From: Catalin Marinas <catalin.marinas@arm.com>
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
linux-arm-kernel@lists.infradead.org, will@kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH 1/5] arm64: vdso: don't free unallocated pages
Date: Tue, 14 Apr 2020 16:59:42 +0100 [thread overview]
Message-ID: <20200414155942.GC22140@gaia> (raw)
In-Reply-To: <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com>
On Tue, Apr 14, 2020 at 03:53:45PM +0100, Vincenzo Frascino wrote:
> On 4/14/20 2:27 PM, Mark Rutland wrote:
> > On Tue, Apr 14, 2020 at 01:50:38PM +0100, Vincenzo Frascino wrote:
> >> On 4/14/20 11:42 AM, Mark Rutland wrote:
> >>> The aarch32_vdso_pages[] array never has entries allocated in the C_VVAR
> >>> or C_VDSO slots, and as the array is zero initialized these contain
> >>> NULL.
> >>>
> >>> However in __aarch32_alloc_vdso_pages() when
> >>> aarch32_alloc_kuser_vdso_page() fails we attempt to free the page whose
> >>> struct page is at NULL, which is obviously nonsensical.
> >>
> >> Could you please explain why do you think that free(NULL) is "nonsensical"?
> >
> > Regardless of the below, can you please explain why it is sensical? I'm
> > struggling to follow your argument here.
>
> free(NULL) is a no-operation ("no action occurs") according to the C standard
> (ISO-IEC 9899 paragraph 7.20.3.2). Hence this should not cause any bug if the
> allocator is correctly implemented. From what I can see the implementation of
> the page allocator honors this assumption.
[...]
> > * page_to_virt(NULL) does not have a well-defined result, and
> > page_to_virt() should only be called for a valid struct page pointer.
> > The result of page_to_virt(NULL) may not be a pointer into the linear
> > map as would be expected.
>
> Do you know why this is the case? To be compliant with what the page allocator
> expects page_to_virt(NULL) should be equal to NULL.
Since __free_page(page) (note the two underscores and pointer type) does
not accept a NULL argument, I don't see any reason for page_to_virt() to
accept NULL as a valid argument.
--
Catalin
next prev parent reply other threads:[~2020-04-14 15:59 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200414104252.16061-1-mark.rutland@arm.com>
2020-04-14 10:42 ` [PATCH 1/5] arm64: vdso: don't free unallocated pages Mark Rutland
2020-04-14 12:50 ` Vincenzo Frascino
2020-04-14 13:27 ` Mark Rutland
2020-04-14 14:53 ` Vincenzo Frascino
2020-04-14 15:10 ` Will Deacon
2020-04-15 10:08 ` Catalin Marinas
2020-04-14 15:12 ` Mark Rutland
2020-04-14 15:27 ` Vincenzo Frascino
2020-04-14 15:59 ` Catalin Marinas [this message]
2020-04-15 10:13 ` Catalin Marinas
2020-04-15 13:03 ` Mark Rutland
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