[PATCH 1/2] file-posix: Correctly read max_segments of SG nodes

[Dmitry Fomichev <dmitry.fomichev@wdc.com>]

If scsi-generic driver is in use, an SG node can be specified in
the command line in place of a regular SCSI device. In this case,
sg_get_max_segments() fails to open max_segments entry in sysfs
because /dev/sgX is a character device. As the result, the maximum
transfer size for the device may be calculated incorrectly, causing
I/O errors if the maximum transfer size at the guest ends up to be
larger compared to the host.

Check system device type in sg_get_max_segments() and read the
max_segments value differently if it is a character device.

Reported-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Fixes: 9103f1ceb46614b150bcbc3c9a4fbc72b47fedcc
Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
---
 block/file-posix.c | 55 +++++++++++++++++++++++++++-------------------
 1 file changed, 32 insertions(+), 23 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 094e3b0212..f9e2424e8f 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1108,6 +1108,7 @@ static int sg_get_max_segments(int fd)
     int ret;
     int sysfd = -1;
     long max_segments;
+    unsigned int max_segs;
     struct stat st;
 
     if (fstat(fd, &st)) {
@@ -1115,30 +1116,38 @@ static int sg_get_max_segments(int fd)
         goto out;
     }
 
-    sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
-                                major(st.st_rdev), minor(st.st_rdev));
-    sysfd = open(sysfspath, O_RDONLY);
-    if (sysfd == -1) {
-        ret = -errno;
-        goto out;
+    if (S_ISBLK(st.st_mode)) {
+        sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
+                                    major(st.st_rdev), minor(st.st_rdev));
+        sysfd = open(sysfspath, O_RDONLY);
+        if (sysfd == -1) {
+            ret = -errno;
+            goto out;
+        }
+        do {
+            ret = read(sysfd, buf, sizeof(buf) - 1);
+        } while (ret == -1 && errno == EINTR);
+        if (ret < 0) {
+            ret = -errno;
+            goto out;
+        } else if (ret == 0) {
+            ret = -EIO;
+            goto out;
+        }
+        buf[ret] = 0;
+        /* The file is ended with '\n', pass 'end' to accept that. */
+        ret = qemu_strtol(buf, &end, 10, &max_segments);
+        if (ret == 0 && end && *end == '\n') {
+            ret = max_segments;
+        }
+    } else {
+	ret = ioctl(fd, SG_GET_SG_TABLESIZE, &max_segs);
+        if (ret != 0) {
+            ret = -errno;
+            goto out;
+        }
+        ret = max_segs;
     }
-    do {
-        ret = read(sysfd, buf, sizeof(buf) - 1);
-    } while (ret == -1 && errno == EINTR);
-    if (ret < 0) {
-        ret = -errno;
-        goto out;
-    } else if (ret == 0) {
-        ret = -EIO;
-        goto out;
-    }
-    buf[ret] = 0;
-    /* The file is ended with '\n', pass 'end' to accept that. */
-    ret = qemu_strtol(buf, &end, 10, &max_segments);
-    if (ret == 0 && end && *end == '\n') {
-        ret = max_segments;
-    }
-
 out:
     if (sysfd != -1) {
         close(sysfd);
-- 
2.21.0