Lustre discuss - Sep 2012 - A question about ll_listxattr()

Hi,

I have a question about function ll_listxattr(). According to the standard,
listxattr(2) should return -1if the size of the list buffer is too small to
hold the result. However ll_listxattr() will return a value bigger than
buffer size in some cases.

Let''s assume listxattr(2) returns SIZE when it is called with a large
enough list buffer. If it''s called with a list buffer whose size is
smaller
than SIZE but bigger than SIZE-12, then listxattr(2)  will return SIZE too.
I got the result on Lustre-2.2. I believe it is not a correct result.

The following codes in ll_listxattr() causes this problem:

                if (buffer && (rc + total_len) <= size) {
                        buffer += rc;
                        memcpy(buffer,XATTR_LUSTRE_PREFIX, prefix_len);
                        memcpy(buffer+prefix_len, "lov", name_len);
                        buffer[prefix_len + name_len] = ''\0'';
                }
                rc2 = total_len;

I think when ''(rc + total_len) <= size'' is false and the
buffer is not
NULL, the return valuse should be set to -1.

Is that a problem or a feature?

Thanks,

Li Xi
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On 2012-09-16, at 12:46 PM, Li Xi wrote:
> I have a question about function ll_listxattr(). According to the standard,
listxattr(2) should return -1if the size of the list buffer is too small to hold
the result. However ll_listxattr() will return a value bigger than buffer size
in some cases.
> 
> Let''s assume listxattr(2) returns SIZE when it is called with a
large enough list buffer. If it''s called with a list buffer whose size
is smaller than SIZE but bigger than SIZE-12, then listxattr(2)  will return
SIZE too. I got the result on Lustre-2.2. I believe it is not a correct result.
> 
> The following codes in ll_listxattr() causes this problem:
> 
>                 if (buffer && (rc + total_len) <= size) {
>                         buffer += rc;
>                         memcpy(buffer,XATTR_LUSTRE_PREFIX, prefix_len);
>                         memcpy(buffer+prefix_len, "lov",
name_len);
>                         buffer[prefix_len + name_len] =
''\0'';
>                 }
>                 rc2 = total_len;
> 
> I think when ''(rc + total_len) <= size'' is false and
the buffer is not NULL, the return valuse should be set to -1.
> 
> Is that a problem or a feature?
Definitely looks like a bug.  Could you please file a ticket at
http://bugs.whamcloud.com/ and submit a patch for this?  Please see the
http://wiki.whamcloud.com/display/PUB/Submitting+Changes page for details.

Cheers, Andreas
--
Andreas Dilger                       Whamcloud, Inc.
Principal Lustre Engineer            http://www.whamcloud.com/