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