On Thu, Mar 23, 2017 at 3:17 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Sun, Mar 5, 2017 at 8:18 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>> On Sun, Mar 05, 2017 at 06:33:18PM +0100, Dmitry Vyukov wrote:
>>
>>> Added more debug output.
>>>
>>> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300",
>>> &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0,
>>> 0x1000)
>>>
>>> actually passes name="" because of the overlapping addresses. Flags
>>> contain AT_EMPTY_PATH.
>>
>> Bloody hell... So you end up with name == (char *)&handle->handle_type + 3?
>> Looks like it would be a lot more useful to dump the actual contents of
>> those suckers right before the syscall...
>>
>> Anyway, that explains WTF is going on. The bug is in path_init() and
>> it triggers when you pass something with dentry allocated by d_alloc_pseudo()
>> as dfd, combined with empty pathname. You need to have the file closed
>> by another thread, and have that another thread get out of closing syscall
>> (close(), dup2(), etc.) before the caller of path_init() gets to
>> complete_walk(). We need to make sure that this sucker gets DCACHE_RCUPDATE
>> while it's still guaranteed to be pinned down. Could you try to reproduce
>> with the patch below applied?
>>
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 6f7d96368734..70840281a41c 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -2226,11 +2226,16 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>> nd->path = f.file->f_path;
>> if (flags & LOOKUP_RCU) {
>> rcu_read_lock();
>> - nd->inode = nd->path.dentry->d_inode;
>> - nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
>> + if (unlikely(!(dentry->d_flags & DCACHE_RCUACCESS))) {
>> + spin_lock(&dentry->d_lock);
>> + dentry->d_flags |= DCACHE_RCUACCESS;
>> + spin_unlock(&dentry->d_lock);
>> + }
>> + nd->inode = dentry->d_inode;
>> + nd->seq = read_seqcount_begin(&dentry->d_seq);
>> } else {
>> path_get(&nd->path);
>> - nd->inode = nd->path.dentry->d_inode;
>> + nd->inode = dentry->d_inode;
>> }
>> fdput(f);
>> return s;
>
>
> Al, please send this patch officially. I am running with it since then
> and have not seen the crashes, nor any other issues that look related.
>
> Thanks!
Al, ping. Please send this patch.
