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

[Xavi Hernandez]

Hi David,

On Wed, Jan 10, 2018 at 1:42 PM, David Spisla <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
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