On Tue, Jul 18, 2017 at 9:55 PM, Ard Biesheuvel
[off-list ref] wrote:

On 18 July 2017 at 20:53, Arnd Bergmann [off-list ref] wrote:

quoted

On Fri, Jul 14, 2017 at 2:28 PM, Ard Biesheuvel
[off-list ref] wrote:

quoted

On 14 July 2017 at 10:25, Arnd Bergmann [off-list ref] wrote:

quoted

gcc warns when MODULES_VADDR/END is defined to the same value as
VMALLOC_START/VMALLOC_END, e.g. on x86-32:

fs/proc/kcore.c: In function ‘add_modules_range’:
fs/proc/kcore.c:622:161: error: self-comparison always evaluates to false [-Werror=tautological-compare]
  if (/*MODULES_VADDR != VMALLOC_START && */MODULES_END != VMALLOC_END) {

Does it occur for subtraction as well? Or only for comparison?

This replacement patch would also address the warning:

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 45629f4b5402..35824e986c2c 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c

@@ -623,7 +623,7 @@ static void __init proc_kcore_text_init(void)
 struct kcore_list kcore_modules;
 static void __init add_modules_range(void)
 {
-       if (MODULES_VADDR != VMALLOC_START && MODULES_END != VMALLOC_END) {
+       if (MODULES_VADDR - VMALLOC_START && MODULES_END - VMALLOC_END) {
                kclist_add(&kcore_modules, (void *)MODULES_VADDR,
                        MODULES_END - MODULES_VADDR, KCORE_VMALLOC);
        }

I have also verified that four of the 14 patches are not needed when building
without ccache, this is one of them:

 acpi: thermal: fix gcc-6/ccache warning
 proc/kcore: hide a harmless warning
 SFI: fix tautological-compare warning
 [media] fix warning on v4l2_subdev_call() result interpreted as bool

Not sure what to do with those, we could either ignore them all and
not care about ccache, or we try to address them all in some way.

Any idea why ccache makes a difference here? It is not obvious (not to
me at least)

When ccache is used, the compilation stage is apparently always done on
the preprocessed source file. So instead of parsing (with the integrated
preprocessor)

          if (MODULES_VADDR != VMALLOC_START ...)

the compiler sees

          if (((unsigned long)high_memory + (8 * 1024 * 1024))  !=
              ((unsigned long)high_memory + (8 * 1024 * 1024))  ...)

and it correctly considers the first expression something that one
would write in source code, while -Wtautological-compare
is intended to warn about the second version being always true,
which makes the 'if()' pointless.

       Arnd