From: ebiederm@xmission.com (Eric W. Biederman)
To: Andrew Morton <akpm@linux-foundation.org>
Cc: viro@ZenIV.linux.org.uk, linux-kernel@vger.kernel.org,
linux-pci@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org, tj@kernel.org,
hugh.dickins@tiscali.co.uk, adobriyan@gmail.com,
torvalds@linux-foundation.org, alan@lxorguk.ukuu.org.uk,
gregkh@suse.de, npiggin@suse.de, hch@infradead.org,
ebiederm@aristanetworks.com
Subject: Re: [PATCH 01/23] mm: Introduce revoke_file_mappings.
Date: Mon, 01 Jun 2009 17:12:19 -0700 [thread overview]
Message-ID: <m1vdnfze70.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20090601152553.b2de027a.akpm@linux-foundation.org> (Andrew Morton's message of "Mon\, 1 Jun 2009 15\:25\:53 -0700")
Andrew Morton <akpm@linux-foundation.org> writes:
> On Mon, 1 Jun 2009 14:50:26 -0700
> "Eric W. Biederman" <ebiederm@xmission.com> wrote:
>
>> +static void revoke_vma(struct vm_area_struct *vma)
>
> This looks odd.
>
>> +{
>> + struct file *file = vma->vm_file;
>> + struct address_space *mapping = file->f_mapping;
>> + unsigned long start_addr, end_addr, size;
>> + struct mm_struct *mm;
>> +
>> + start_addr = vma->vm_start;
>> + end_addr = vma->vm_end;
>
> We take a copy of start_addr/end_addr (and this end_addr value is never used)
A foolish consistency.
>> + /* Switch out the locks so I can maninuplate this under the mm sem.
>> + * Needed so I can call vm_ops->close.
>> + */
>> + mm = vma->vm_mm;
>> + atomic_inc(&mm->mm_users);
>> + spin_unlock(&mapping->i_mmap_lock);
>> +
>> + /* Block page faults and other code modifying the mm. */
>> + down_write(&mm->mmap_sem);
>> +
>> + /* Lookup a vma for my file address */
>> + vma = find_vma(mm, start_addr);
>
> Then we look up a vma. Is there reason to believe that this will
> differ from the incoming arg which we just overwrote? Maybe the code
> is attempting to handle racing concurrent mmap/munmap activity? If so,
> what are the implications of this?
Yes it is. The file based index is only safe while we hold the i_mmap_lock.
The manipulation that needs to happen under the mmap_sem.
So I drop all of the locks and restart. And use the time honored
kernel practice of relooking up the thing I am going to manipulate.
As long as it is for the same file I don't care.
> I _think_ that what the function is attempting to do is "unmap the vma
> which covers the address at vma->start_addr". If so, why not just pass
> it that virtual address?
Actually it is unmapping a vma for the file I am revoking. I hand it
one and then it does an address space jig.
> Anyway, it's all a bit obscure and I do think that the semantics and
> behaviour should be carefully explained in a comment, no?
>
>> + if (vma->vm_file != file)
>> + goto out;
>
> This strengthens the theory that some sort of race-management is
> happening here.
Totally. I dropped all of my locks so I am having to restart in
a different locking context.
>> + start_addr = vma->vm_start;
>> + end_addr = vma->vm_end;
>> + size = end_addr - start_addr;
>> +
>> + /* Unlock the pages */
>> + if (mm->locked_vm && (vma->vm_flags & VM_LOCKED)) {
>> + mm->locked_vm -= vma_pages(vma);
>> + vma->vm_flags &= ~VM_LOCKED;
>> + }
>> +
>> + /* Unmap the vma */
>> + zap_page_range(vma, start_addr, size, NULL);
>> +
>> + /* Unlink the vma from the file */
>> + unlink_file_vma(vma);
>> +
>> + /* Close the vma */
>> + if (vma->vm_ops && vma->vm_ops->close)
>> + vma->vm_ops->close(vma);
>> + fput(vma->vm_file);
>> + vma->vm_file = NULL;
>> + if (vma->vm_flags & VM_EXECUTABLE)
>> + removed_exe_file_vma(vma->vm_mm);
>> +
>> + /* Repurpose the vma */
>> + vma->vm_private_data = NULL;
>> + vma->vm_ops = &revoked_vm_ops;
>> + vma->vm_flags &= ~(VM_NONLINEAR | VM_CAN_NONLINEAR);
>> +out:
>> + up_write(&mm->mmap_sem);
>> + spin_lock(&mapping->i_mmap_lock);
>> +}
>
> Also, I'm not a bit fan of the practice of overwriting the value of a
> formal argument, especially in a function which is this large and
> complex. It makes the code harder to follow, because the one variable
> holds two conceptually different things within the span of the same
> function. And it adds risk that someone will will later access a field
> of *vma and it will be the wrong vma. Worse, the bug is only exposed
> under exeedingly rare conditions.
>
> So.. Use a new local, please.
We can never legitimately have more than one vma manipulated in this function.
As for the rest. I guess I just assumed that the reader of the code would
have a basic understanding of the locking rules for those data structures.
Certainly the worst thing I suffer from is being close to the code,
and not realizing which pieces are not obvious to a naive observer.
Eric
next prev parent reply other threads:[~2009-06-02 0:12 UTC|newest]
Thread overview: 99+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-04-11 12:01 [RFC][PATCH 0/9] File descriptor hot-unplug support Eric W. Biederman
2009-04-11 12:03 ` [RFC][PATCH 1/9] mm: Introduce remap_file_mappings Eric W. Biederman
2009-04-11 12:05 ` [RFC][PATCH 2/9] mm: Implement generic support for revoking a mapping Eric W. Biederman
2009-04-11 12:06 ` [RFC][PATCH 3/9] sysfs: Use remap_file_mappings Eric W. Biederman
2009-04-11 12:07 ` [RFC][PATCH 4/9] vfs: Generalize the file_list Eric W. Biederman
2009-04-11 12:08 ` [RFC][PATCH 5/9] vfs: Introduce basic infrastructure for revoking a file Eric W. Biederman
2009-04-14 22:12 ` Jonathan Corbet
2009-04-15 2:55 ` Eric W. Biederman
2009-04-11 12:10 ` [RFC][PATCH 6/9] vfs: Utilize fops_read_lock where appropriate Eric W. Biederman
2009-04-11 12:11 ` [RFC][PATCH 7/9] vfs: Optimize fops_read_lock Eric W. Biederman
2009-04-11 12:13 ` [RFC][PATCH 8/9] vfs: Implement generic revoked file operations Eric W. Biederman
2009-04-12 18:56 ` Jamie Lokier
2009-04-12 20:04 ` Eric W. Biederman
2009-04-12 20:31 ` Jamie Lokier
2009-04-12 21:53 ` Eric W. Biederman
2009-04-12 20:54 ` Eric W. Biederman
2009-04-12 21:02 ` Jamie Lokier
2009-04-12 23:06 ` Eric W. Biederman
2009-04-11 12:14 ` [RFC][PATCH 9/9] proc: Use the generic vfs revoke facility that now exists Eric W. Biederman
2009-04-11 15:58 ` [RFC][PATCH 0/9] File descriptor hot-unplug support Al Viro
2009-04-11 16:49 ` Eric W. Biederman
2009-04-11 16:56 ` Al Viro
2009-04-11 23:57 ` Eric W. Biederman
2009-04-12 20:21 ` Eric W. Biederman
2009-04-14 3:16 ` Tejun Heo
2009-04-14 7:39 ` Eric W. Biederman
2009-04-14 7:45 ` Tejun Heo
2009-04-14 8:27 ` Eric W. Biederman
2009-04-14 8:49 ` Tejun Heo
2009-04-14 15:07 ` Jamie Lokier
2009-04-14 19:09 ` Eric W. Biederman
2009-06-01 21:45 ` [PATCH 0/23] File descriptor hot-unplug support v2 Eric W. Biederman
2009-06-01 21:50 ` [PATCH 01/23] mm: Introduce revoke_file_mappings Eric W. Biederman
2009-06-01 22:25 ` Andrew Morton
2009-06-02 0:12 ` Eric W. Biederman [this message]
2009-06-01 21:50 ` [PATCH 02/23] vfs: Implement unpoll_file Eric W. Biederman
2009-06-06 8:08 ` Al Viro
2009-06-01 21:50 ` [PATCH 03/23] vfs: Generalize the file_list Eric W. Biederman
2009-06-02 7:06 ` Nick Piggin
2009-06-05 19:33 ` Eric W. Biederman
2009-06-09 10:38 ` Nick Piggin
2009-06-09 18:38 ` Eric W. Biederman
2009-06-10 6:05 ` Nick Piggin
2009-06-01 21:50 ` [PATCH 04/23] vfs: Introduce infrastructure for revoking a file Eric W. Biederman
2009-06-02 5:16 ` Pekka Enberg
2009-06-02 6:51 ` Eric W. Biederman
2009-06-02 7:08 ` Pekka Enberg
2009-06-02 7:14 ` Nick Piggin
2009-06-02 17:06 ` Linus Torvalds
2009-06-02 20:52 ` Eric W. Biederman
2009-06-03 6:37 ` Nick Piggin
2009-06-02 22:56 ` Eric W. Biederman
2009-06-03 6:38 ` Nick Piggin
2009-06-05 9:03 ` Miklos Szeredi
2009-06-05 19:06 ` Eric W. Biederman
2009-06-01 21:50 ` [PATCH 05/23] vfs: Teach lseek to use file_hotplug_lock Eric W. Biederman
2009-06-01 21:50 ` [PATCH 06/23] vfs: Teach read/write to use file_hotplug_read_lock Eric W. Biederman
2009-06-01 21:50 ` [PATCH 07/23] vfs: Teach sendfile,splice,tee,and vmsplice to use file_hotplug_lock Eric W. Biederman
2009-06-03 23:39 ` Badari Pulavarty
2009-06-05 19:37 ` Eric W. Biederman
2009-06-01 21:50 ` [PATCH 08/23] vfs: Teach readdir " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 09/23] vfs: Teach poll and select " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 10/23] vfs: Teach do_path_lookup " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 11/23] mm: Teach mmap " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 12/23] vfs: Teach fcntl " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 13/23] vfs: Teach ioctl " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 14/23] vfs: Teach flock " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 15/23] vfs: Teach fallocate, and filp_close " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 16/23] vfs: Teach fstatfs, fstatfs64, ftruncate, fchdir, fchmod, fchown " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 17/23] proc: Teach /proc/<pid>/fd " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 18/23] vfs: Teach epoll " Eric W. Biederman
2009-06-02 16:51 ` Davide Libenzi
2009-06-02 21:23 ` Eric W. Biederman
2009-06-02 21:52 ` Davide Libenzi
2009-06-02 22:51 ` Eric W. Biederman
2009-06-03 14:57 ` Davide Libenzi
2009-06-03 20:53 ` Eric W. Biederman
2009-06-04 0:50 ` Davide Libenzi
2009-06-04 1:42 ` Eric W. Biederman
2009-06-01 21:50 ` [PATCH 19/23] eventpoll: Fix comment Eric W. Biederman
2009-06-01 21:50 ` [PATCH 20/23] vfs: Teach aio to use file_hotplug_lock Eric W. Biederman
2009-06-01 21:50 ` [PATCH 21/23] vfs: Teach fsync " Eric W. Biederman
2009-06-01 21:50 ` [PATCH 22/23] vfs: Teach fadvice to file_hotplug_lock Eric W. Biederman
2009-06-01 21:50 ` [PATCH 23/23] vfs: Teach readahead to use the file_hotplug_lock Eric W. Biederman
2009-06-03 23:25 ` Badari Pulavarty
2009-06-06 8:03 ` [PATCH 0/23] File descriptor hot-unplug support v2 Al Viro
2009-06-08 9:41 ` Miklos Szeredi
2009-06-08 10:24 ` Jamie Lokier
2009-06-08 16:29 ` Al Viro
2009-06-08 16:44 ` Miklos Szeredi
2009-06-08 17:50 ` Al Viro
2009-06-08 18:01 ` Linus Torvalds
2009-06-08 18:50 ` Al Viro
2009-06-08 19:18 ` Linus Torvalds
2009-06-09 6:42 ` Eric W. Biederman
2009-06-09 10:52 ` Nick Piggin
2009-06-09 5:50 ` Miklos Szeredi
2009-06-09 6:31 ` Eric W. Biederman
2009-06-09 6:22 ` Eric W. Biederman
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