable as an extern variable. Then, refer to the value of the linker > symbol using the & operator. Because the variable is at a valid data > address, we know that a data pointer can represent the value. > Linker symbols for an arbitrary address: In C code, declare this as an > extern symbol. The type does not matter. If you are using GCC > extensions, declare it as "extern void". > > Indeed, it seems that Clang is happier with that pattern: > https://godbolt.org/z/sW3t5W > > Looking at __stop___trace_bprintk_fmt in particular: > > kernel/trace/trace.h > 1923:extern const char *__stop___trace_bprintk_fmt[]; Godbolt says clang is happy if it is written as: if (&__stop___trace_bprintk_fmt[0] != &__start___trace_bprintk_fmt[0]) Which is probably the best compromise. The type here is const char *[], so it would be a shame to see it go. I think this warning is specific to arrays and is designed to detect programmer errors like: int a[1]; int b[1]; return a < b; Where the author intended to use memcmp() Jason
