lguest: documentation IV: Launcher
Documentation: The Launcher
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 2cea0c8..1eb05f9 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -208,24 +208,39 @@
return 1;
}
+/*L:305
+ * Dealing With Guest Memory.
+ *
+ * When the Guest gives us (what it thinks is) a physical address, we can use
+ * the normal copy_from_user() & copy_to_user() on that address: remember,
+ * Guest physical == Launcher virtual.
+ *
+ * But we can't trust the Guest: it might be trying to access the Launcher
+ * code. We have to check that the range is below the pfn_limit the Launcher
+ * gave us. We have to make sure that addr + len doesn't give us a false
+ * positive by overflowing, too. */
int lguest_address_ok(const struct lguest *lg,
unsigned long addr, unsigned long len)
{
return (addr+len) / PAGE_SIZE < lg->pfn_limit && (addr+len >= addr);
}
-/* Just like get_user, but don't let guest access lguest binary. */
+/* This is a convenient routine to get a 32-bit value from the Guest (a very
+ * common operation). Here we can see how useful the kill_lguest() routine we
+ * met in the Launcher can be: we return a random value (0) instead of needing
+ * to return an error. */
u32 lgread_u32(struct lguest *lg, unsigned long addr)
{
u32 val = 0;
- /* Don't let them access lguest binary */
+ /* Don't let them access lguest binary. */
if (!lguest_address_ok(lg, addr, sizeof(val))
|| get_user(val, (u32 __user *)addr) != 0)
kill_guest(lg, "bad read address %#lx", addr);
return val;
}
+/* Same thing for writing a value. */
void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
{
if (!lguest_address_ok(lg, addr, sizeof(val))
@@ -233,6 +248,9 @@
kill_guest(lg, "bad write address %#lx", addr);
}
+/* This routine is more generic, and copies a range of Guest bytes into a
+ * buffer. If the copy_from_user() fails, we fill the buffer with zeroes, so
+ * the caller doesn't end up using uninitialized kernel memory. */
void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
{
if (!lguest_address_ok(lg, addr, bytes)
@@ -243,6 +261,7 @@
}
}
+/* Similarly, our generic routine to copy into a range of Guest bytes. */
void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
unsigned bytes)
{
@@ -250,6 +269,7 @@
|| copy_to_user((void __user *)addr, b, bytes) != 0)
kill_guest(lg, "bad write address %#lx len %u", addr, bytes);
}
+/* (end of memory access helper routines) :*/
static void set_ts(void)
{