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[PATCH gnumach] Implement mig_deallocate to free memory when kernel serv

From: Flavio Cruz
Subject: [PATCH gnumach] Implement mig_deallocate to free memory when kernel server RPC succeeds
Date: Mon, 1 May 2023 22:52:19 -0400

In case the kernel interfaces use dynamically sized strings, we will
end up calling mig_deallocate to free the out of line string that was copied
into the kernel. As a matter of contract such type of data is freed
automatically either in kernel code when the RPC fails or in the MiG
stub if it succeeds.

This was tested by changing task_set_name to use dynamic strings and making
sure out of line data is passed when strlen(name) > 4.
 kern/ipc_mig.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kern/ipc_mig.c b/kern/ipc_mig.c
index cb0b2c83..a145f8c5 100644
--- a/kern/ipc_mig.c
+++ b/kern/ipc_mig.c
@@ -303,6 +303,16 @@ mig_strncpy(char *dest, const char *src, int len)
        return dest - dest_;
+/* Called by MiG to deallocate memory, which in this case happens
+ * to be kernel memory. */
+mig_deallocate(vm_address_t addr, vm_size_t size)
+       (void) size;
+       /* We do the same thing as in ipc_kmsg_clean_body. */
+       vm_map_copy_discard((vm_map_copy_t) addr);
 #define        fast_send_right_lookup(name, port, abort)                       
 MACRO_BEGIN                                                            \
        ipc_space_t space = current_space();                            \

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