[PATCH] xenfs/xensyms: respect hypervisor's "next" indication

[Jan Beulich]

The interface specifies the symnum field as an input and output; the
hypervisor sets it to the next sequential symbol's index. xensyms_next()
incrementing the position explicitly (and xensyms_next_sym()
decrementing it to "rewind") is only correct as long as the sequence of
symbol indexes is non-sparse. Use the hypervisor-supplied value instead
to update the position in xensyms_next(), and use the saved incoming
index in xensyms_next_sym().

Fixes: a11f4f0a4e18 ("xen: xensyms support")
Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
Not sure if this is to be considered stable (backport) material.

--- a/drivers/xen/xenfs/xensyms.c
+++ b/drivers/xen/xenfs/xensyms.c
@@ -48,7 +48,7 @@ static int xensyms_next_sym(struct xensy
                        return -ENOMEM;
 
                set_xen_guest_handle(symdata->name, xs->name);
-               symdata->symnum--; /* Rewind */
+               symdata->symnum = symnum; /* Rewind */
 
                ret = HYPERVISOR_platform_op(&xs->op);
                if (ret < 0)
@@ -78,7 +78,7 @@ static void *xensyms_next(struct seq_fil
 {
        struct xensyms *xs = m->private;
 
-       xs->op.u.symdata.symnum = ++(*pos);
+       *pos = xs->op.u.symdata.symnum;
 
        if (xensyms_next_sym(xs))
                return NULL;