Patchwork [V11,13/17] block: dump to buffer for bdrv_snapshot_dump() and bdrv_image_info_dump()

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Submitter Wayne Xia
Date April 2, 2013, 11:47 a.m.
Message ID <1364903250-10429-14-git-send-email-xiawenc@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/232984/
State New
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Comments

Wayne Xia - April 2, 2013, 11:47 a.m.
This patch would allow hmp use them just like qemu-img, and avoid
string truncation.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
---
 block/qapi.c         |   65 +++++++++++++++++++++++++++-----------------------
 include/block/qapi.h |    4 +-
 qemu-img.c           |   22 ++++++++++++----
 savevm.c             |   11 ++++++--
 4 files changed, 61 insertions(+), 41 deletions(-)
Stefan Hajnoczi - April 8, 2013, 1:36 p.m.
On Tue, Apr 02, 2013 at 07:47:26PM +0800, Wenchao Xia wrote:
> diff --git a/qemu-img.c b/qemu-img.c
> index 5b229a9..032f68c 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1558,18 +1558,24 @@ static void dump_snapshots(BlockDriverState *bs)
>  {
>      QEMUSnapshotInfo *sn_tab, *sn;
>      int nb_sns, i;
> -    char buf[256];
> +    GString *buf = g_string_new(NULL);
>  
>      nb_sns = bdrv_snapshot_list(bs, &sn_tab);
>      if (nb_sns <= 0)
>          return;

Leaks buf.

> diff --git a/savevm.c b/savevm.c
> index e4e0008..ce0bbe1 100644
> --- a/savevm.c
> +++ b/savevm.c
> @@ -2466,7 +2466,7 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>      int nb_sns, i, ret, available;
>      int total;
>      int *available_snapshots;
> -    char buf[256];
> +    GString *buf = NULL;
>  
>      bs = bdrv_snapshots();
>      if (!bs) {
> @@ -2509,11 +2509,16 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>      }
>  
>      if (total > 0) {

Please declare buf here since it is only used in this scope.
Markus Armbruster - April 10, 2013, 4:56 p.m.
Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Tue, Apr 02, 2013 at 07:47:26PM +0800, Wenchao Xia wrote:
>> diff --git a/qemu-img.c b/qemu-img.c
>> index 5b229a9..032f68c 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -1558,18 +1558,24 @@ static void dump_snapshots(BlockDriverState *bs)
>>  {
>>      QEMUSnapshotInfo *sn_tab, *sn;
>>      int nb_sns, i;
>> -    char buf[256];
>> +    GString *buf = g_string_new(NULL);
>>  
>>      nb_sns = bdrv_snapshot_list(bs, &sn_tab);
>>      if (nb_sns <= 0)
>>          return;
>
> Leaks buf.

Trivially avoided my moving the initialization behind the return.

>> diff --git a/savevm.c b/savevm.c
>> index e4e0008..ce0bbe1 100644
>> --- a/savevm.c
>> +++ b/savevm.c
>> @@ -2466,7 +2466,7 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>>      int nb_sns, i, ret, available;
>>      int total;
>>      int *available_snapshots;
>> -    char buf[256];
>> +    GString *buf = NULL;
>>  
>>      bs = bdrv_snapshots();
>>      if (!bs) {
>> @@ -2509,11 +2509,16 @@ void do_info_snapshots(Monitor *mon, const
>> QDict *qdict)
>>      }
>>  
>>      if (total > 0) {
>
> Please declare buf here since it is only used in this scope.

Matter of taste.  I'd keep it where it is :)
Markus Armbruster - April 10, 2013, 5:05 p.m.
Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:

>   This patch would allow hmp use them just like qemu-img, and avoid
> string truncation.
>
> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
> ---
>  block/qapi.c         |   65 +++++++++++++++++++++++++++-----------------------
>  include/block/qapi.h |    4 +-
>  qemu-img.c           |   22 ++++++++++++----
>  savevm.c             |   11 ++++++--
>  4 files changed, 61 insertions(+), 41 deletions(-)
>
> diff --git a/block/qapi.c b/block/qapi.c
> index df63c97..e27519e 100644
> --- a/block/qapi.c
> +++ b/block/qapi.c
> @@ -327,7 +327,7 @@ BlockInfoList *qmp_query_block(Error **errp)
>      return NULL;
>  }
>  
> -char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
> +void bdrv_snapshot_dump(GString *buf, QEMUSnapshotInfo *sn)
>  {
>      char buf1[128], date_buf[128], clock_buf[128];
>      struct tm tm;

I like this change, because it replaces an arbitrarily sized buffer by a
flexible one.

> @@ -335,9 +335,8 @@ char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
>      int64_t secs;
>  
>      if (!sn) {
> -        snprintf(buf, buf_size,
> -                 "%-10s%-20s%7s%20s%15s",
> -                 "ID", "TAG", "VM SIZE", "DATE", "VM CLOCK");
> +        g_string_append_printf(buf, "%-10s%-20s%7s%20s%15s",
> +                               "ID", "TAG", "VM SIZE", "DATE", "VM CLOCK");
>      } else {
>          ti = sn->date_sec;
>          localtime_r(&ti, &tm);
> @@ -350,17 +349,17 @@ char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
>                   (int)((secs / 60) % 60),
>                   (int)(secs % 60),
>                   (int)((sn->vm_clock_nsec / 1000000) % 1000));
> -        snprintf(buf, buf_size,
> -                 "%-10s%-20s%7s%20s%15s",
> -                 sn->id_str, sn->name,
> -                 get_human_readable_size(buf1, sizeof(buf1), sn->vm_state_size),
> -                 date_buf,
> -                 clock_buf);
> +        g_string_append_printf(buf,
> +                               "%-10s%-20s%7s%20s%15s",
> +                               sn->id_str, sn->name,
> +                               get_human_readable_size(buf1, sizeof(buf1),
> +                                                       sn->vm_state_size),
> +                               date_buf,
> +                               clock_buf);
>      }
> -    return buf;
>  }
>  
> -void bdrv_image_info_dump(ImageInfo *info)
> +void bdrv_image_info_dump(GString *buf, ImageInfo *info)
>  {
>      char size_buf[128], dsize_buf[128];
>      if (!info->has_actual_size) {
> @@ -370,43 +369,48 @@ void bdrv_image_info_dump(ImageInfo *info)

I don't like this change, because it introduces buffering for no
discernible reason.  Unless you can show me one, I'd like you to keep
printing directly.

>                                  info->actual_size);
>      }
>      get_human_readable_size(size_buf, sizeof(size_buf), info->virtual_size);
> -    printf("image: %s\n"
> -           "file format: %s\n"
> -           "virtual size: %s (%" PRId64 " bytes)\n"
> -           "disk size: %s\n",
> -           info->filename, info->format, size_buf,
> -           info->virtual_size,
> -           dsize_buf);
> +    g_string_append_printf(buf,
> +                           "image: %s\n"
> +                           "file format: %s\n"
> +                           "virtual size: %s (%" PRId64 " bytes)\n"
> +                           "disk size: %s\n",
> +                           info->filename, info->format, size_buf,
> +                           info->virtual_size,
> +                           dsize_buf);
>  
>      if (info->has_encrypted && info->encrypted) {
> -        printf("encrypted: yes\n");
> +        g_string_append_printf(buf, "encrypted: yes\n");
>      }
>  
>      if (info->has_cluster_size) {
> -        printf("cluster_size: %" PRId64 "\n", info->cluster_size);
> +        g_string_append_printf(buf, "cluster_size: %" PRId64 "\n",
> +                               info->cluster_size);
>      }
>  
>      if (info->has_dirty_flag && info->dirty_flag) {
> -        printf("cleanly shut down: no\n");
> +        g_string_append_printf(buf, "cleanly shut down: no\n");
>      }
>  
>      if (info->has_backing_filename) {
> -        printf("backing file: %s", info->backing_filename);
> +        g_string_append_printf(buf, "backing file: %s",
> +                               info->backing_filename);
>          if (info->has_full_backing_filename) {
> -            printf(" (actual path: %s)", info->full_backing_filename);
> +            g_string_append_printf(buf, " (actual path: %s)",
> +                                   info->full_backing_filename);
>          }
> -        putchar('\n');
> +        g_string_append_printf(buf, "\n");
>          if (info->has_backing_filename_format) {
> -            printf("backing file format: %s\n", info->backing_filename_format);
> +            g_string_append_printf(buf, "backing file format: %s\n",
> +                                   info->backing_filename_format);
>          }
>      }
>  
>      if (info->has_snapshots) {
>          SnapshotInfoList *elem;
> -        char buf[256];
>  
> -        printf("Snapshot list:\n");
> -        printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
> +        g_string_append_printf(buf, "Snapshot list:\n");
> +        bdrv_snapshot_dump(buf, NULL);
> +        g_string_append_printf(buf, "\n");
>  
>          /* Ideally bdrv_snapshot_dump() would operate on SnapshotInfoList but
>           * we convert to the block layer's native QEMUSnapshotInfo for now.
> @@ -422,7 +426,8 @@ void bdrv_image_info_dump(ImageInfo *info)
>  
>              pstrcpy(sn.id_str, sizeof(sn.id_str), elem->value->id);
>              pstrcpy(sn.name, sizeof(sn.name), elem->value->name);
> -            printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), &sn));
> +            bdrv_snapshot_dump(buf, &sn);
> +            g_string_append_printf(buf, "\n");
>          }
>      }
>  }
> diff --git a/include/block/qapi.h b/include/block/qapi.h
> index 237660f..dc1e090 100644
> --- a/include/block/qapi.h
> +++ b/include/block/qapi.h
> @@ -38,6 +38,6 @@ int bdrv_query_image_info(BlockDriverState *bs,
>  void bdrv_query_info(BlockDriverState *bs,
>                       BlockInfo **p_info,
>                       Error **errp);
> -char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
> -void bdrv_image_info_dump(ImageInfo *info);
> +void bdrv_snapshot_dump(GString *buf, QEMUSnapshotInfo *sn);
> +void bdrv_image_info_dump(GString *buf, ImageInfo *info);
>  #endif
> diff --git a/qemu-img.c b/qemu-img.c
> index 5b229a9..032f68c 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1558,18 +1558,24 @@ static void dump_snapshots(BlockDriverState *bs)
>  {
>      QEMUSnapshotInfo *sn_tab, *sn;
>      int nb_sns, i;
> -    char buf[256];
> +    GString *buf = g_string_new(NULL);
>  
>      nb_sns = bdrv_snapshot_list(bs, &sn_tab);
>      if (nb_sns <= 0)
>          return;
> -    printf("Snapshot list:\n");
> -    printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
> +    g_string_append_printf(buf, "Snapshot list:\n");
> +    bdrv_snapshot_dump(buf, NULL);
> +    g_string_append_printf(buf, "\n");
>      for(i = 0; i < nb_sns; i++) {
>          sn = &sn_tab[i];
> -        printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), sn));
> +        bdrv_snapshot_dump(buf, sn);
> +        g_string_append_printf(buf, "\n");
>      }
> +
> +    printf("%s", buf->str);
> +    g_string_free(buf, true);
>      g_free(sn_tab);
> +
>  }
>  
>  static void dump_json_image_info_list(ImageInfoList *list)
> @@ -1610,15 +1616,19 @@ static void dump_human_image_info_list(ImageInfoList *list)
>  {
>      ImageInfoList *elem;
>      bool delim = false;
> +    GString *buf = g_string_new(NULL);
>  
>      for (elem = list; elem; elem = elem->next) {
>          if (delim) {
> -            printf("\n");
> +            g_string_append_printf(buf, "\n");
>          }
>          delim = true;
>  
> -        bdrv_image_info_dump(elem->value);
> +        bdrv_image_info_dump(buf, elem->value);
>      }
> +
> +    printf("%s", buf->str);
> +    g_string_free(buf, true);
>  }
>  
>  static gboolean str_equal_func(gconstpointer a, gconstpointer b)
> diff --git a/savevm.c b/savevm.c
> index e4e0008..ce0bbe1 100644
> --- a/savevm.c
> +++ b/savevm.c
> @@ -2466,7 +2466,7 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>      int nb_sns, i, ret, available;
>      int total;
>      int *available_snapshots;
> -    char buf[256];
> +    GString *buf = NULL;

Useless initialization.  But if you keep bdrv_snapshot_dump() printing,
it all goes away.

>  
>      bs = bdrv_snapshots();
>      if (!bs) {
> @@ -2509,11 +2509,16 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>      }
>  
>      if (total > 0) {
> -        monitor_printf(mon, "%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
> +        buf = g_string_new(NULL);
> +        bdrv_snapshot_dump(buf, NULL);
> +        g_string_append_printf(buf, "\n");
>          for (i = 0; i < total; i++) {
>              sn = &sn_tab[available_snapshots[i]];
> -            monitor_printf(mon, "%s\n", bdrv_snapshot_dump(buf, sizeof(buf), sn));
> +            bdrv_snapshot_dump(buf, sn);
> +            g_string_append_printf(buf, "\n");
>          }
> +        monitor_printf(mon, "%s", buf->str);
> +        g_string_free(buf, true);
>      } else {
>          monitor_printf(mon, "There is no suitable snapshot available\n");
>      }
Wayne Xia - April 11, 2013, 6:35 a.m.
>>   
>> -void bdrv_image_info_dump(ImageInfo *info)
>> +void bdrv_image_info_dump(GString *buf, ImageInfo *info)
>>   {
>>       char size_buf[128], dsize_buf[128];
>>       if (!info->has_actual_size) {
>> @@ -370,43 +369,48 @@ void bdrv_image_info_dump(ImageInfo *info)
> 
> I don't like this change, because it introduces buffering for no
> discernible reason.  Unless you can show me one, I'd like you to keep
> printing directly.
> 
  HMP code later need to call this function, and then print buf to
monitor console, which is the goal of this patch.

>>                                   info->actual_size);
>>       }

>>   
>>   static gboolean str_equal_func(gconstpointer a, gconstpointer b)
>> diff --git a/savevm.c b/savevm.c
>> index e4e0008..ce0bbe1 100644
>> --- a/savevm.c
>> +++ b/savevm.c
>> @@ -2466,7 +2466,7 @@ void do_info_snapshots(Monitor *mon, const QDict *qdict)
>>       int nb_sns, i, ret, available;
>>       int total;
>>       int *available_snapshots;
>> -    char buf[256];
>> +    GString *buf = NULL;
> 
> Useless initialization.  But if you keep bdrv_snapshot_dump() printing,
> it all goes away.
> 
  will remove.
Markus Armbruster - April 11, 2013, 12:05 p.m.
Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:

>>>   
>>> -void bdrv_image_info_dump(ImageInfo *info)
>>> +void bdrv_image_info_dump(GString *buf, ImageInfo *info)
>>>   {
>>>       char size_buf[128], dsize_buf[128];
>>>       if (!info->has_actual_size) {
>>> @@ -370,43 +369,48 @@ void bdrv_image_info_dump(ImageInfo *info)
>> 
>> I don't like this change, because it introduces buffering for no
>> discernible reason.  Unless you can show me one, I'd like you to keep
>> printing directly.
>> 
>   HMP code later need to call this function, and then print buf to
> monitor console, which is the goal of this patch.

Aha, the actual problem is "print either to stdout or to current
monitor", so that we can share code among qemu-img and monitor commands.
Such sharing is good, and the problem is real.

bdrv_snapshot_dump() solves it this way: print to a fixed-size buffer,
which the caller either prints to stdout (qemu-img) or to the current
monitor (info snapshots).

Your patch makes the buffer flexible.  Improvement.

But in my opinion, the detour through the buffer is silly.  Why not
simply use a print function that does the right thing?

We already have such a function for "print either to stderr or to
current monitor": error_printf().  Could easily add one for stdout.

[...]
Wayne Xia - April 12, 2013, 4:51 a.m.
于 2013-4-11 20:05, Markus Armbruster 写道:
> Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:
> 
>>>>    
>>>> -void bdrv_image_info_dump(ImageInfo *info)
>>>> +void bdrv_image_info_dump(GString *buf, ImageInfo *info)
>>>>    {
>>>>        char size_buf[128], dsize_buf[128];
>>>>        if (!info->has_actual_size) {
>>>> @@ -370,43 +369,48 @@ void bdrv_image_info_dump(ImageInfo *info)
>>>
>>> I don't like this change, because it introduces buffering for no
>>> discernible reason.  Unless you can show me one, I'd like you to keep
>>> printing directly.
>>>
>>    HMP code later need to call this function, and then print buf to
>> monitor console, which is the goal of this patch.
> 
> Aha, the actual problem is "print either to stdout or to current
> monitor", so that we can share code among qemu-img and monitor commands.
> Such sharing is good, and the problem is real.
> 
> bdrv_snapshot_dump() solves it this way: print to a fixed-size buffer,
> which the caller either prints to stdout (qemu-img) or to the current
> monitor (info snapshots).
> 
> Your patch makes the buffer flexible.  Improvement.
> 
> But in my opinion, the detour through the buffer is silly.  Why not
> simply use a print function that does the right thing?
> 
> We already have such a function for "print either to stderr or to
> current monitor": error_printf().  Could easily add one for stdout.
> 
> [...]
> 
  Indeed it is a better way, how about:
void message_printf(const char *fmt, ...);
I am not sure where should this function be placed,
hmp.c?
Markus Armbruster - April 12, 2013, 11:51 a.m.
Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:

> 于 2013-4-11 20:05, Markus Armbruster 写道:
>> Wenchao Xia <xiawenc@linux.vnet.ibm.com> writes:
>> 
>>>>>    
>>>>> -void bdrv_image_info_dump(ImageInfo *info)
>>>>> +void bdrv_image_info_dump(GString *buf, ImageInfo *info)
>>>>>    {
>>>>>        char size_buf[128], dsize_buf[128];
>>>>>        if (!info->has_actual_size) {
>>>>> @@ -370,43 +369,48 @@ void bdrv_image_info_dump(ImageInfo *info)
>>>>
>>>> I don't like this change, because it introduces buffering for no
>>>> discernible reason.  Unless you can show me one, I'd like you to keep
>>>> printing directly.
>>>>
>>>    HMP code later need to call this function, and then print buf to
>>> monitor console, which is the goal of this patch.
>> 
>> Aha, the actual problem is "print either to stdout or to current
>> monitor", so that we can share code among qemu-img and monitor commands.
>> Such sharing is good, and the problem is real.
>> 
>> bdrv_snapshot_dump() solves it this way: print to a fixed-size buffer,
>> which the caller either prints to stdout (qemu-img) or to the current
>> monitor (info snapshots).
>> 
>> Your patch makes the buffer flexible.  Improvement.
>> 
>> But in my opinion, the detour through the buffer is silly.  Why not
>> simply use a print function that does the right thing?
>> 
>> We already have such a function for "print either to stderr or to
>> current monitor": error_printf().  Could easily add one for stdout.
>> 
>> [...]
>> 
>   Indeed it is a better way, how about:
> void message_printf(const char *fmt, ...);
> I am not sure where should this function be placed,
> hmp.c?

I figure it needs to go in a file that's linked into the utilities, too.
I think these are in make variables util-obj-y, stub-obj-y.

I'd be tempted to put it into qemu-error.c.  If you don't like that
because it's not about errors, consider renaming the file :)

Patch

diff --git a/block/qapi.c b/block/qapi.c
index df63c97..e27519e 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -327,7 +327,7 @@  BlockInfoList *qmp_query_block(Error **errp)
     return NULL;
 }
 
-char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
+void bdrv_snapshot_dump(GString *buf, QEMUSnapshotInfo *sn)
 {
     char buf1[128], date_buf[128], clock_buf[128];
     struct tm tm;
@@ -335,9 +335,8 @@  char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
     int64_t secs;
 
     if (!sn) {
-        snprintf(buf, buf_size,
-                 "%-10s%-20s%7s%20s%15s",
-                 "ID", "TAG", "VM SIZE", "DATE", "VM CLOCK");
+        g_string_append_printf(buf, "%-10s%-20s%7s%20s%15s",
+                               "ID", "TAG", "VM SIZE", "DATE", "VM CLOCK");
     } else {
         ti = sn->date_sec;
         localtime_r(&ti, &tm);
@@ -350,17 +349,17 @@  char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn)
                  (int)((secs / 60) % 60),
                  (int)(secs % 60),
                  (int)((sn->vm_clock_nsec / 1000000) % 1000));
-        snprintf(buf, buf_size,
-                 "%-10s%-20s%7s%20s%15s",
-                 sn->id_str, sn->name,
-                 get_human_readable_size(buf1, sizeof(buf1), sn->vm_state_size),
-                 date_buf,
-                 clock_buf);
+        g_string_append_printf(buf,
+                               "%-10s%-20s%7s%20s%15s",
+                               sn->id_str, sn->name,
+                               get_human_readable_size(buf1, sizeof(buf1),
+                                                       sn->vm_state_size),
+                               date_buf,
+                               clock_buf);
     }
-    return buf;
 }
 
-void bdrv_image_info_dump(ImageInfo *info)
+void bdrv_image_info_dump(GString *buf, ImageInfo *info)
 {
     char size_buf[128], dsize_buf[128];
     if (!info->has_actual_size) {
@@ -370,43 +369,48 @@  void bdrv_image_info_dump(ImageInfo *info)
                                 info->actual_size);
     }
     get_human_readable_size(size_buf, sizeof(size_buf), info->virtual_size);
-    printf("image: %s\n"
-           "file format: %s\n"
-           "virtual size: %s (%" PRId64 " bytes)\n"
-           "disk size: %s\n",
-           info->filename, info->format, size_buf,
-           info->virtual_size,
-           dsize_buf);
+    g_string_append_printf(buf,
+                           "image: %s\n"
+                           "file format: %s\n"
+                           "virtual size: %s (%" PRId64 " bytes)\n"
+                           "disk size: %s\n",
+                           info->filename, info->format, size_buf,
+                           info->virtual_size,
+                           dsize_buf);
 
     if (info->has_encrypted && info->encrypted) {
-        printf("encrypted: yes\n");
+        g_string_append_printf(buf, "encrypted: yes\n");
     }
 
     if (info->has_cluster_size) {
-        printf("cluster_size: %" PRId64 "\n", info->cluster_size);
+        g_string_append_printf(buf, "cluster_size: %" PRId64 "\n",
+                               info->cluster_size);
     }
 
     if (info->has_dirty_flag && info->dirty_flag) {
-        printf("cleanly shut down: no\n");
+        g_string_append_printf(buf, "cleanly shut down: no\n");
     }
 
     if (info->has_backing_filename) {
-        printf("backing file: %s", info->backing_filename);
+        g_string_append_printf(buf, "backing file: %s",
+                               info->backing_filename);
         if (info->has_full_backing_filename) {
-            printf(" (actual path: %s)", info->full_backing_filename);
+            g_string_append_printf(buf, " (actual path: %s)",
+                                   info->full_backing_filename);
         }
-        putchar('\n');
+        g_string_append_printf(buf, "\n");
         if (info->has_backing_filename_format) {
-            printf("backing file format: %s\n", info->backing_filename_format);
+            g_string_append_printf(buf, "backing file format: %s\n",
+                                   info->backing_filename_format);
         }
     }
 
     if (info->has_snapshots) {
         SnapshotInfoList *elem;
-        char buf[256];
 
-        printf("Snapshot list:\n");
-        printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
+        g_string_append_printf(buf, "Snapshot list:\n");
+        bdrv_snapshot_dump(buf, NULL);
+        g_string_append_printf(buf, "\n");
 
         /* Ideally bdrv_snapshot_dump() would operate on SnapshotInfoList but
          * we convert to the block layer's native QEMUSnapshotInfo for now.
@@ -422,7 +426,8 @@  void bdrv_image_info_dump(ImageInfo *info)
 
             pstrcpy(sn.id_str, sizeof(sn.id_str), elem->value->id);
             pstrcpy(sn.name, sizeof(sn.name), elem->value->name);
-            printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), &sn));
+            bdrv_snapshot_dump(buf, &sn);
+            g_string_append_printf(buf, "\n");
         }
     }
 }
diff --git a/include/block/qapi.h b/include/block/qapi.h
index 237660f..dc1e090 100644
--- a/include/block/qapi.h
+++ b/include/block/qapi.h
@@ -38,6 +38,6 @@  int bdrv_query_image_info(BlockDriverState *bs,
 void bdrv_query_info(BlockDriverState *bs,
                      BlockInfo **p_info,
                      Error **errp);
-char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
-void bdrv_image_info_dump(ImageInfo *info);
+void bdrv_snapshot_dump(GString *buf, QEMUSnapshotInfo *sn);
+void bdrv_image_info_dump(GString *buf, ImageInfo *info);
 #endif
diff --git a/qemu-img.c b/qemu-img.c
index 5b229a9..032f68c 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1558,18 +1558,24 @@  static void dump_snapshots(BlockDriverState *bs)
 {
     QEMUSnapshotInfo *sn_tab, *sn;
     int nb_sns, i;
-    char buf[256];
+    GString *buf = g_string_new(NULL);
 
     nb_sns = bdrv_snapshot_list(bs, &sn_tab);
     if (nb_sns <= 0)
         return;
-    printf("Snapshot list:\n");
-    printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
+    g_string_append_printf(buf, "Snapshot list:\n");
+    bdrv_snapshot_dump(buf, NULL);
+    g_string_append_printf(buf, "\n");
     for(i = 0; i < nb_sns; i++) {
         sn = &sn_tab[i];
-        printf("%s\n", bdrv_snapshot_dump(buf, sizeof(buf), sn));
+        bdrv_snapshot_dump(buf, sn);
+        g_string_append_printf(buf, "\n");
     }
+
+    printf("%s", buf->str);
+    g_string_free(buf, true);
     g_free(sn_tab);
+
 }
 
 static void dump_json_image_info_list(ImageInfoList *list)
@@ -1610,15 +1616,19 @@  static void dump_human_image_info_list(ImageInfoList *list)
 {
     ImageInfoList *elem;
     bool delim = false;
+    GString *buf = g_string_new(NULL);
 
     for (elem = list; elem; elem = elem->next) {
         if (delim) {
-            printf("\n");
+            g_string_append_printf(buf, "\n");
         }
         delim = true;
 
-        bdrv_image_info_dump(elem->value);
+        bdrv_image_info_dump(buf, elem->value);
     }
+
+    printf("%s", buf->str);
+    g_string_free(buf, true);
 }
 
 static gboolean str_equal_func(gconstpointer a, gconstpointer b)
diff --git a/savevm.c b/savevm.c
index e4e0008..ce0bbe1 100644
--- a/savevm.c
+++ b/savevm.c
@@ -2466,7 +2466,7 @@  void do_info_snapshots(Monitor *mon, const QDict *qdict)
     int nb_sns, i, ret, available;
     int total;
     int *available_snapshots;
-    char buf[256];
+    GString *buf = NULL;
 
     bs = bdrv_snapshots();
     if (!bs) {
@@ -2509,11 +2509,16 @@  void do_info_snapshots(Monitor *mon, const QDict *qdict)
     }
 
     if (total > 0) {
-        monitor_printf(mon, "%s\n", bdrv_snapshot_dump(buf, sizeof(buf), NULL));
+        buf = g_string_new(NULL);
+        bdrv_snapshot_dump(buf, NULL);
+        g_string_append_printf(buf, "\n");
         for (i = 0; i < total; i++) {
             sn = &sn_tab[available_snapshots[i]];
-            monitor_printf(mon, "%s\n", bdrv_snapshot_dump(buf, sizeof(buf), sn));
+            bdrv_snapshot_dump(buf, sn);
+            g_string_append_printf(buf, "\n");
         }
+        monitor_printf(mon, "%s", buf->str);
+        g_string_free(buf, true);
     } else {
         monitor_printf(mon, "There is no suitable snapshot available\n");
     }