On Tue, Jul 18, 2017 at 9:55 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > On 18 July 2017 at 20:53, Arnd Bergmann <arnd@xxxxxxxx> wrote: >> On Fri, Jul 14, 2017 at 2:28 PM, Ard Biesheuvel >> <ard.biesheuvel@xxxxxxxxxx> wrote: >>> On 14 July 2017 at 10:25, Arnd Bergmann <arnd@xxxxxxxx> wrote: >>>> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html