Re: [PATCH V6 2/2] mm/debug: Add tests validating architecture page table helpers

[Anshuman Khandual]

On 10/15/2019 11:39 PM, Qian Cai wrote:
> On Tue, 2019-10-15 at 14:51 +0530, Anshuman Khandual wrote:
>> +static unsigned long __init get_random_vaddr(void)
>> +{
>> +	unsigned long random_vaddr, random_pages, total_user_pages;
>> +
>> +	total_user_pages = (TASK_SIZE - FIRST_USER_ADDRESS) / PAGE_SIZE;
>> +
>> +	random_pages = get_random_long() % total_user_pages;
>> +	random_vaddr = FIRST_USER_ADDRESS + random_pages * PAGE_SIZE;
>> +
>> +	WARN_ON(random_vaddr > TASK_SIZE);
>> +	WARN_ON(random_vaddr < FIRST_USER_ADDRESS);
> 
> It would be nice if this patch does not introduce a new W=1 GCC warning here on
> x86 because FIRST_USER_ADDRESS is 0, and GCC think the code is dumb because
> "random_vaddr" is unsigned,
> 
> In file included from ./arch/x86/include/asm/bug.h:83,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂfrom ./include/linux/bug.h:5,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂfrom ./include/linux/mmdebug.h:5,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂfrom ./include/linux/gfp.h:5,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂfrom mm/debug_vm_pgtable.c:13:
> mm/debug_vm_pgtable.c: In function âget_random_vaddrâ:
> mm/debug_vm_pgtable.c:359:23: warning: comparison of unsigned expression < 0 is
> always false [-Wtype-limits]
> Â WARN_ON(random_vaddr < FIRST_USER_ADDRESS);
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ^
> ./include/asm-generic/bug.h:113:25: note: in definition of macro âWARN_ONâ
> Â int __ret_warn_on = !!(condition);ÂÂÂÂ\
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ^~~~~~~~~

The test checks against an erroneous unsigned long overflow when
FIRST_USER_ADDRESS is not 0 but a positive number. Wondering if
the compiler will still complain if we merge both the WARN_ON()
checks as || on a single statement.