Re: [Xen-devel] [PATCH] xl: Return error when no userdata exists

[Jim Fehlig <jfehlig@xxxxxxxxxx>]

Ian Jackson wrote:
> Jim Fehlig writes ("[Xen-devel] [PATCH] xl: Return error when no userdata 
> exists"):
>   
>> xl: Return error when no userdata exists
>>
>> The libvirt libxenlight driver will store its own userdata with
>> id 'libvirt-xml', but currently libxl_userdata_retrieve() does
>> not fail on non-existent userdata due to inverted error check.
>>     
>
> Nonexistent userdata is the same as zero-length userdata, according to
> the specification.  From libxl.h:
>
>   int libxl_userdata_store(libxl_ctx *ctx, uint32_t domid,
>                                 const char *userdata_userid,
>                                 const uint8_t *data, int datalen);
>     /* If datalen==0, data is not used and the user data for
>      * that domain and userdata_userid is deleted. */
>
>   int libxl_userdata_retrieve(libxl_ctx *ctx, uint32_t domid,
>                                    const char *userdata_userid,
>                                    uint8_t **data_r, int *datalen_r);
>     /* On successful return, *data_r is from malloc.
>      * If there is no data for that domain and userdata_userid,
>      * *data_r and *datalen_r will be set to 0.
>      * data_r and datalen_r may be 0.
>      * On error return, *data_r and *datalen_r are undefined.
>      */
>
> So I think to detect absence of userdata you should be checking
> *data_r or *datalen_r on successful return from
> libxl_userdata_retrieve.
>   

Okay.

> You are however correct in that there is a bug in the error handling:
> if libxl_read_file_contents fails (ie, e!=0) for a reason other than
> ENOENT, libxl_userdata_retrieve improperly ignores it.
>   

Right.

> I think the patch below fixes this.
>
> Do you agree ?
>   

Yes.

Acked-by: Jim Fehlig <jfehlig@xxxxxxxxxx>

Thanks,
Jim

> Thanks,
> Ian.
>
>
> libxl: correct error path in libxl_userdata_retrieve
>
> Firstly, if libxl_read_file_contents fails, it doesn't really leave
> *data and *datalen_r undefined - it leaves them unchanged.  Tighten up
> the spec for the benefit of libxl_userdata_retrieve.
>
> Secondly, libxl_userdata_retrieve ignored errors, assuming they were
> all ENOENT.  Instead it should fail on unexpected errors.
>
> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>
>
> diff -r 6067a17114bc tools/libxl/libxl_dom.c
> --- a/tools/libxl/libxl_dom.c Thu Jan 27 16:17:27 2011 +0000
> +++ b/tools/libxl/libxl_dom.c Thu Jan 27 18:55:41 2011 +0000
> @@ -671,7 +671,10 @@ int libxl_userdata_retrieve(libxl_ctx *c
>      }
>  
>      e = libxl_read_file_contents(ctx, filename, data_r ? &data : 0, 
> &datalen);
> -
> +    if (e && errno != ENOENT) {
> +        rc = ERROR_FAIL;
> +        goto out;
> +    }
>      if (!e && !datalen) {
>          LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "userdata file %s is empty", 
> filename);
>          if (data_r) assert(!*data_r);
> diff -r 6067a17114bc tools/libxl/libxl_utils.h
> --- a/tools/libxl/libxl_utils.h       Thu Jan 27 16:17:27 2011 +0000
> +++ b/tools/libxl/libxl_utils.h       Thu Jan 27 18:55:41 2011 +0000
> @@ -36,7 +36,7 @@ int libxl_read_file_contents(libxl_ctx *
>    /* Reads the contents of the plain file filename into a mallocd
>     * buffer.  Returns 0 or errno.  Any errors other than ENOENT are logged.
>     * If the file is empty, *data_r and *datalen_r are set to 0.
> -   * On error, *data_r and *datalen_r are undefined.
> +   * On error, *data_r and *datalen_r are unchanged.
>     * data_r and/or datalen_r may be 0.
>     */
>  
>
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>   

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