docs: Update description of 'user=username' for '-run-with'

The description of '-runas' and '-run-with' didn't explain that QEMU
will use setuid/setgid to implement the option, so the user might get
confused if using 'elevateprivileges=deny' as well.

Since '-runas' is going to be deprecated and replaced by '-run-with'
in the coming qemu9.1, add the message there.

Signed-off-by: Boqiao Fu <bfu@redhat.com>
Link: https://lore.kernel.org/r/CAFRHJ6J9uMk+HMZL+W+KE1yoRCOLPgbPUVVDku55sdXYiGXXHg@mail.gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/qemu-options.hx b/qemu-options.hx
index ad6521ef5e7e5b7e34ac7feb0326e22d1c9f3e1f..694fa37f284d14c362487ff4b8610b49a7dd8703 100644 (file)
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -5024,8 +5024,11 @@ SRST
     in combination with -runas.
 
     ``user=username`` or ``user=uid:gid`` can be used to drop root privileges
-    by switching to the specified user (via username) or user and group
-    (via uid:gid) immediately before starting guest execution.
+    before starting guest execution. QEMU will use the ``setuid`` and ``setgid``
+    system calls to switch to the specified identity.  Note that the
+    ``user=username`` syntax will also apply the full set of supplementary
+    groups for the user, whereas the ``user=uid:gid`` will use only the
+    ``gid`` group.
 ERST
 #endif