Re: ioemu: Re-enable guest boot with blktap devices ([Xen-devel] Weekly VMX status report. Xen: #18577 & Xen0: #696)

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Yosuke Iwamatsu writes ("ioemu: Re-enable guest boot with blktap devices 
([Xen-devel] Weekly VMX status report. Xen: #18577 & Xen0: #696)"):
> Bug #1367 looks caused by the xenstore path checking code recently added
> to ioemu-remote. I saw the same problem and found that the code didn't
> consider the case blktap devices were attached to the guest. The patch
> below should avoid the problem.

Thanks for the report.  Could you try the attached patch instead and
let me know whether it fixes the problem ?

Keir Fraser writes ("Re: ioemu: Re-enable guest boot with blktap devices 
([Xen-devel] Weekly VMX status report. Xen: #18577 & Xen0: #696)"):
> Alternatively could just check a shorter path prefix (just /local/domain/0/
> would be sufficient I think). Full path checking is obviously inherently a
> bit more fragile.

Well, yes, but this is a security check and I think those are really
supposed to be brittle.  So I'd prefer to make us check that the path
is definitely completely of an expected form.

Ian.

commit 629adb3f5244169731ff18b16ae919641d81ad76
Author: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Date:   Tue Oct 14 11:46:53 2008 +0100

    Fix blktap device backend patch check
    
    Regarding http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1367,
    it appears that the expected backend path check is too strict for'
    blktap devices.  Therefore if the devtype is `vbd' we allow the
    backend to be `tap'.
    
    Thanks to report and inspiration from Yosuke Iwamatsu.
    
    Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

diff --git a/xenstore.c b/xenstore.c
index f5aa8a7..5cd5063 100644
--- a/xenstore.c
+++ b/xenstore.c
@@ -191,17 +191,36 @@ static void xenstore_get_backend_path(char **backend, 
const char *devtype,
     backend_dompath = xs_get_domain_path(xsh, domid_backend);
     if (!backend_dompath) goto out;
     
-    if (pasprintf(&expected_backend, "%s/backend/%s/%lu/%s",
-                  backend_dompath, devtype, frontend_domid, inst_danger)
-        == -1) goto out;
+    const char *expected_devtypes[3];
+    const char **expected_devtype = expected_devtypes;
+
+    *expected_devtype++ = devtype;
+    if (!strcmp(devtype, "vbd")) *expected_devtype++ = "tap";
+    *expected_devtype = 0;
+    assert(expected_devtype <
+           expected_devtypes + ARRAY_SIZE(expected_devtypes));
+
+    for (expected_devtype = expected_devtypes;
+         *expected_devtype;
+         expected_devtype++) {
+    
+        if (pasprintf(&expected_backend, "%s/backend/%s/%lu/%s",
+                      backend_dompath, *expected_devtype,
+                      frontend_domid, inst_danger)
+            == -1) goto out;
 
-    if (strcmp(bpath, expected_backend)) {
-        fprintf(stderr, "frontend `%s' expected backend `%s' got `%s',"
-                " ignoring\n", frontend_path, expected_backend, bpath);
-        errno = EINVAL;
-        goto out;
+        if (!strcmp(bpath, expected_backend))
+            goto found;
     }
 
+    fprintf(stderr, "frontend `%s' devtype `%s' expected backend `%s'"
+            " got `%s', ignoring\n",
+            frontend_path, devtype, expected_backend, bpath);
+    errno = EINVAL;
+    goto out;
+
+ found:
+
     if (pasprintf(&backend_frontend_path, "%s/frontend", bpath)
         == -1) goto out;

-- 

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