[Qemu-devel] [PATCH] A different way to ask for readonly drive

[Naphtali Sprei <nsprei@redhat.com>]

Hi,
After feedback from Red Hat guys, I've decided to slightly modify the approach to drive's readonly.
The new approach also addresses the silent fall-back to open the drives' file as read-only when read-write fails
(permission denied) that causes unexpected behavior.
Instead of the 'readonly' boolean flag, another flag introduced (a replacement), 'read_write' with three values [on|off|try]:
read_write=on : open with read and write permission, no fall-back to read-only
read_write=off: open with read-only permission
read_write=try: open with read and write permission and if fails, fall-back to read-only (the default if nothing specified)

Suggestions for better naming for flag/values welcomed.

I've tried to explicitly pass the required flags for the bdrv_open function in callers, but probably missed some.

 Naphtali



Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
---
 block.c       |   29 +++++++++++++++++------------
 block.h       |    7 +++++--
 hw/xen_disk.c |    3 ++-
 monitor.c     |    2 +-
 qemu-config.c |    4 ++--
 qemu-img.c    |   14 ++++++++------
 qemu-nbd.c    |    2 +-
 vl.c          |   42 +++++++++++++++++++++++++++++++++---------
 8 files changed, 69 insertions(+), 34 deletions(-)

diff --git a/block.c b/block.c
index 3f3496e..75788ca 100644
--- a/block.c
+++ b/block.c
@@ -356,7 +356,7 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags)
 int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
                BlockDriver *drv)
 {
-    int ret, open_flags, try_rw;
+    int ret, open_flags;
     char tmp_filename[PATH_MAX];
     char backing_filename[PATH_MAX];
 
@@ -378,7 +378,7 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
 
         /* if there is a backing file, use it */
         bs1 = bdrv_new("");
-        ret = bdrv_open2(bs1, filename, 0, drv);
+        ret = bdrv_open2(bs1, filename, BDRV_O_RDONLY, drv);
         if (ret < 0) {
             bdrv_delete(bs1);
             return ret;
@@ -445,19 +445,22 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         bs->enable_write_cache = 1;
 
     /* Note: for compatibility, we open disk image files as RDWR, and
-       RDONLY as fallback */
-    try_rw = !bs->read_only || bs->is_temporary;
-    if (!(flags & BDRV_O_FILE))
-        open_flags = (try_rw ? BDRV_O_RDWR : 0) |
-            (flags & (BDRV_O_CACHE_MASK|BDRV_O_NATIVE_AIO));
-    else
+       RDONLY as fallback (if flag BDRV_O_RO_FBCK is on) */
+    bs->read_only = BDRV_FLAGS_IS_RO(flags);
+    if (!(flags & BDRV_O_FILE)) {
+        open_flags = (flags & (BDRV_O_ACCESS | BDRV_O_CACHE_MASK|BDRV_O_NATIVE_AIO));
+        if (bs->is_temporary) { /* snapshot */
+            open_flags |= BDRV_O_RDWR;
+        }
+    } else
         open_flags = flags & ~(BDRV_O_FILE | BDRV_O_SNAPSHOT);
     if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv))
         ret = -ENOTSUP;
     else
         ret = drv->bdrv_open(bs, filename, open_flags);
-    if ((ret == -EACCES || ret == -EPERM) && !(flags & BDRV_O_FILE)) {
-        ret = drv->bdrv_open(bs, filename, open_flags & ~BDRV_O_RDWR);
+    if ((ret == -EACCES || ret == -EPERM) && !(flags & BDRV_O_FILE) &&
+        (flags & BDRV_O_RO_FBCK)) {
+        ret = drv->bdrv_open(bs, filename, (open_flags & ~BDRV_O_RDWR) | BDRV_O_RDONLY);
         bs->read_only = 1;
     }
     if (ret < 0) {
@@ -481,12 +484,14 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
         /* if there is a backing file, use it */
         BlockDriver *back_drv = NULL;
         bs->backing_hd = bdrv_new("");
-        /* pass on read_only property to the backing_hd */
-        bs->backing_hd->read_only = bs->read_only;
         path_combine(backing_filename, sizeof(backing_filename),
                      filename, bs->backing_file);
         if (bs->backing_format[0] != '\0')
             back_drv = bdrv_find_format(bs->backing_format);
+        /* pass on ro flags to the backing_hd */
+        bs->backing_hd->read_only =  BDRV_FLAGS_IS_RO(flags);
+        open_flags &= ~BDRV_O_ACCESS;
+        open_flags |= (flags & BDRV_O_ACCESS);
         ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
                          back_drv);
         if (ret < 0) {
diff --git a/block.h b/block.h
index fa51ddf..b32c6a4 100644
--- a/block.h
+++ b/block.h
@@ -28,8 +28,9 @@ typedef struct QEMUSnapshotInfo {
 } QEMUSnapshotInfo;
 
 #define BDRV_O_RDONLY      0x0000
-#define BDRV_O_RDWR        0x0002
-#define BDRV_O_ACCESS      0x0003
+#define BDRV_O_RDWR        0x0001
+#define BDRV_O_ACCESS      0x0001
+#define BDRV_O_RO_FBCK     0x0002
 #define BDRV_O_CREAT       0x0004 /* create an empty file */
 #define BDRV_O_SNAPSHOT    0x0008 /* open the file read only and save writes in a snapshot */
 #define BDRV_O_FILE        0x0010 /* open as a raw file (do not try to
@@ -41,6 +42,8 @@ typedef struct QEMUSnapshotInfo {
 #define BDRV_O_NATIVE_AIO  0x0080 /* use native AIO instead of the thread pool */
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
+#define BDRV_O_DFLT_OPEN   (BDRV_O_RDWR | BDRV_O_RO_FBCK)
+#define BDRV_FLAGS_IS_RO(flags) ((flags & BDRV_O_ACCESS) == BDRV_O_RDONLY)
 
 #define BDRV_SECTOR_BITS   9
 #define BDRV_SECTOR_SIZE   (1 << BDRV_SECTOR_BITS)
diff --git a/hw/xen_disk.c b/hw/xen_disk.c
index 5c55251..13688db 100644
--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -610,7 +610,8 @@ static int blk_init(struct XenDevice *xendev)
     /* read-only ? */
     if (strcmp(blkdev->mode, "w") == 0) {
 	mode   = O_RDWR;
-	qflags = BDRV_O_RDWR;
+        /* for compatibility, also allow readonly fallback, for now */
+	qflags = BDRV_O_RDWR | BDRV_O_RO_FBCK;
     } else {
 	mode   = O_RDONLY;
 	qflags = BDRV_O_RDONLY;
diff --git a/monitor.c b/monitor.c
index d97d529..440e17e 100644
--- a/monitor.c
+++ b/monitor.c
@@ -910,7 +910,7 @@ static void do_change_block(Monitor *mon, const char *device,
     }
     if (eject_device(mon, bs, 0) < 0)
         return;
-    bdrv_open2(bs, filename, 0, drv);
+    bdrv_open2(bs, filename, BDRV_O_DFLT_OPEN, drv);
     monitor_read_bdrv_key_start(mon, bs, NULL, NULL);
 }
 
diff --git a/qemu-config.c b/qemu-config.c
index c3203c8..b559459 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -76,8 +76,8 @@ QemuOptsList qemu_drive_opts = {
             .type = QEMU_OPT_STRING,
             .help = "pci address (virtio only)",
         },{
-            .name = "readonly",
-            .type = QEMU_OPT_BOOL,
+            .name = "read_write",
+            .type = QEMU_OPT_STRING,
         },
         { /* end if list */ }
     },
diff --git a/qemu-img.c b/qemu-img.c
index 1d97f2e..dee3e60 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -201,7 +201,7 @@ static BlockDriverState *bdrv_new_open(const char *filename,
     } else {
         drv = NULL;
     }
-    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
+    if (bdrv_open2(bs, filename, BRDV_O_FLAGS | BDRV_O_DFLT_OPEN, drv) < 0) {
         error("Could not open '%s'", filename);
     }
     if (bdrv_is_encrypted(bs)) {
@@ -406,7 +406,7 @@ static int img_check(int argc, char **argv)
     } else {
         drv = NULL;
     }
-    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
+    if (bdrv_open2(bs, filename, BRDV_O_FLAGS | BDRV_O_RDONLY, drv) < 0) {
         error("Could not open '%s'", filename);
     }
     ret = bdrv_check(bs);
@@ -465,7 +465,7 @@ static int img_commit(int argc, char **argv)
     } else {
         drv = NULL;
     }
-    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
+    if (bdrv_open2(bs, filename, BRDV_O_FLAGS | BDRV_O_RDWR, drv) < 0) {
         error("Could not open '%s'", filename);
     }
     ret = bdrv_commit(bs);
@@ -884,7 +884,7 @@ static int img_info(int argc, char **argv)
     } else {
         drv = NULL;
     }
-    if (bdrv_open2(bs, filename, BRDV_O_FLAGS, drv) < 0) {
+    if (bdrv_open2(bs, filename, BRDV_O_FLAGS | BDRV_O_RDONLY, drv) < 0) {
         error("Could not open '%s'", filename);
     }
     bdrv_get_format(bs, fmt_name, sizeof(fmt_name));
@@ -932,10 +932,11 @@ static int img_snapshot(int argc, char **argv)
     BlockDriverState *bs;
     QEMUSnapshotInfo sn;
     char *filename, *snapshot_name = NULL;
-    int c, ret;
+    int c, ret, bdrv_oflags;
     int action = 0;
     qemu_timeval tv;
 
+    bdrv_oflags = BDRV_O_RDWR; /* but no read-only fallback */
     /* Parse commandline parameters */
     for(;;) {
         c = getopt(argc, argv, "la:c:d:h");
@@ -951,6 +952,7 @@ static int img_snapshot(int argc, char **argv)
                 return 0;
             }
             action = SNAPSHOT_LIST;
+            bdrv_oflags = BDRV_O_RDONLY; /* no need for RW */
             break;
         case 'a':
             if (action) {
@@ -988,7 +990,7 @@ static int img_snapshot(int argc, char **argv)
     if (!bs)
         error("Not enough memory");
 
-    if (bdrv_open2(bs, filename, 0, NULL) < 0) {
+    if (bdrv_open2(bs, filename, bdrv_oflags, NULL) < 0) {
         error("Could not open '%s'", filename);
     }
 
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 6cdb834..64f4c68 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -212,7 +212,7 @@ int main(int argc, char **argv)
     int opt_ind = 0;
     int li;
     char *end;
-    int flags = 0;
+    int flags = BDRV_O_DFLT_OPEN;
     int partition = -1;
     int ret;
     int shared = 1;
diff --git a/vl.c b/vl.c
index c0d98f5..cef2d67 100644
--- a/vl.c
+++ b/vl.c
@@ -2090,6 +2090,7 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
                       int *fatal_error)
 {
     const char *buf;
+    const char *rw_buf = 0;
     const char *file = NULL;
     char devname[128];
     const char *serial;
@@ -2104,12 +2105,12 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
     int index;
     int cache;
     int aio = 0;
-    int ro = 0;
     int bdrv_flags;
     int on_read_error, on_write_error;
     const char *devaddr;
     DriveInfo *dinfo;
     int snapshot = 0;
+    int read_write, ro_fallback;
 
     *fatal_error = 1;
 
@@ -2137,7 +2138,7 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
     secs  = qemu_opt_get_number(opts, "secs", 0);
 
     snapshot = qemu_opt_get_bool(opts, "snapshot", 0);
-    ro = qemu_opt_get_bool(opts, "readonly", 0);
+    rw_buf = qemu_opt_get(opts, "read_write");
 
     file = qemu_opt_get(opts, "file");
     serial = qemu_opt_get(opts, "serial");
@@ -2420,6 +2421,29 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
         *fatal_error = 0;
         return NULL;
     }
+
+    read_write = 1;
+    ro_fallback = 1;
+    if (rw_buf) {
+        if (!strcmp(rw_buf, "off")) {
+            read_write = 0;
+            ro_fallback = 0;
+            if (type != IF_SCSI && type != IF_VIRTIO && type != IF_FLOPPY) {
+                fprintf(stderr, "qemu: read_write=off flag not supported for drive of this interface\n");
+                return NULL;
+            }
+        } else if (!strcmp(rw_buf, "on")) {
+            read_write = 1;
+            ro_fallback = 0;
+        } else if (!strcmp(rw_buf, "try")) { /* default, but keep it explicit */
+            read_write = 1;
+            ro_fallback = 1;
+        } else {
+            fprintf(stderr, "qemu: '%s' invalid read_write option (valid values: [on|off|try] )\n", buf);
+            return NULL;
+        }
+    }
+    
     bdrv_flags = 0;
     if (snapshot) {
         bdrv_flags |= BDRV_O_SNAPSHOT;
@@ -2436,16 +2460,16 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
         bdrv_flags &= ~BDRV_O_NATIVE_AIO;
     }
 
-    if (ro == 1) {
-        if (type == IF_IDE) {
-            fprintf(stderr, "qemu: readonly flag not supported for drive with ide interface\n");
-            return NULL;
-        }
-        (void)bdrv_set_read_only(dinfo->bdrv, 1);
+    if (read_write) {
+        bdrv_flags |= BDRV_O_RDWR;
+    }
+    if (ro_fallback) {
+        bdrv_flags |= BDRV_O_RO_FBCK;
     }
+  
 
     if (bdrv_open2(dinfo->bdrv, file, bdrv_flags, drv) < 0) {
-        fprintf(stderr, "qemu: could not open disk image %s: %s\n",
+        fprintf(stderr, "qemu: could not open disk image %s with requested permission: %s\n",
                         file, strerror(errno));
         return NULL;
     }
-- 
1.6.3.3