[Gluster-devel] Simulating some kind of "virtual file"

[David Spisla]

Hello Amar, Xavi

Von: Amar Tumballi [mailto:atumball at redhat.com]
Gesendet: Mittwoch, 10. Januar 2018 14:16
An: Xavi Hernandez <jahernan at redhat.com>; David Spisla <david.spisla at iternity.com>
Cc: gluster-devel at gluster.org
Betreff: Re: [Gluster-devel] Simulating some kind of "virtual file"

Check the files in $mountpoint/.meta/ directory. These are all virtual. And meta xlator gives a very good idea about how to handle virtual files (and directories).

-Amar
[David Spisla] Sounds good. Thank you

On Wed, Jan 10, 2018 at 6:36 PM, Xavi Hernandez <jahernan at redhat.com<mailto:jahernan at redhat.com>> wrote:
Hi David,

On Wed, Jan 10, 2018 at 1:42 PM, David Spisla <david.spisla at iternity.com<mailto:david.spisla at iternity.com>> wrote:
[David Spisla] I tried this:
char *new_path = malloc(1+len_path-5);
memcpy(new_path, loc->path, len_path-5);
new_path[strlen(new_path)] = '\0';
loc->name = new_path + (len_path - len_name);

First of all, you should always use memory allocation functions from gluster. This includes GF_MALLOC(), gf_strdup(), gf_asprintf() and several other variants. You can look at libglusterfs/src/mem-pool.h to see all available options.

The second problem I see is that memcpy() doesn't write a terminating null character, so when you compute strlen() afterwards, it will return invalid length, or even try to access invalid memory, causing a crash.

You should do something like this (assuming both loc->path and loc->name are not NULL and skipping many necessary checks):

len_path = strlen(loc->path);
len_name = strlen(loc->name);
new_path = GF_MALLOC(len_path - 4, gf_common_mt_char);
memcpy(new_path, loc->path, len_path - 5);
new_path[len_path - 5] = 0;
loc->name = new_path + len_path - len_name;

This should work fine.

Xavi
[David Spisla] Yes, this worls fine. Thank you 😊. By the way, is there a way inside gluster xlator to get access to xattr or attr of a file. In the lookup function there is only the struct loc, but I am missing there the files gfid. It seems to be null always. I could use syncop_getxattr() with the parameter loc, but the gfid is missing. Can I get the gfid if I have only loc->path and loc-> name? It is like a conversion from files path to files gfid.

One of the main purposes of the 'lookup' fop is to resolve a given path to an existing gfid, so you won't find any gfid in the lookup request (unless it's a revalidate request). You need to look at the response (cbk) of the lookup to get the real gfid. If the request succeeds, you can find the gfid in buf->ia_gfid of the lookup callback.

Other fops that receive a loc_t structure are normally called after a successful lookup, so loc->gfid and/or loc->inode->gfid should be set (unfortunately there isn't an homogeneous management of loc_t structures by xlators, so not always both fields are set).

You can also request additional xattrs in the lookup request by adding them to the xdata dictionary. Their values will be returned in the xdata argument of the lookup callback.

Xavi
[David Spisla] Ok, so there is no chance to get that gfid in an initial lookup. Another problem seems to be that there is no loc parameter in lookup_cbk function. I have the buf->gfid and inode, but there is no loc with the path and name oft he file. I I want to use syncop_getxattr but I have no loc as parameter. Can I get a loc struct with the gfid?

David

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--
Amar Tumballi (amarts)
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