Allow guest to specify syscall vector to use.

(Based on Ron Minnich's LGUEST_PLAN9_SYSCALL patch).

This patch allows Guests to specify what system call vector they want,
and we try to reserve it.  We only allow one non-Linux system call
vector, to try to avoid DoS on the Host.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 48 insertions, 1 deletions

diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index fdefc0afc38c..a57d757eab6e 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -12,8 +12,14 @@
  * them first, so we also have a way of "reflecting" them into the Guest as if
  * they had been delivered to it directly. :*/
 #include <linux/uaccess.h>
+#include <linux/interrupt.h>
+#include <linux/module.h>
 #include "lg.h"
 
+/* Allow Guests to use a non-128 (ie. non-Linux) syscall trap. */
+static unsigned int syscall_vector = SYSCALL_VECTOR;
+module_param(syscall_vector, uint, 0444);
+
 /* The address of the interrupt handler is split into two bits: */
 static unsigned long idt_address(u32 lo, u32 hi)
 {
@@ -183,6 +189,47 @@ void maybe_do_interrupt(struct lguest *lg)
          * timer interrupt. */
         write_timestamp(lg);
 }
+/*:*/
+
+/* Linux uses trap 128 for system calls.  Plan9 uses 64, and Ron Minnich sent
+ * me a patch, so we support that too.  It'd be a big step for lguest if half
+ * the Plan 9 user base were to start using it.
+ *
+ * Actually now I think of it, it's possible that Ron *is* half the Plan 9
+ * userbase.  Oh well. */
+static bool could_be_syscall(unsigned int num)
+{
+        /* Normal Linux SYSCALL_VECTOR or reserved vector? */
+        return num == SYSCALL_VECTOR || num == syscall_vector;
+}
+
+/* The syscall vector it wants must be unused by Host. */
+bool check_syscall_vector(struct lguest *lg)
+{
+        u32 vector;
+
+        if (get_user(vector, &lg->lguest_data->syscall_vec))
+                return false;
+
+        return could_be_syscall(vector);
+}
+
+int init_interrupts(void)
+{
+        /* If they want some strange system call vector, reserve it now */
+        if (syscall_vector != SYSCALL_VECTOR
+            && test_and_set_bit(syscall_vector, used_vectors)) {
+                printk("lg: couldn't reserve syscall %u\n", syscall_vector);
+                return -EBUSY;
+        }
+        return 0;
+}
+
+void free_interrupts(void)
+{
+        if (syscall_vector != SYSCALL_VECTOR)
+                clear_bit(syscall_vector, used_vectors);
+}
 
 /*H:220 Now we've got the routines to deliver interrupts, delivering traps
  * like page fault is easy.  The only trick is that Intel decided that some
@@ -224,7 +271,7 @@ static int direct_trap(unsigned int num)
 {
         /* Hardware interrupts don't go to the Guest at all (except system
          * call). */
-        if (num >= FIRST_EXTERNAL_VECTOR && num != SYSCALL_VECTOR)
+        if (num >= FIRST_EXTERNAL_VECTOR && !could_be_syscall(num))
                 return 0;
 
         /* The Host needs to see page faults (for shadow paging and to save the