All of lore.kernel.org
 help / color / mirror / Atom feed
From: Catalin Marinas <catalin.marinas@arm.com>
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	will@kernel.org, stable@vger.kernel.org,
	linux-arm-kernel@lists.infradead.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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

WARNING: multiple messages have this Message-ID (diff)
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

  parent reply	other threads:[~2020-04-14 16:00 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-14 10:42 [PATCH 0/5] arm64: vdso: cleanups Mark Rutland
2020-04-14 10:42 ` [PATCH 1/5] arm64: vdso: don't free unallocated pages Mark Rutland
2020-04-14 10:42   ` Mark Rutland
2020-04-14 12:50   ` Vincenzo Frascino
2020-04-14 12:50     ` Vincenzo Frascino
2020-04-14 13:27     ` Mark Rutland
2020-04-14 13:27       ` Mark Rutland
2020-04-14 14:53       ` Vincenzo Frascino
2020-04-14 14:53         ` Vincenzo Frascino
2020-04-14 15:10         ` Will Deacon
2020-04-14 15:10           ` Will Deacon
2020-04-15 10:08           ` Catalin Marinas
2020-04-15 10:08             ` Catalin Marinas
2020-04-14 15:12         ` Mark Rutland
2020-04-14 15:12           ` Mark Rutland
2020-04-14 15:27           ` Vincenzo Frascino
2020-04-14 15:27             ` Vincenzo Frascino
2020-04-14 15:59         ` Catalin Marinas [this message]
2020-04-14 15:59           ` Catalin Marinas
2020-04-15 10:13   ` Catalin Marinas
2020-04-15 10:13     ` Catalin Marinas
2020-04-15 13:03     ` Mark Rutland
2020-04-15 13:03       ` Mark Rutland
2020-04-14 10:42 ` [PATCH 2/5] arm64: vdso: remove aarch32_vdso_pages[] Mark Rutland
2020-04-14 10:42 ` [PATCH 3/5] arm64: vdso: simplify arch_vdso_type ifdeffery Mark Rutland
2020-04-14 10:42 ` [PATCH 4/5] arm64: vdso: use consistent 'abi' nomenclature Mark Rutland
2020-04-14 10:42 ` [PATCH 5/5] arm64: vdso: use consistent 'map' nomenclature Mark Rutland
2020-04-14 15:20 ` [PATCH 0/5] arm64: vdso: cleanups Will Deacon
2020-04-14 15:43   ` Mark Rutland
2020-04-14 15:52     ` Will Deacon
2020-04-14 16:27       ` Mark Rutland
2020-04-28 12:49 ` Will Deacon
2020-04-28 12:52   ` Mark Rutland

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200414155942.GC22140@gaia \
    --to=catalin.marinas@arm.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=mark.rutland@arm.com \
    --cc=stable@vger.kernel.org \
    --cc=vincenzo.frascino@arm.com \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.