[Qemu-devel] [PATCH 3/4] raw-posix: Introduce hdev_is_sg()

[Dimitris Aragiorgis <dimara@arrikto.com>]

Until now, an SG device was identified only by checking if its path
started with "/dev/sg". Then, hdev_open() set bs->sg accordingly.
This is very fragile, e.g. it fails with symlinks or relative paths.
We should rely on the actual properties of the device instead of the
specified file path.

Test for an SG device (e.g. /dev/sg0) by ensuring that all of the
following holds:

 - The device supports the SG_GET_VERSION_NUM ioctl
 - The device supports the SG_GET_SCSI_ID ioctl
 - The specified file name corresponds to a character device

Signed-off-by: Dimitris Aragiorgis <dimara@arrikto.com>
---
 block/raw-posix.c |   39 ++++++++++++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 11 deletions(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 24b061f..d35e5ac 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -57,6 +57,7 @@
 #include <linux/fd.h>
 #include <linux/fs.h>
 #include <linux/hdreg.h>
+#include <scsi/sg.h>
 #ifdef __s390__
 #include <asm/dasd.h>
 #endif
@@ -2073,15 +2074,38 @@ static void hdev_parse_filename(const char *filename, QDict *options,
     qdict_put_obj(options, "filename", QOBJECT(qstring_from_str(filename)));
 }
 
+static int hdev_is_sg(BlockDriverState *bs)
+{
+
+#if defined(__linux__)
+
+    struct stat st;
+    struct sg_scsi_id scsiid;
+    int sg_version;
+
+    if (!bdrv_ioctl(bs, SG_GET_VERSION_NUM, &sg_version) &&
+        !bdrv_ioctl(bs, SG_GET_SCSI_ID, &scsiid) &&
+        stat(bs->filename, &st) >= 0 && S_ISCHR(st.st_mode)) {
+        DEBUG_BLOCK_PRINT("SG device found: type=%d, version=%d\n",
+            scsiid.scsi_type, sg_version);
+        return 1;
+    }
+
+#endif
+
+    return 0;
+}
+
 static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp)
 {
     BDRVRawState *s = bs->opaque;
     Error *local_err = NULL;
     int ret;
-    const char *filename = qdict_get_str(options, "filename");
 
 #if defined(__APPLE__) && defined(__MACH__)
+    const char *filename = qdict_get_str(options, "filename");
+
     if (strstart(filename, "/dev/cdrom", NULL)) {
         kern_return_t kernResult;
         io_iterator_t mediaIterator;
@@ -2110,16 +2134,6 @@ static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
 #endif
 
     s->type = FTYPE_FILE;
-#if defined(__linux__)
-    {
-        char resolved_path[ MAXPATHLEN ], *temp;
-
-        temp = realpath(filename, resolved_path);
-        if (temp && strstart(temp, "/dev/sg", NULL)) {
-            bs->sg = 1;
-        }
-    }
-#endif
 
     ret = raw_open_common(bs, options, flags, 0, &local_err);
     if (ret < 0) {
@@ -2129,6 +2143,9 @@ static int hdev_open(BlockDriverState *bs, QDict *options, int flags,
         return ret;
     }
 
+    /* Since this does ioctl the device must be already opened */
+    bs->sg = hdev_is_sg(bs);
+
     if (flags & BDRV_O_RDWR) {
         ret = check_hdev_writable(s);
         if (ret < 0) {
-- 
1.7.10.4