vfio: selftests: Introduce a sysfs lib

Introduce a sysfs library to handle the common reads/writes to the
PCI sysfs files, for example, getting the total number of VFs supported
by the device via /sys/bus/pci/devices/$BDF/sriov_totalvfs. The library
will be used in the upcoming test patch to configure the VFs for a given
PF device.

Since readlink() is quite commonly used in the lib, introduce and use
readlink_safe() to take care of potential buffer overrun errors and to
safely terminate the buffer with '\0'.

Opportunistically, move vfio_pci_get_group_from_dev() to this library as
it falls under the same bucket. Rename it to sysfs_iommu_group_get() to
align with other function names.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Tested-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20260505212838.1698034-4-rananta@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

tools/testing/selftests/vfio/lib/include/libvfio.h

@@ -5,6 +5,7 @@
 #include <libvfio/assert.h>
 #include <libvfio/iommu.h>
 #include <libvfio/iova_allocator.h>
+#include <libvfio/sysfs.h>
 #include <libvfio/vfio_pci_device.h>
 #include <libvfio/vfio_pci_driver.h>
 

tools/testing/selftests/vfio/lib/include/libvfio/sysfs.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_SYSFS_H
+#define SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_SYSFS_H
+
+int sysfs_sriov_totalvfs_get(const char *bdf);
+int sysfs_sriov_numvfs_get(const char *bdf);
+void sysfs_sriov_numvfs_set(const char *bdf, int numvfs);
+char *sysfs_sriov_vf_bdf_get(const char *pf_bdf, int i);
+int sysfs_iommu_group_get(const char *bdf);
+char *sysfs_driver_get(const char *bdf);
+
+#endif /* SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_SYSFS_H */

tools/testing/selftests/vfio/lib/libvfio.mk

@@ -6,6 +6,7 @@ LIBVFIO_SRCDIR := $(selfdir)/vfio/lib
 LIBVFIO_C := iommu.c
 LIBVFIO_C += iova_allocator.c
 LIBVFIO_C += libvfio.c
+LIBVFIO_C += sysfs.c
 LIBVFIO_C += vfio_pci_device.c
 LIBVFIO_C += vfio_pci_driver.c
 

tools/testing/selftests/vfio/lib/sysfs.c

@@ -0,0 +1,150 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <fcntl.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/limits.h>
+
+#include <libvfio.h>
+
+#define readlink_safe(_path, _buf) ({					\
+	int __ret;							\
+									\
+	_Static_assert(!__builtin_types_compatible_p(			\
+		__typeof__(_buf), char *),				\
+	    "readlink_safe: _buf must be an array, not a pointer");	\
+									\
+	__ret = readlink(_path, _buf, sizeof(_buf) - 1);		\
+	if (__ret != -1)						\
+		_buf[__ret] = 0;					\
+	__ret;								\
+})
+
+static void readlink_base(const char *path, const char *data_fmt, void *out_data)
+{
+	char rl_path[PATH_MAX];
+	int ret;
+
+	ret = readlink_safe(path, rl_path);
+	VFIO_ASSERT_NE(ret, -1);
+
+	ret = sscanf(basename(rl_path), data_fmt, out_data);
+	VFIO_ASSERT_EQ(ret, 1);
+}
+
+static int sysfs_val_get_int(const char *component, const char *name,
+			     const char *file)
+{
+	char path[PATH_MAX];
+	char buf[32];
+	int ret;
+	int fd;
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/%s/%s/%s", component, name, file);
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return fd;
+
+	VFIO_ASSERT_GT(read(fd, buf, ARRAY_SIZE(buf)), 0);
+	VFIO_ASSERT_EQ(close(fd), 0);
+
+	errno = 0;
+	ret = strtol(buf, NULL, 0);
+	VFIO_ASSERT_EQ(errno, 0, "sysfs path \"%s\" is not an integer: \"%s\"\n", path, buf);
+
+	return ret;
+}
+
+static void sysfs_val_set(const char *component, const char *name,
+			  const char *file, const char *val)
+{
+	char path[PATH_MAX];
+	int fd;
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/%s/%s/%s", component, name, file);
+	VFIO_ASSERT_GT(fd = open(path, O_WRONLY), 0);
+
+	VFIO_ASSERT_EQ(write(fd, val, strlen(val)), strlen(val));
+	VFIO_ASSERT_EQ(close(fd), 0);
+}
+
+static int sysfs_device_val_get(const char *bdf, const char *file)
+{
+	return sysfs_val_get_int("devices", bdf, file);
+}
+
+static void sysfs_device_val_set(const char *bdf, const char *file, const char *val)
+{
+	sysfs_val_set("devices", bdf, file, val);
+}
+
+static void sysfs_device_val_set_int(const char *bdf, const char *file, int val)
+{
+	char val_str[32];
+
+	snprintf_assert(val_str, sizeof(val_str), "%d", val);
+	sysfs_device_val_set(bdf, file, val_str);
+}
+
+int sysfs_sriov_totalvfs_get(const char *bdf)
+{
+	return sysfs_device_val_get(bdf, "sriov_totalvfs");
+}
+
+int sysfs_sriov_numvfs_get(const char *bdf)
+{
+	return sysfs_device_val_get(bdf, "sriov_numvfs");
+}
+
+void sysfs_sriov_numvfs_set(const char *bdf, int numvfs)
+{
+	sysfs_device_val_set_int(bdf, "sriov_numvfs", numvfs);
+}
+
+char *sysfs_sriov_vf_bdf_get(const char *pf_bdf, int i)
+{
+	char path[PATH_MAX];
+	char *out_vf_bdf;
+
+	/* Fit "0000:00:00.0" */
+	out_vf_bdf = calloc(16, sizeof(char));
+	VFIO_ASSERT_NOT_NULL(out_vf_bdf);
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/devices/%s/virtfn%d", pf_bdf, i);
+	readlink_base(path, "%s", out_vf_bdf);
+
+	return out_vf_bdf;
+}
+
+int sysfs_iommu_group_get(const char *bdf)
+{
+	char path[PATH_MAX];
+	int group;
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/devices/%s/iommu_group", bdf);
+	readlink_base(path, "%d", &group);
+
+	return group;
+}
+
+char *sysfs_driver_get(const char *bdf)
+{
+	char driver_path[PATH_MAX];
+	char path[PATH_MAX];
+	char *out_driver;
+	int ret;
+
+	snprintf_assert(path, PATH_MAX, "/sys/bus/pci/devices/%s/driver", bdf);
+	ret = readlink_safe(path, driver_path);
+	if (ret == -1) {
+		if (errno == ENOENT)
+			return NULL;
+
+		VFIO_FAIL("Failed to read %s\n", path);
+	}
+
+	out_driver = strdup(basename(driver_path));
+	VFIO_ASSERT_NOT_NULL(out_driver);
+
+	return out_driver;
+}

tools/testing/selftests/vfio/lib/vfio_pci_device.c

@@ -25,8 +25,6 @@
 #include "kselftest.h"
 #include <libvfio.h>
 
-#define PCI_SYSFS_PATH	"/sys/bus/pci/devices"
-
 static void vfio_pci_irq_set(struct vfio_pci_device *device,
 			     u32 index, u32 vector, u32 count, int *fds)
 {
@@ -204,24 +202,6 @@ void vfio_pci_device_reset(struct vfio_pci_device *device)
 	ioctl_assert(device->fd, VFIO_DEVICE_RESET, NULL);
 }
 
-static unsigned int vfio_pci_get_group_from_dev(const char *bdf)
-{
-	char dev_iommu_group_path[PATH_MAX] = {0};
-	char sysfs_path[PATH_MAX] = {0};
-	unsigned int group;
-	int ret;
-
-	snprintf_assert(sysfs_path, PATH_MAX, "%s/%s/iommu_group", PCI_SYSFS_PATH, bdf);
-
-	ret = readlink(sysfs_path, dev_iommu_group_path, sizeof(dev_iommu_group_path));
-	VFIO_ASSERT_NE(ret, -1, "Failed to get the IOMMU group for device: %s\n", bdf);
-
-	ret = sscanf(basename(dev_iommu_group_path), "%u", &group);
-	VFIO_ASSERT_EQ(ret, 1, "Failed to get the IOMMU group for device: %s\n", bdf);
-
-	return group;
-}
-
 static void vfio_pci_group_setup(struct vfio_pci_device *device, const char *bdf)
 {
 	struct vfio_group_status group_status = {
@@ -230,7 +210,7 @@ static void vfio_pci_group_setup(struct vfio_pci_device *device, const char *bdf
 	char group_path[32];
 	int group;
 
-	group = vfio_pci_get_group_from_dev(bdf);
+	group = sysfs_iommu_group_get(bdf);
 	snprintf_assert(group_path, sizeof(group_path), "/dev/vfio/%d", group);
 
 	device->group_fd = open(group_path, O_RDWR);