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

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Message-ID: <ee707646-0196-63bb-45cc-6b949ae9530e@arm.com>
Date:   Wed, 24 Jul 2019 17:36:45 +0100
From:   Steven Price <steven.price@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v9 19/21] mm: Add generic ptdump

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@....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