From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
Paul Durrant <paul@xen.org>
Subject: Re: [Xen-devel] [PATCH v7 1/3] AMD/IOMMU: allocate one device table per PCI segment
Date: Fri, 4 Oct 2019 18:28:11 +0100 [thread overview]
Message-ID: <f7f47cb5-a08b-ca08-93df-73427d94b847@citrix.com> (raw)
In-Reply-To: <44d87efa-36af-7c06-6530-a1691ab55a83@suse.com>
On 04/10/2019 14:30, Jan Beulich wrote:
> On 04.10.2019 15:18, Andrew Cooper wrote:
>> On 26/09/2019 15:28, Jan Beulich wrote:
>>> @@ -1068,8 +1067,29 @@ static void * __init allocate_ppr_log(st
>>> IOMMU_PPR_LOG_DEFAULT_ENTRIES, "PPR Log");
>>> }
>>>
>>> +/*
>>> + * Within ivrs_mappings[] we allocate an extra array element to store
>>> + * - segment number,
>>> + * - device table.
>>> + */
>>> +#define IVRS_MAPPINGS_SEG(m) (m)[ivrs_bdf_entries].dte_requestor_id
>>> +#define IVRS_MAPPINGS_DEVTAB(m) (m)[ivrs_bdf_entries].intremap_table
>>> +
>>> +static void __init free_ivrs_mapping(void *ptr)
>>> +{
>>> + const struct ivrs_mappings *ivrs_mappings = ptr;
>> How absolutely certain are we that ptr will never be NULL?
> As certain as we can be by never installing a NULL pointer into the
> radix tree, and by observing that neither radix_tree_destroy() nor
> radix_tree_node_destroy() would ever call the callback for a NULL
> node.
>
>> It might be better to rename this to radix_tree_free_ivrs_mappings() to
>> make it clear who calls it, and also provide a hint as to why the
>> parameter is void.
> I'm not happy to add a radix_tree_ prefix; I'd be fine with adding
> e.g. do_ instead, in case this provides enough of a hint for your
> taste that this is actually a callback function.
How about a _callback() suffix? I'm looking to make it obvious that you
code shouldn't simply call it directly.
A "do_" prefix, in particular, provides no useful information to the reader.
>>> @@ -1082,13 +1102,15 @@ static int __init amd_iommu_init_one(str
>>> if ( intr && !set_iommu_interrupt_handler(iommu) )
>>> goto error_out;
>>>
>>> - /* To make sure that device_table.buffer has been successfully allocated */
>>> - if ( device_table.buffer == NULL )
>>> + /* Make sure that the device table has been successfully allocated. */
>>> + ivrs_mappings = get_ivrs_mappings(iommu->seg);
>>> + if ( !IVRS_MAPPINGS_DEVTAB(ivrs_mappings) )
>> This is still going to crash with a NULL pointer deference in the case
>> described by the comment. (Then again, it may not crash, and hit
>> userspace at the 64M mark.)
>>
>> You absolutely need to check ivrs_mappings being non NULL before using
>> IVRS_MAPPINGS_DEVTAB(), or perhaps roll the check into the macro.
> I can only repeat what I've said in reply to your respective v6 remark:
> We won't come here for an IOMMU which didn't have its ivrs_mappings
> successfully allocated.
Right, but to a first approximation, I don't care. I can picture
exactly what Coverity will say about this, in that radix_tree_lookup()
may return NULL, and it is used here unconditionally where in most other
contexts, the pointer gets checked before use.
> You also seem to be mixing up this and the
> device table allocation - the comment refers to the latter, while your
> NULL deref concern is about the former. (If you go through the code
> you'll find that we have numerous other places utilizing the fact that
> get_ivrs_mappings() can't fail in cases like the one above.)
The existing code being terrible isn't a reasonable justification for
adding to the mess.
It appears we have:
1x assert not null
14x blind use
3x check
which isn't a great statement about the quality of the code.
Seeing as we are pushed to the deadline for 4.13, begrudgingly A-by
(preferably with the _callback() suffix), but I'm still not happy with
the overall quality of the code. At least it isn't getting
substantially worse as a consequence here.
~Andrew
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next prev parent reply other threads:[~2019-10-04 17:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-26 14:26 [Xen-devel] [PATCH v7 0/3] AMD IOMMU: further improvements Jan Beulich
2019-09-26 14:28 ` [Xen-devel] [PATCH v7 1/3] AMD/IOMMU: allocate one device table per PCI segment Jan Beulich
2019-10-04 13:18 ` Andrew Cooper
2019-10-04 13:30 ` Jan Beulich
2019-10-04 17:28 ` Andrew Cooper [this message]
2019-10-07 10:03 ` Jan Beulich
2019-10-07 10:19 ` Jürgen Groß
2019-10-07 10:49 ` Jan Beulich
2019-10-07 11:25 ` Jürgen Groß
2019-10-10 5:57 ` Jan Beulich
2019-10-10 6:12 ` Jürgen Groß
2019-09-26 14:29 ` [Xen-devel] [PATCH v7 2/3] AMD/IOMMU: allow callers to request allocate_buffer() to skip its memset() Jan Beulich
2019-10-04 13:26 ` Andrew Cooper
2019-10-04 13:33 ` Jan Beulich
2019-09-26 14:29 ` [Xen-devel] [PATCH v7 3/3] AMD/IOMMU: pre-fill all DTEs right after table allocation Jan Beulich
2019-10-04 13:43 ` Andrew Cooper
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