[PATCH] block: Make BSG detection more sane in hdev_open()

["Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx>]

From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>

Greetings hch, tomo and Co,

This patch changes the Linux BSG backstore detect logic in hdev_open()
in order to determine when to actually set 'bs->sg = BDS_BSG;' by obtaining
the BSG major from a SysFS attribute in /sys/class/bsg/$H:C:T:L/dev,
instead of the original hardcoded 254 major check.

This patch has been tested with Linux KVM guest v2.6.26 using the megasas
HBA emulation SGL passthrough from a TCM_Loop IBLOCK backstore running on
v2.6.35 x86_64.

Thanks to Mark Harvey for initially contributing code for doing this with
tgt.git/usr/bs_sg.c!

Thanks!

Signed-off-by: Nicholas A. Bellinger <nab@xxxxxxxxxxxxxxx>
---
 block/raw-posix.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++----
 1 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index e7afc4a..487e7f0 100644
+++ b/block/raw-posix.c
@@ -842,7 +842,10 @@ static int hdev_open(BlockDriverState *bs, const char *filename, int flags)
 {
     BDRVRawState *s = bs->opaque;
 #if defined(__linux__)
-    struct stat st;
+    struct stat st, st2;
+    FILE *file;
+    char major[8], dev[64], path[128], *p, *buf;
+    int ch, i;
 #endif
 
 #ifdef CONFIG_COCOA
@@ -881,11 +884,51 @@ static int hdev_open(BlockDriverState *bs, const char *filename, int flags)
     if (S_ISCHR(st.st_mode)) {
         if (major(st.st_rdev) == SCSI_GENERIC_MAJOR) {
             bs->sg = BDS_SCSI_GENERIC;
-        } else if (major(st.st_rdev) == 254) {
-            /* This is not yet defined in include/linux/major.h.. */
-            bs->sg = BDS_BSG;
+        } else { /* Extract major for BSG backstore usage */
+            memset(major, 0, 8);
+            memset(dev, 0, 64);
+            memset(path, 0, 128);
+
+            buf = strdup(filename);
+            if (!(buf))
+                goto out;
+            /*
+             * Locate the device name from the path, we are interested
+             * in the last strsep() token..
+             */
+            while ((p = strsep(&buf, "/")))
+                snprintf(dev, 64, "%s", p);
+            /*
+             * Check to sure the sysfs entry exists before calling open
+             */
+            snprintf(path, 128, "/sys/class/bsg/%s/dev", dev);
+            if (stat(path, &st2) < 0)
+                goto out;
+
+            file = fopen(path, "r");
+            if (!(file)) {
+                printf("fopen() failed for BSG sysfs path: %s\n", path);
+                goto out;
+            }
+            ch = fgetc(file);
+            for (i = 0; i < 7; i++) {
+                if (ch == ':') {
+                    major[i] = '\0';
+                    break;
+                }
+                major[i] = ch;
+                ch = fgetc(file);
+            }
+            fclose(file);
+            /*
+             * If the major returned by /sys/class/bsg/$H:C:T:L/dev matches
+             * stat(), then we signal BDS_BSG usage.
+             */ 
+            if (major(st.st_rdev) == atoi(major))
+                bs->sg = BDS_BSG;
         }
     }
+out:
 #endif
 
     return raw_open_common(bs, filename, flags, 0);
-- 
1.5.6.5

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