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From: Fabiano Rosas <farosas@suse.de>
To: qemu-devel@nongnu.org
Cc: Peter Xu <peterx@redhat.com>,
Thomas Huth <thuth@redhat.com>
Subject: [PATCH 3/7] migration: Document the effect of vmstate_info_nullptr
Date: Tue, 7 Jan 2025 16:50:21 -0300
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The migration stream lacks magic numbers at some key points. It's easy
to mis-parse data. Unfortunately, the VMS_NULLPTR_MARKER continues
with the trend. A '0' byte is ambiguous and could be interpreted as a
valid 0x30.
It is maybe not worth trying to change this while keeping backward
compatibility, so add some words of documentation to clarify.
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
migration/vmstate-types.c | 6 ++++++
scripts/analyze-migration.py | 9 +++++++++
2 files changed, 15 insertions(+)
diff --git a/migration/vmstate-types.c b/migration/vmstate-types.c
index e83bfccb9e..08ed059f87 100644
--- a/migration/vmstate-types.c
+++ b/migration/vmstate-types.c
@@ -339,6 +339,12 @@ static int put_nullptr(QEMUFile *f, void *pv, size_t s=
ize,
=20
const VMStateInfo vmstate_info_nullptr =3D {
.name =3D "uint64",
+
+ /*
+ * Ideally these would actually read/write the size of a pointer,
+ * but we're stuck with just a byte now for backward
+ * compatibility.
+ */
.get =3D get_nullptr,
.put =3D put_nullptr,
};
diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py
index f2457b1dde..4292fde424 100755
--- a/scripts/analyze-migration.py
+++ b/scripts/analyze-migration.py
@@ -388,12 +388,21 @@ def read(self):
return self.data
=20
class VMSDFieldUInt(VMSDFieldInt):
+ NULL_PTR_MARKER =3D 0x30
+
def __init__(self, desc, file):
super(VMSDFieldUInt, self).__init__(desc, file)
=20
def read(self):
super(VMSDFieldUInt, self).read()
self.data =3D self.udata
+
+ if self.data =3D=3D self.NULL_PTR_MARKER:
+ # The migration stream encodes NULL pointers as '0' so any
+ # 0x30 in the stream could be a NULL. There's not much we
+ # can do without breaking backward compatibility.
+ pass
+
return self.data
=20
class VMSDFieldIntLE(VMSDFieldInt):
--=20
2.35.3