Re: git: f08247fd888e - main - Assert that mbufs are writable if we write to them

On 9/12/24 05:03, John Baldwin wrote:
> I think part of the motivation for marking M_EXTPG as read-only is that you can't "write"
> to m_data via mtod() or the like.  That said, M_EXTPG aren't really read-only.  It
> depends on the backing store.  M_EXTPG were first merged into FreeBSD prior to KTLS to
> support sendfile, and in that case, they should be M_RDONLY because they alias pages
> from the file's VM object.  However, M_EXTPG mbufs allocated via functions like
> m_uiotombuf_nomap should not be M_RDONLY.  I think this originated in the original
> import of KTLS which doesn't push setting M_RDONLY out to the callers of mb_alloc_extpgs,
> and a few other places that hardcode M_RDONLY with M_EXTPG (_mb_unmapped_to_ext should
> preserve M_RDONLY from the original mbuf instead of forcing M_RDONLY).
> 
> I can take a stab at a patch but won't have time to really test it until after Euro.

Patch available below.  Compile tested but not run-tested:

https://github.com/freebsd/freebsd-src/compare/main...bsdjhb:freebsd:m_extpg_rdonly

> On 9/11/24 16:56, Drew Gallatin wrote:
>> M_EXTPGS mbufs are marked read-only because they refer to external data.   The original crypto code, (before kTLS was converted to OCF), used to just build an iovec using PHYS_TO_DMAP() on the page array.  I think this case was missed during the conversion to OCF.
>>
>> I'm not sure what the best thing to do is, as they should be read only, except this one specific case.... I'd be tempted to just nerf the KASSERT for EXTPGS.
>>
>> On Wed, Sep 11, 2024, at 11:02 AM, Kristof Provost wrote:
>>> On 11 Sep 2024, at 16:45, Mark Johnston wrote:
>>>> On Wed, Sep 11, 2024 at 11:18:26AM +0000, Kristof Provost wrote:
>>>>> The branch main has been updated by kp:
>>>>>
>>>>> URL: https://cgit.FreeBSD.org/src/commit/?id=f08247fd888e6f7db0ecf2aaa39377144ac40b4c
>>>>>
>>>>> commit f08247fd888e6f7db0ecf2aaa39377144ac40b4c
>>>>> Author:     Kristof Provost <kp@FreeBSD.org>
>>>>> AuthorDate: 2024-09-10 20:15:31 +0000
>>>>> Commit:     Kristof Provost <kp@FreeBSD.org>
>>>>> CommitDate: 2024-09-11 11:17:48 +0000
>>>>>
>>>>>       Assert that mbufs are writable if we write to them
>>>>>
>>>>>       m_copyback() modifies the mbuf, so it must be a writable mbuf.
>>>>
>>>> This change still triggers a panic for me when running KTLS tests.  I
>>>> note that EXTPG mbufs always have M_RDONLY set, but I'm not quite sure
>>>> why.  I suspect such mbufs need special handling with respect to the new
>>>> assertion.
>>>>
>>>> syzbot also triggered this panic:
>>>> https://syzkaller.appspot.com/bug?extid=58c918369f9dc323409d
>>>>
>>> Yeah, I saw that one before I went out for a bike ride.
>>>
>>> Clearly something is wrong. Either ktls is using read-only buffers or the M_WRITABLE() macro isn’t quite smart enough to spot this specific case.
>>>
>>> I’m not familiar enough with ktls to easily tell which.
>>>
>>> I’ll back this assertion change out for now, so we’re not panicing test machines while we figure this out.
>>>
>>> Best regards,
>>> Kristof
>>>
>>
> 

-- 
John Baldwin