Re: [PATCH v9 19/21] mm: Add generic ptdump

From: Steven Price <steven.price@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: x86@kernel.org, "Arnd Bergmann" <arnd@arndb.de>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Andy Lutomirski" <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"James Morse" <james.morse@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Will Deacon" <will@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org, "Liang,
	Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v9 19/21] mm: Add generic ptdump
Date: Wed, 24 Jul 2019 17:36:45 +0100	[thread overview]
Message-ID: <ee707646-0196-63bb-45cc-6b949ae9530e@arm.com> (raw)
In-Reply-To: <20190723095747.GB8085@lakrids.cambridge.arm.com>

On 23/07/2019 10:57, Mark Rutland wrote:
> On Mon, Jul 22, 2019 at 04:42:08PM +0100, Steven Price wrote:
>> Add a generic version of page table dumping that architectures can
>> opt-in to
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
> 
> [...]
> 
>> +#ifdef CONFIG_KASAN
>> +/*
>> + * This is an optimization for KASAN=y case. Since all kasan page tables
>> + * eventually point to the kasan_early_shadow_page we could call note_page()
>> + * right away without walking through lower level page tables. This saves
>> + * us dozens of seconds (minutes for 5-level config) while checking for
>> + * W+X mapping or reading kernel_page_tables debugfs file.
>> + */
>> +static inline bool kasan_page_table(struct ptdump_state *st, void *pt,
>> +				    unsigned long addr)
>> +{
>> +	if (__pa(pt) == __pa(kasan_early_shadow_pmd) ||
>> +#ifdef CONFIG_X86
>> +	    (pgtable_l5_enabled() &&
>> +			__pa(pt) == __pa(kasan_early_shadow_p4d)) ||
>> +#endif
>> +	    __pa(pt) == __pa(kasan_early_shadow_pud)) {
>> +		st->note_page(st, addr, 5, pte_val(kasan_early_shadow_pte[0]));
>> +		return true;
>> +	}
>> +	return false;
> 
> Having you tried this with CONFIG_DEBUG_VIRTUAL?
> 
> The kasan_early_shadow_pmd is a kernel object rather than a linear map
> object, so you should use __pa_symbol for that.

Thanks for pointing that out - it is indeed broken on arm64. This was
moved from x86 where CONFIG_DEBUG_VIRTUAL doesn't seem to pick this up.
There is actually a problem here that 'pt' might not be in the linear
map (so __pa(pt) barfs on arm64 as well as kasan_early_shadow_p?d).

It looks like having the comparisons of the form "pt ==
lm_alias(kasan_early_shadow_p?d)" is probably best.

> It's a bit horrid to have to test multiple levels in one function; can't
> we check the relevant level inline in each of the test_p?d funcs?
> 
> They're optional anyway, so they only need to be defined for
> CONFIG_KASAN.

Good point - removing the test_p?d callbacks when !CONFIG_KASAN
simplifies the code.

Thanks,

Steve