[Qemu-devel] [PATCH for-3.2 v3 0/3] HMP/snapshot changes - do not use ID anymore

[Qemu-devel] [PATCH for-3.2 v3 0/3] HMP/snapshot changes - do not use ID anymore

[Daniel Henrique Barboza]

changes in v3:
- rebased to v3.1.0-rc0 tag
- hmp-commands.hx documentation now mentions the change of semantics
starting version 3.2.
- previous version link:
http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00629.html


It is not uncommon to see bugs being opened by testers that attempt to
create VM snapshots using HMP. It turns out that "0" and "1" are quite
common snapshot names and they trigger a lot of bugs. I gave an example
in the commit message of patch 1, but to sum up here: QEMU treats the
input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. It
is documented as such, but this can lead to strange situations.

Given that it is strange for an API to consider a parameter to be 2 fields
at the same time, and inadvently treating them as one or the other, and
that removing the ID field is too drastic, my idea here is to keep the
ID field for internal control, but do not let the user set it.

This series simplifies the meaning of savevm/loadvm/delvm to be up to
par to what the QEMU code (and Libvirt) is already doing: snapshot
operations using "tag" semantics only, leaving the "id" to be
automatically calculated by the block drivers and used internally
only.

This change of semantics does not affect existing snapshots. What
changes is that any HMP operations with them will use the
updated semantics.


Daniel Henrique Barboza (3):
  block/snapshot.c: eliminate use of ID input in snapshot operations
  block/snapshot: remove bdrv_snapshot_delete_by_id_or_name
  qcow2-snapshot: remove redundant find_snapshot_by_id_and_name call

 block/qcow2-snapshot.c   |  5 -----
 block/snapshot.c         | 25 +++----------------------
 hmp-commands.hx          | 32 ++++++++++++++++++++------------
 include/block/snapshot.h |  3 ---
 qemu-img.c               | 15 +++++++++++----
 5 files changed, 34 insertions(+), 46 deletions(-)

-- 
2.17.2

[Qemu-devel] [PATCH for-3.2 v3 1/3] block/snapshot.c: eliminate use of ID input in snapshot operations

[Daniel Henrique Barboza]

At this moment, QEMU attempts to create/load/delete snapshots
by using either an ID (id_str) or a name. The problem is that the code
isn't consistent of whether the entered argument is an ID or a name,
causing unexpected behaviors.

For example, when creating snapshots via savevm <arg>, what happens is that
"arg" is treated as both name and id_str. In a guest without snapshots, create
a single snapshot via savevm:

(qemu) savevm 0
(qemu) info snapshots
List of snapshots present on all disks:
ID        TAG                 VM SIZE                DATE       VM CLOCK
--        0                      741M 2018-07-31 13:39:56   00:41:25.313

A snapshot with name "0" is created. ID is hidden from the user, but the
ID is a non-zero integer that starts at "1". Thus, this snapshot has
id_str=1, TAG="0". Creating a second snapshot with arg = 1, the first one
is deleted:

(qemu) savevm 1
(qemu) info snapshots
List of snapshots present on all disks:
ID        TAG                 VM SIZE                DATE       VM CLOCK
--        1                      741M 2018-07-31 13:42:14   00:41:55.252

What happened?

- when creating the second snapshot, a verification is done inside
bdrv_all_delete_snapshot to delete any existing snapshots that matches an
string argument. Here, the code calls bdrv_all_delete_snapshot("1", ...);

- bdrv_all_delete_snapshot calls bdrv_snapshot_find(..., "1") for each
BlockDriverState of the guest. And this is where things goes tilting:
bdrv_snapshot_find does a search by both id_str and name. It finds
out that there is a snapshot that has id_str = 1, stores a reference
to the snapshot in the sn_info pointer and then returns match found;

- since a match was found, a call to bdrv_snapshot_delete_by_id_or_name() is
made. This function ignores the pointer written by bdrv_snapshot_find. Instead,
it deletes the snapshot using bdrv_snapshot_delete() calling it first with
id_str = 1. If it fails to delete, then it calls it again with name = 1.

- after all that, QEMU creates the new snapshot, that has id_str = 1 and
name = 1. The user is left wondering that happened with the first snapshot
created. Similar bugs can be triggered when using loadvm and delvm.

Before contemplating discarding the use of ID input in these operations,
I've searched the code of what would be the implications. My findings
are:

- the RBD and Sheepdog drivers don't care. Both uses the 'name' field as
key in their logic, making id_str = name when appropriate.
replay-snapshot.c does not make any special use of id_str;

- qcow2 uses id_str as an unique identifier but it is automatically
calculated, not being influenced by user input. Other than that, there are
no distinguish operations made only with id_str;

- in blockdev.c, the delete operation uses a match of both id_str AND
name. Given that id_str is either a copy of 'name' or auto-generated,
we're fine here.

This gives motivation to not consider ID as a valid user input in HMP
commands - sticking with 'name' input only is more consistent. To
accomplish that, the following changes were made in this patch:

- bdrv_snapshot_find() does not match for id_str anymore, only 'name'. The
function is called in save_snapshot(), load_snapshot(), bdrv_all_delete_snapshot()
and bdrv_all_find_snapshot(). This change makes the search function more
predictable and does not change the behavior of any underlying code that uses
these affected functions, which are related to HMP (which is fine) and the
main loop inside vl.c (which doesn't care about it anyways);

- bdrv_all_delete_snapshot() does not call bdrv_snapshot_delete_by_id_or_name
anymore. Instead, it uses the pointer returned by bdrv_snapshot_find to
erase the snapshot with the exact match of id_str an name. This function
is called in save_snapshot and hmp_delvm, thus this change  produces the
intended effect;

- documentation changes to reflect the new behavior. I consider this to
be an API fix instead of an API change - the user was already creating
snapshots using 'name', but now he/she will also enjoy a consistent
behavior.

Ideally we would get rid of the id_str field entirely, but this would have
repercussions on existing snapshots. Another day perhaps.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 block/snapshot.c |  5 +++--
 hmp-commands.hx  | 32 ++++++++++++++++++++------------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/block/snapshot.c b/block/snapshot.c
index 3218a542df..e371d2243d 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -63,7 +63,7 @@ int bdrv_snapshot_find(BlockDriverState *bs, QEMUSnapshotInfo *sn_info,
     }
     for (i = 0; i < nb_sns; i++) {
         sn = &sn_tab[i];
-        if (!strcmp(sn->id_str, name) || !strcmp(sn->name, name)) {
+        if (!strcmp(sn->name, name)) {
             *sn_info = *sn;
             ret = 0;
             break;
@@ -448,7 +448,8 @@ int bdrv_all_delete_snapshot(const char *name, BlockDriverState **first_bad_bs,
         aio_context_acquire(ctx);
         if (bdrv_can_snapshot(bs) &&
                 bdrv_snapshot_find(bs, snapshot, name) >= 0) {
-            ret = bdrv_snapshot_delete_by_id_or_name(bs, name, err);
+            ret = bdrv_snapshot_delete(bs, snapshot->id_str,
+                                       snapshot->name, err);
         }
         aio_context_release(ctx);
         if (ret < 0) {
diff --git a/hmp-commands.hx b/hmp-commands.hx
index db0c681f74..4f96a38890 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -350,49 +350,57 @@ ETEXI
     {
         .name       = "savevm",
         .args_type  = "name:s?",
-        .params     = "[tag|id]",
-        .help       = "save a VM snapshot. If no tag or id are provided, a new snapshot is created",
+        .params     = "tag",
+        .help       = "save a VM snapshot. If no tag is provided, a new snapshot is created",
         .cmd        = hmp_savevm,
     },
 
 STEXI
-@item savevm [@var{tag}|@var{id}]
+@item savevm @var{tag}
 @findex savevm
 Create a snapshot of the whole virtual machine. If @var{tag} is
 provided, it is used as human readable identifier. If there is already
-a snapshot with the same tag or ID, it is replaced. More info at
+a snapshot with the same tag, it is replaced. More info at
 @ref{vm_snapshots}.
+
+Since 3.2, savevm stopped allowing the snapshot id to be set, accepting
+only @var{tag} as parameter.
 ETEXI
 
     {
         .name       = "loadvm",
         .args_type  = "name:s",
-        .params     = "tag|id",
-        .help       = "restore a VM snapshot from its tag or id",
+        .params     = "tag",
+        .help       = "restore a VM snapshot from its tag",
         .cmd        = hmp_loadvm,
         .command_completion = loadvm_completion,
     },
 
 STEXI
-@item loadvm @var{tag}|@var{id}
+@item loadvm @var{tag}
 @findex loadvm
 Set the whole virtual machine to the snapshot identified by the tag
-@var{tag} or the unique snapshot ID @var{id}.
+@var{tag}.
+
+Since 3.2, loadvm stopped accepting snapshot id as parameter.
 ETEXI
 
     {
         .name       = "delvm",
         .args_type  = "name:s",
-        .params     = "tag|id",
-        .help       = "delete a VM snapshot from its tag or id",
+        .params     = "tag",
+        .help       = "delete a VM snapshot from its tag",
         .cmd        = hmp_delvm,
         .command_completion = delvm_completion,
     },
 
 STEXI
-@item delvm @var{tag}|@var{id}
+@item delvm @var{tag}
 @findex delvm
-Delete the snapshot identified by @var{tag} or @var{id}.
+Delete the snapshot identified by @var{tag}.
+
+Since 3.2, delvm stopped deleting snapshots by snapshot id, accepting
+only @var{tag} as parameter.
 ETEXI
 
     {
-- 
2.17.2

Re: [Qemu-devel] [PATCH for-3.2 v3 1/3] block/snapshot.c: eliminate use of ID input in snapshot operations

[Dr. David Alan Gilbert]

* Daniel Henrique Barboza (danielhb413@gmail.com) wrote:
> At this moment, QEMU attempts to create/load/delete snapshots
> by using either an ID (id_str) or a name. The problem is that the code
> isn't consistent of whether the entered argument is an ID or a name,
> causing unexpected behaviors.
> 
> For example, when creating snapshots via savevm <arg>, what happens is that
> "arg" is treated as both name and id_str. In a guest without snapshots, create
> a single snapshot via savevm:

I'm OK with the HMP side, and I think OK with the idea so:

Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

but this is mainly block code, so let Kevin or Max review it fully.

Dave

> (qemu) savevm 0
> (qemu) info snapshots
> List of snapshots present on all disks:
> ID        TAG                 VM SIZE                DATE       VM CLOCK
> --        0                      741M 2018-07-31 13:39:56   00:41:25.313
> 
> A snapshot with name "0" is created. ID is hidden from the user, but the
> ID is a non-zero integer that starts at "1". Thus, this snapshot has
> id_str=1, TAG="0". Creating a second snapshot with arg = 1, the first one
> is deleted:
> 
> (qemu) savevm 1
> (qemu) info snapshots
> List of snapshots present on all disks:
> ID        TAG                 VM SIZE                DATE       VM CLOCK
> --        1                      741M 2018-07-31 13:42:14   00:41:55.252
> 
> What happened?
> 
> - when creating the second snapshot, a verification is done inside
> bdrv_all_delete_snapshot to delete any existing snapshots that matches an
> string argument. Here, the code calls bdrv_all_delete_snapshot("1", ...);
> 
> - bdrv_all_delete_snapshot calls bdrv_snapshot_find(..., "1") for each
> BlockDriverState of the guest. And this is where things goes tilting:
> bdrv_snapshot_find does a search by both id_str and name. It finds
> out that there is a snapshot that has id_str = 1, stores a reference
> to the snapshot in the sn_info pointer and then returns match found;
> 
> - since a match was found, a call to bdrv_snapshot_delete_by_id_or_name() is
> made. This function ignores the pointer written by bdrv_snapshot_find. Instead,
> it deletes the snapshot using bdrv_snapshot_delete() calling it first with
> id_str = 1. If it fails to delete, then it calls it again with name = 1.
> 
> - after all that, QEMU creates the new snapshot, that has id_str = 1 and
> name = 1. The user is left wondering that happened with the first snapshot
> created. Similar bugs can be triggered when using loadvm and delvm.
> 
> Before contemplating discarding the use of ID input in these operations,
> I've searched the code of what would be the implications. My findings
> are:
> 
> - the RBD and Sheepdog drivers don't care. Both uses the 'name' field as
> key in their logic, making id_str = name when appropriate.
> replay-snapshot.c does not make any special use of id_str;
> 
> - qcow2 uses id_str as an unique identifier but it is automatically
> calculated, not being influenced by user input. Other than that, there are
> no distinguish operations made only with id_str;
> 
> - in blockdev.c, the delete operation uses a match of both id_str AND
> name. Given that id_str is either a copy of 'name' or auto-generated,
> we're fine here.
> 
> This gives motivation to not consider ID as a valid user input in HMP
> commands - sticking with 'name' input only is more consistent. To
> accomplish that, the following changes were made in this patch:
> 
> - bdrv_snapshot_find() does not match for id_str anymore, only 'name'. The
> function is called in save_snapshot(), load_snapshot(), bdrv_all_delete_snapshot()
> and bdrv_all_find_snapshot(). This change makes the search function more
> predictable and does not change the behavior of any underlying code that uses
> these affected functions, which are related to HMP (which is fine) and the
> main loop inside vl.c (which doesn't care about it anyways);
> 
> - bdrv_all_delete_snapshot() does not call bdrv_snapshot_delete_by_id_or_name
> anymore. Instead, it uses the pointer returned by bdrv_snapshot_find to
> erase the snapshot with the exact match of id_str an name. This function
> is called in save_snapshot and hmp_delvm, thus this change  produces the
> intended effect;
> 
> - documentation changes to reflect the new behavior. I consider this to
> be an API fix instead of an API change - the user was already creating
> snapshots using 'name', but now he/she will also enjoy a consistent
> behavior.
> 
> Ideally we would get rid of the id_str field entirely, but this would have
> repercussions on existing snapshots. Another day perhaps.
> 
> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> ---
>  block/snapshot.c |  5 +++--
>  hmp-commands.hx  | 32 ++++++++++++++++++++------------
>  2 files changed, 23 insertions(+), 14 deletions(-)
> 
> diff --git a/block/snapshot.c b/block/snapshot.c
> index 3218a542df..e371d2243d 100644
> --- a/block/snapshot.c
> +++ b/block/snapshot.c
> @@ -63,7 +63,7 @@ int bdrv_snapshot_find(BlockDriverState *bs, QEMUSnapshotInfo *sn_info,
>      }
>      for (i = 0; i < nb_sns; i++) {
>          sn = &sn_tab[i];
> -        if (!strcmp(sn->id_str, name) || !strcmp(sn->name, name)) {
> +        if (!strcmp(sn->name, name)) {
>              *sn_info = *sn;
>              ret = 0;
>              break;
> @@ -448,7 +448,8 @@ int bdrv_all_delete_snapshot(const char *name, BlockDriverState **first_bad_bs,
>          aio_context_acquire(ctx);
>          if (bdrv_can_snapshot(bs) &&
>                  bdrv_snapshot_find(bs, snapshot, name) >= 0) {
> -            ret = bdrv_snapshot_delete_by_id_or_name(bs, name, err);
> +            ret = bdrv_snapshot_delete(bs, snapshot->id_str,
> +                                       snapshot->name, err);
>          }
>          aio_context_release(ctx);
>          if (ret < 0) {
> diff --git a/hmp-commands.hx b/hmp-commands.hx
> index db0c681f74..4f96a38890 100644
> --- a/hmp-commands.hx
> +++ b/hmp-commands.hx
> @@ -350,49 +350,57 @@ ETEXI
>      {
>          .name       = "savevm",
>          .args_type  = "name:s?",
> -        .params     = "[tag|id]",
> -        .help       = "save a VM snapshot. If no tag or id are provided, a new snapshot is created",
> +        .params     = "tag",
> +        .help       = "save a VM snapshot. If no tag is provided, a new snapshot is created",
>          .cmd        = hmp_savevm,
>      },
>  
>  STEXI
> -@item savevm [@var{tag}|@var{id}]
> +@item savevm @var{tag}
>  @findex savevm
>  Create a snapshot of the whole virtual machine. If @var{tag} is
>  provided, it is used as human readable identifier. If there is already
> -a snapshot with the same tag or ID, it is replaced. More info at
> +a snapshot with the same tag, it is replaced. More info at
>  @ref{vm_snapshots}.
> +
> +Since 3.2, savevm stopped allowing the snapshot id to be set, accepting
> +only @var{tag} as parameter.
>  ETEXI
>  
>      {
>          .name       = "loadvm",
>          .args_type  = "name:s",
> -        .params     = "tag|id",
> -        .help       = "restore a VM snapshot from its tag or id",
> +        .params     = "tag",
> +        .help       = "restore a VM snapshot from its tag",
>          .cmd        = hmp_loadvm,
>          .command_completion = loadvm_completion,
>      },
>  
>  STEXI
> -@item loadvm @var{tag}|@var{id}
> +@item loadvm @var{tag}
>  @findex loadvm
>  Set the whole virtual machine to the snapshot identified by the tag
> -@var{tag} or the unique snapshot ID @var{id}.
> +@var{tag}.
> +
> +Since 3.2, loadvm stopped accepting snapshot id as parameter.
>  ETEXI
>  
>      {
>          .name       = "delvm",
>          .args_type  = "name:s",
> -        .params     = "tag|id",
> -        .help       = "delete a VM snapshot from its tag or id",
> +        .params     = "tag",
> +        .help       = "delete a VM snapshot from its tag",
>          .cmd        = hmp_delvm,
>          .command_completion = delvm_completion,
>      },
>  
>  STEXI
> -@item delvm @var{tag}|@var{id}
> +@item delvm @var{tag}
>  @findex delvm
> -Delete the snapshot identified by @var{tag} or @var{id}.
> +Delete the snapshot identified by @var{tag}.
> +
> +Since 3.2, delvm stopped deleting snapshots by snapshot id, accepting
> +only @var{tag} as parameter.
>  ETEXI
>  
>      {
> -- 
> 2.17.2
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: [Qemu-devel] [PATCH for-3.2 v3 1/3] block/snapshot.c: eliminate use of ID input in snapshot operations

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1079 bytes --]

On 11/7/18 7:09 AM, Daniel Henrique Barboza wrote:
> At this moment, QEMU attempts to create/load/delete snapshots
> by using either an ID (id_str) or a name. The problem is that the code
> isn't consistent of whether the entered argument is an ID or a name,
> causing unexpected behaviors.
> 

>  STEXI
> -@item savevm [@var{tag}|@var{id}]
> +@item savevm @var{tag}
>  @findex savevm
>  Create a snapshot of the whole virtual machine. If @var{tag} is
>  provided, it is used as human readable identifier. If there is already
> -a snapshot with the same tag or ID, it is replaced. More info at
> +a snapshot with the same tag, it is replaced. More info at
>  @ref{vm_snapshots}.
> +
> +Since 3.2, savevm stopped allowing the snapshot id to be set, accepting

s/3.2/4.0/

> +only @var{tag} as parameter.

> +Since 3.2, delvm stopped deleting snapshots by snapshot id, accepting
> +only @var{tag} as parameter.

and again

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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Re: [Qemu-devel] [PATCH for-3.2 v3 1/3] block/snapshot.c: eliminate use of ID input in snapshot operations

[Kevin Wolf]

[-- Attachment #1: Type: text/plain, Size: 1132 bytes --]

Am 15.02.2019 um 17:21 hat Eric Blake geschrieben:
> On 11/7/18 7:09 AM, Daniel Henrique Barboza wrote:
> > At this moment, QEMU attempts to create/load/delete snapshots
> > by using either an ID (id_str) or a name. The problem is that the code
> > isn't consistent of whether the entered argument is an ID or a name,
> > causing unexpected behaviors.
> > 
> 
> >  STEXI
> > -@item savevm [@var{tag}|@var{id}]
> > +@item savevm @var{tag}
> >  @findex savevm
> >  Create a snapshot of the whole virtual machine. If @var{tag} is
> >  provided, it is used as human readable identifier. If there is already
> > -a snapshot with the same tag or ID, it is replaced. More info at
> > +a snapshot with the same tag, it is replaced. More info at
> >  @ref{vm_snapshots}.
> > +
> > +Since 3.2, savevm stopped allowing the snapshot id to be set, accepting
> 
> s/3.2/4.0/
> 
> > +only @var{tag} as parameter.
> 
> > +Since 3.2, delvm stopped deleting snapshots by snapshot id, accepting
> > +only @var{tag} as parameter.
> 
> and again

Sorry, I should have mentioned that I changed this while applying.

Kevin

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[Qemu-devel] [PATCH for-3.2 v3 2/3] block/snapshot: remove bdrv_snapshot_delete_by_id_or_name

[Daniel Henrique Barboza]

After the previous patch, the only instance of this function left
is inside qemu-img.c.

qemu-img is using it inside the 'img_snapshot' function to delete
snapshots in the SNAPSHOT_DELETE case, based on a "snapshot_name"
string that refers to the tag, not ID, of the QEMUSnapshotInfo struct.
This can be verified by checking the SNAPSHOT_CREATE case that
comes shortly before SNAPSHOT_DELETE. In that case, the same
"snapshot_name" variable is being strcpy to the 'name' field
of the QEMUSnapshotInfo struct sn:

pstrcpy(sn.name, sizeof(sn.name), snapshot_name);

Based on that, it is unlikely that "snapshot_name" might contain
an "id" in SNAPSHOT_DELETE.

This patch changes SNAPSHOT_DELETE to use snapshot_find() and
snapshot_delete() instead of bdrv_snapshot_delete_by_id_or_name.
After that, there is no instances left of bdrv_snapshot_delete_by_id_or_name
in the code, so it is safe to remove it entirely.

Suggested-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 block/snapshot.c         | 20 --------------------
 include/block/snapshot.h |  3 ---
 qemu-img.c               | 15 +++++++++++----
 3 files changed, 11 insertions(+), 27 deletions(-)

diff --git a/block/snapshot.c b/block/snapshot.c
index e371d2243d..f2f48f926a 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -301,26 +301,6 @@ int bdrv_snapshot_delete(BlockDriverState *bs,
     return ret;
 }
 
-int bdrv_snapshot_delete_by_id_or_name(BlockDriverState *bs,
-                                       const char *id_or_name,
-                                       Error **errp)
-{
-    int ret;
-    Error *local_err = NULL;
-
-    ret = bdrv_snapshot_delete(bs, id_or_name, NULL, &local_err);
-    if (ret == -ENOENT || ret == -EINVAL) {
-        error_free(local_err);
-        local_err = NULL;
-        ret = bdrv_snapshot_delete(bs, NULL, id_or_name, &local_err);
-    }
-
-    if (ret < 0) {
-        error_propagate(errp, local_err);
-    }
-    return ret;
-}
-
 int bdrv_snapshot_list(BlockDriverState *bs,
                        QEMUSnapshotInfo **psn_info)
 {
diff --git a/include/block/snapshot.h b/include/block/snapshot.h
index f73d1094af..b5d5084a12 100644
--- a/include/block/snapshot.h
+++ b/include/block/snapshot.h
@@ -61,9 +61,6 @@ int bdrv_snapshot_delete(BlockDriverState *bs,
                          const char *snapshot_id,
                          const char *name,
                          Error **errp);
-int bdrv_snapshot_delete_by_id_or_name(BlockDriverState *bs,
-                                       const char *id_or_name,
-                                       Error **errp);
 int bdrv_snapshot_list(BlockDriverState *bs,
                        QEMUSnapshotInfo **psn_info);
 int bdrv_snapshot_load_tmp(BlockDriverState *bs,
diff --git a/qemu-img.c b/qemu-img.c
index 4c96db7ba4..7b4910adcf 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3121,11 +3121,18 @@ static int img_snapshot(int argc, char **argv)
         break;
 
     case SNAPSHOT_DELETE:
-        bdrv_snapshot_delete_by_id_or_name(bs, snapshot_name, &err);
-        if (err) {
-            error_reportf_err(err, "Could not delete snapshot '%s': ",
-                              snapshot_name);
+        ret = bdrv_snapshot_find(bs, &sn, snapshot_name);
+        if (ret < 0) {
+            error_report("Could not delete snapshot '%s': snapshot not "
+                         "found", snapshot_name);
             ret = 1;
+        } else {
+            ret = bdrv_snapshot_delete(bs, sn.id_str, sn.name, &err);
+            if (ret < 0) {
+                error_reportf_err(err, "Could not delete snapshot '%s': ",
+                                  snapshot_name);
+                ret = 1;
+            }
         }
         break;
     }
-- 
2.17.2

[Qemu-devel] [PATCH for-3.2 v3 3/3] qcow2-snapshot: remove redundant find_snapshot_by_id_and_name call

[Daniel Henrique Barboza]

In qcow2_snapshot_create there is the following code block:

    /* Generate an ID */
    find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str));

    /* Check that the ID is unique */
    if (find_snapshot_by_id_and_name(bs, sn_info->id_str, NULL) >= 0) {
        return -EEXIST;
    }

find_new_snapshot_id cycles through all snapshots, getting the id_str
as an unsigned long int, calculating the max id_max value of all the
existing id_strs and writing in the id_str pointer id_max + 1:

    for(i = 0; i < s->nb_snapshots; i++) {
        sn = s->snapshots + i;
        id = strtoul(sn->id_str, NULL, 10);
        if (id > id_max)
            id_max = id;
    }
    snprintf(id_str, id_str_size, "%lu", id_max + 1);

Here, sn_info->id_str will have the unique value id_max + 1. Right
after that, find_snapshot_by_id_and_name is called with
id = sn_info->id_str and name = NULL. This will cause the function
to execute the following:

    } else if (id) {
        for (i = 0; i < s->nb_snapshots; i++) {
            if (!strcmp(s->snapshots[i].id_str, id)) {
                return i;
            }
        }
    }

In short, we're searching the existing snapshots to see if sn_info->id_str
matches any existing id, right after we set in the previous line a
sn_info->id_str value that is already unique.

The first code block goes way back to commit 585f8587ad, a 2006 commit from
Fabrice Bellard that simply says "new qcow2 disk image format". No more
info is provided about this logic in any subsequent commits that moved
this code block around.

I can't say about the original design, but the current logic is redundant.
bdrv_snapshot_create is called in aio_context lock, forbidding any
concurrent call to accidentally create a new snapshot between
the find_new_snapshot_id and find_snapshot_by_id_and_name calls. What
we're ending up doing is to cycle through the snapshots two times
for no viable reason.

This patch eliminates the redundancy by removing the 'id is unique'
check that calls find_snapshot_by_id_and_name.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 block/qcow2-snapshot.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index bb6a5b7516..20e8472191 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -358,11 +358,6 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
     /* Generate an ID */
     find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str));
 
-    /* Check that the ID is unique */
-    if (find_snapshot_by_id_and_name(bs, sn_info->id_str, NULL) >= 0) {
-        return -EEXIST;
-    }
-
     /* Populate sn with passed data */
     sn->id_str = g_strdup(sn_info->id_str);
     sn->name = g_strdup(sn_info->name);
-- 
2.17.2

Re: [Qemu-devel] [PATCH for-3.2 v3 0/3] HMP/snapshot changes - do not use ID anymore

[Daniel Henrique Barboza]

Ping

On 11/7/18 11:09 AM, Daniel Henrique Barboza wrote:
> changes in v3:
> - rebased to v3.1.0-rc0 tag
> - hmp-commands.hx documentation now mentions the change of semantics
> starting version 3.2.
> - previous version link:
> http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00629.html
>
>
> It is not uncommon to see bugs being opened by testers that attempt to
> create VM snapshots using HMP. It turns out that "0" and "1" are quite
> common snapshot names and they trigger a lot of bugs. I gave an example
> in the commit message of patch 1, but to sum up here: QEMU treats the
> input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. It
> is documented as such, but this can lead to strange situations.
>
> Given that it is strange for an API to consider a parameter to be 2 fields
> at the same time, and inadvently treating them as one or the other, and
> that removing the ID field is too drastic, my idea here is to keep the
> ID field for internal control, but do not let the user set it.
>
> This series simplifies the meaning of savevm/loadvm/delvm to be up to
> par to what the QEMU code (and Libvirt) is already doing: snapshot
> operations using "tag" semantics only, leaving the "id" to be
> automatically calculated by the block drivers and used internally
> only.
>
> This change of semantics does not affect existing snapshots. What
> changes is that any HMP operations with them will use the
> updated semantics.
>
>
> Daniel Henrique Barboza (3):
>    block/snapshot.c: eliminate use of ID input in snapshot operations
>    block/snapshot: remove bdrv_snapshot_delete_by_id_or_name
>    qcow2-snapshot: remove redundant find_snapshot_by_id_and_name call
>
>   block/qcow2-snapshot.c   |  5 -----
>   block/snapshot.c         | 25 +++----------------------
>   hmp-commands.hx          | 32 ++++++++++++++++++++------------
>   include/block/snapshot.h |  3 ---
>   qemu-img.c               | 15 +++++++++++----
>   5 files changed, 34 insertions(+), 46 deletions(-)
>

Re: [Qemu-devel] [PATCH for-3.2 v3 0/3] HMP/snapshot changes - do not use ID anymore

[Daniel Henrique Barboza]

Ping

I believe all the discussion that happened in v2 applies here as well. Do we
have a plan for this series? Should I add something else (warning message
or doc note) to indicate a deprecation of the old meaning of the arguments?


Thanks,


DHB

On 11/7/18 11:09 AM, Daniel Henrique Barboza wrote:
> changes in v3:
> - rebased to v3.1.0-rc0 tag
> - hmp-commands.hx documentation now mentions the change of semantics
> starting version 3.2.
> - previous version link:
> http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00629.html
>
>
> It is not uncommon to see bugs being opened by testers that attempt to
> create VM snapshots using HMP. It turns out that "0" and "1" are quite
> common snapshot names and they trigger a lot of bugs. I gave an example
> in the commit message of patch 1, but to sum up here: QEMU treats the
> input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. It
> is documented as such, but this can lead to strange situations.
>
> Given that it is strange for an API to consider a parameter to be 2 fields
> at the same time, and inadvently treating them as one or the other, and
> that removing the ID field is too drastic, my idea here is to keep the
> ID field for internal control, but do not let the user set it.
>
> This series simplifies the meaning of savevm/loadvm/delvm to be up to
> par to what the QEMU code (and Libvirt) is already doing: snapshot
> operations using "tag" semantics only, leaving the "id" to be
> automatically calculated by the block drivers and used internally
> only.
>
> This change of semantics does not affect existing snapshots. What
> changes is that any HMP operations with them will use the
> updated semantics.
>
>
> Daniel Henrique Barboza (3):
>    block/snapshot.c: eliminate use of ID input in snapshot operations
>    block/snapshot: remove bdrv_snapshot_delete_by_id_or_name
>    qcow2-snapshot: remove redundant find_snapshot_by_id_and_name call
>
>   block/qcow2-snapshot.c   |  5 -----
>   block/snapshot.c         | 25 +++----------------------
>   hmp-commands.hx          | 32 ++++++++++++++++++++------------
>   include/block/snapshot.h |  3 ---
>   qemu-img.c               | 15 +++++++++++----
>   5 files changed, 34 insertions(+), 46 deletions(-)
>

Re: [Qemu-devel] [PATCH for-3.2 v3 0/3] HMP/snapshot changes - do not use ID anymore

[Kevin Wolf]

Am 07.11.2018 um 14:09 hat Daniel Henrique Barboza geschrieben:
> changes in v3:
> - rebased to v3.1.0-rc0 tag
> - hmp-commands.hx documentation now mentions the change of semantics
> starting version 3.2.
> - previous version link:
> http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00629.html
> 
> 
> It is not uncommon to see bugs being opened by testers that attempt to
> create VM snapshots using HMP. It turns out that "0" and "1" are quite
> common snapshot names and they trigger a lot of bugs. I gave an example
> in the commit message of patch 1, but to sum up here: QEMU treats the
> input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. It
> is documented as such, but this can lead to strange situations.
> 
> Given that it is strange for an API to consider a parameter to be 2 fields
> at the same time, and inadvently treating them as one or the other, and
> that removing the ID field is too drastic, my idea here is to keep the
> ID field for internal control, but do not let the user set it.
> 
> This series simplifies the meaning of savevm/loadvm/delvm to be up to
> par to what the QEMU code (and Libvirt) is already doing: snapshot
> operations using "tag" semantics only, leaving the "id" to be
> automatically calculated by the block drivers and used internally
> only.
> 
> This change of semantics does not affect existing snapshots. What
> changes is that any HMP operations with them will use the
> updated semantics.

Thanks, applied to the block branch.

Kevin