[PATCH] tools/virtio: Test virtual address range detection

[David Woodhouse]

As things stand, an application which wants to use vhost with a trivial
1:1 mapping of its virtual address space is forced to jump through hoops
to detect what the address range might be. The VHOST_SET_MEM_TABLE ioctl
helpfully doesn't fail immediately; you only get a failure *later* when
you attempt to set the backend, if the table *could* map to an address
which is out of range, even if no out-of-range address is actually
being referenced.

Since userspace is growing workarounds for this lovely kernel API, let's
ensure that we have a regression test that does things basically the same
way as https://gitlab.com/openconnect/openconnect/-/commit/443edd9d8826
does.

This is untested as I can't actually get virtio_test to work at all; it
just seems to deadlock on a spinlock. But it's getting the right answer
for the virtio range on x86_64 at least.

Signed-off-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
---

Please, tell me I don't need to do this. But if I *do*, it needs a
regression test in-kernel.

 tools/virtio/virtio_test.c | 109 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 106 insertions(+), 3 deletions(-)

diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
index cb3f29c09aff..e40eeeb05b71 100644
--- a/tools/virtio/virtio_test.c
+++ b/tools/virtio/virtio_test.c
@@ -11,6 +11,7 @@
 #include <sys/ioctl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <sys/mman.h>
 #include <fcntl.h>
 #include <stdbool.h>
 #include <linux/virtio_types.h>
@@ -124,6 +125,109 @@ static void vq_info_add(struct vdev_info *dev, int num)
 	dev->nvqs++;
 }
 
+/*
+ * This is awful. The kernel doesn't let us just ask for a 1:1 mapping of
+ * our virtual address space; we have to *know* the minimum and maximum
+ * addresses. We can't test it directly with VHOST_SET_MEM_TABLE because
+ * that actually succeeds, and the failure only occurs later when we try
+ * to use a buffer at an address that *is* valid, but our memory table
+ * *could* point to addresses that aren't. Ewww.
+ *
+ * So... attempt to work out what TASK_SIZE is for the kernel we happen
+ * to be running on right now...
+ */
+
+static int testaddr(unsigned long addr)
+{
+	void *res = mmap((void *)addr, getpagesize(), PROT_NONE,
+			 MAP_FIXED|MAP_ANONYMOUS, -1, 0);
+	if (res == MAP_FAILED) {
+		if (errno == EEXIST || errno == EINVAL)
+			return 1;
+
+		/* We get ENOMEM for a bad virtual address */
+		return 0;
+	}
+	/* It shouldn't actually succeed without either MAP_SHARED or
+	 * MAP_PRIVATE in the flags, but just in case... */
+	munmap((void *)addr, getpagesize());
+	return 1;
+}
+
+static int find_vmem_range(struct vhost_memory *vmem)
+{
+	const unsigned long page_size = getpagesize();
+	unsigned long top;
+	unsigned long bottom;
+
+	top = -page_size;
+
+	if (testaddr(top)) {
+		vmem->regions[0].memory_size = top;
+		goto out;
+	}
+
+	/* 'top' is the lowest address known *not* to work */
+	bottom = top;
+	while (1) {
+		bottom >>= 1;
+		bottom &= ~(page_size - 1);
+		assert(bottom);
+
+		if (testaddr(bottom))
+			break;
+		top = bottom;
+	}
+
+	/* It's often a page or two below the boundary */
+	top -= page_size;
+	if (testaddr(top)) {
+		vmem->regions[0].memory_size = top;
+		goto out;
+	}
+	top -= page_size;
+	if (testaddr(top)) {
+		vmem->regions[0].memory_size = top;
+		goto out;
+	}
+
+	/* Now, bottom is the highest address known to work,
+	   and we must search between it and 'top' which is
+	   the lowest address known not to. */
+	while (bottom + page_size != top) {
+		unsigned long test = bottom + (top - bottom) / 2;
+		test &= ~(page_size - 1);
+
+		if (testaddr(test)) {
+			bottom = test;
+			continue;
+		}
+		test -= page_size;
+		if (testaddr(test)) {
+			vmem->regions[0].memory_size = test;
+			goto out;
+		}
+
+		test -= page_size;
+		if (testaddr(test)) {
+			vmem->regions[0].memory_size = test;
+			goto out;
+		}
+		top = test;
+	}
+	vmem->regions[0].memory_size = bottom;
+
+ out:
+	vmem->regions[0].guest_phys_addr = page_size;
+	vmem->regions[0].userspace_addr = page_size;
+	printf("Detected virtual address range 0x%lx-0x%lx\n",
+	       page_size,
+	       (unsigned long)(page_size + vmem->regions[0].memory_size));
+
+	return 0;
+}
+
+
 static void vdev_info_init(struct vdev_info* dev, unsigned long long features)
 {
 	int r;
@@ -143,9 +247,8 @@ static void vdev_info_init(struct vdev_info* dev, unsigned long long features)
 	memset(dev->mem, 0, offsetof(struct vhost_memory, regions) +
                           sizeof dev->mem->regions[0]);
 	dev->mem->nregions = 1;
-	dev->mem->regions[0].guest_phys_addr = (long)dev->buf;
-	dev->mem->regions[0].userspace_addr = (long)dev->buf;
-	dev->mem->regions[0].memory_size = dev->buf_size;
+	r = find_vmem_range(dev->mem);
+	assert(r >= 0);
 	r = ioctl(dev->control, VHOST_SET_MEM_TABLE, dev->mem);
 	assert(r >= 0);
 }

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