extlinux: try to get the disk offset from sysfs

It is possible(?) that HDIO_GETGEO can't return the full offset, and
in either case it is too small -- only 32 bits on 32-bit platforms.
Thus query sysfs for the real value, if available.

sysfs also contains information for slave devices -- this is going to
really matter for the md/lvm issues.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/extlinux/main.c b/extlinux/main.c
index 002cecd..30422c2 100755 (executable)
--- a/extlinux/main.c
+++ b/extlinux/main.c
@@ -100,6 +100,32 @@ struct geometry_table {
     struct hd_geometry g;
 };
 
+static int sysfs_get_offset(int devfd, unsigned long *start)
+{
+    struct stat st;
+    char sysfs_name[128];
+    FILE *f;
+    int rv;
+
+    if (fstat(devfd, &st))
+       return -1;
+
+    if ((size_t)snprintf(sysfs_name, sizeof sysfs_name,
+                        "/sys/dev/block/%u:%u/start",
+                        major(st.st_dev), minor(st.st_dev))
+       >= sizeof sysfs_name)
+       return -1;
+
+    f = fopen(sysfs_name, "r");
+    if (!f)
+       return -1;
+
+    rv = fscanf(f, "%lu", start);
+    fclose(f);
+
+    return (rv == 1) ? 0 : -1;
+}
+
 /* Standard floppy disk geometries, plus LS-120.  Zipdisk geometry
    (x/64/32) is the final fallback.  I don't know what LS-240 has
    as its geometry, since I don't have one and don't know anyone that does,
@@ -123,24 +149,25 @@ int get_geometry(int devfd, uint64_t totalbytes, struct hd_geometry *geo)
     struct loop_info li;
     struct loop_info64 li64;
     const struct geometry_table *gp;
+    int rv = 0;
 
     memset(geo, 0, sizeof *geo);
 
     if (!ioctl(devfd, HDIO_GETGEO, &geo)) {
-       return 0;
+       goto ok;
     } else if (!ioctl(devfd, FDGETPRM, &fd_str)) {
        geo->heads = fd_str.head;
        geo->sectors = fd_str.sect;
        geo->cylinders = fd_str.track;
        geo->start = 0;
-       return 0;
+       goto ok;
     }
 
     /* Didn't work.  Let's see if this is one of the standard geometries */
     for (gp = standard_geometries; gp->bytes; gp++) {
        if (gp->bytes == totalbytes) {
            memcpy(geo, &gp->g, sizeof *geo);
-           return 0;
+           goto ok;
        }
     }
 
@@ -153,19 +180,25 @@ int get_geometry(int devfd, uint64_t totalbytes, struct hd_geometry *geo)
     geo->cylinders = totalbytes / (geo->heads * geo->sectors << SECTOR_SHIFT);
     geo->start = 0;
 
-    if (!opt.sectors && !opt.heads)
+    if (!opt.sectors && !opt.heads) {
        fprintf(stderr,
                "Warning: unable to obtain device geometry (defaulting to %d heads, %d sectors)\n"
                "         (on hard disks, this is usually harmless.)\n",
                geo->heads, geo->sectors);
+       rv = 1;                 /* Suboptimal result */
+    }
 
+ok:
     /* If this is a loopback device, try to set the start */
     if (!ioctl(devfd, LOOP_GET_STATUS64, &li64))
        geo->start = li64.lo_offset >> SECTOR_SHIFT;
     else if (!ioctl(devfd, LOOP_GET_STATUS, &li))
        geo->start = (unsigned int)li.lo_offset >> SECTOR_SHIFT;
+    else if (!sysfs_get_offset(devfd, &geo->start)) {
+       /* OK */
+    }
 
-    return 1;
+    return rv;
 }
 
 /*