Re: [Qemu-devel] [RFC v2 05/33] migration: better error handling with QEMUFile

[Peter Xu]

On Thu, Sep 21, 2017 at 06:51:37PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) wrote:
> > If the postcopy down due to some reason, we can always see this on dst:
> > 
> >   qemu-system-x86_64: RP: Received invalid message 0x0000 length 0x0000
> > 
> > However in most cases that's not the real issue. The problem is that
> > qemu_get_be16() has no way to show whether the returned data is valid or
> > not, and we are _always_ assuming it is valid. That's possibly not wise.
> > 
> > The best approach to solve this would be: refactoring QEMUFile interface
> > to allow the APIs to return error if there is. However it needs quite a
> > bit of work and testing. For now, let's explicitly check the validity
> > first before using the data in all places for qemu_get_*().
> > 
> > This patch tries to fix most of the cases I can see. Only if we are with
> > this, can we make sure we are processing the valid data, and also can we
> > make sure we can capture the channel down events correctly.
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  migration/migration.c |  5 +++++
> >  migration/ram.c       | 22 ++++++++++++++++++----
> >  migration/savevm.c    | 41 +++++++++++++++++++++++++++++++++++++++--
> >  3 files changed, 62 insertions(+), 6 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index c818412..92bf9b8 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -1543,6 +1543,11 @@ static void *source_return_path_thread(void *opaque)
> >          header_type = qemu_get_be16(rp);
> >          header_len = qemu_get_be16(rp);
> >  
> > +        if (qemu_file_get_error(rp)) {
> > +            mark_source_rp_bad(ms);
> > +            goto out;
> > +        }
> > +
> >          if (header_type >= MIG_RP_MSG_MAX ||
> >              header_type == MIG_RP_MSG_INVALID) {
> >              error_report("RP: Received invalid message 0x%04x length 
> > 0x%04x",
> > diff --git a/migration/ram.c b/migration/ram.c
> > index affb20c..7e20097 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2417,7 +2417,7 @@ static int ram_load_postcopy(QEMUFile *f)
> >      void *last_host = NULL;
> >      bool all_zero = false;
> >  
> > -    while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
> > +    while (!(flags & RAM_SAVE_FLAG_EOS)) {
> 
> With this change, I don't see what's checking the result of:
>    ret = postcopy_place_page(...)
> at the bottom of the loop.

Indeed.

I'll add the check after postcopy_place_page().

> 
> >          ram_addr_t addr;
> >          void *host = NULL;
> >          void *page_buffer = NULL;
> > @@ -2426,6 +2426,16 @@ static int ram_load_postcopy(QEMUFile *f)
> >          uint8_t ch;
> >  
> >          addr = qemu_get_be64(f);
> > +
> > +        /*
> > +         * If qemu file error, we should stop here, and then "addr"
> > +         * may be invalid
> > +         */
> > +        ret = qemu_file_get_error(f);
> > +        if (ret) {
> > +            break;
> > +        }
> > +
> >          flags = addr & ~TARGET_PAGE_MASK;
> >          addr &= TARGET_PAGE_MASK;
> >  
> > @@ -2506,6 +2516,13 @@ static int ram_load_postcopy(QEMUFile *f)
> >              error_report("Unknown combination of migration flags: %#x"
> >                           " (postcopy mode)", flags);
> >              ret = -EINVAL;
> > +            break;
> > +        }
> > +
> > +        /* Detect for any possible file errors */
> > +        if (qemu_file_get_error(f)) {
> > +            ret = qemu_file_get_error(f);
> > +            break;
> >          }
> >  
> >          if (place_needed) {
> > @@ -2520,9 +2537,6 @@ static int ram_load_postcopy(QEMUFile *f)
> >                                            place_source, block);
> >              }
> >          }
> > -        if (!ret) {
> > -            ret = qemu_file_get_error(f);
> > -        }
> >      }
> >  
> >      return ret;
> > diff --git a/migration/savevm.c b/migration/savevm.c
> > index fdd15fa..7172f14 100644
> > --- a/migration/savevm.c
> > +++ b/migration/savevm.c
> > @@ -1720,6 +1720,11 @@ static int loadvm_process_command(QEMUFile *f)
> >      cmd = qemu_get_be16(f);
> >      len = qemu_get_be16(f);
> >  
> > +    /* Check validity before continue processing of cmds */
> > +    if (qemu_file_get_error(f)) {
> > +        return qemu_file_get_error(f);
> > +    }
> > +
> >      trace_loadvm_process_command(cmd, len);
> >      if (cmd >= MIG_CMD_MAX || cmd == MIG_CMD_INVALID) {
> >          error_report("MIG_CMD 0x%x unknown (len 0x%x)", cmd, len);
> > @@ -1785,6 +1790,7 @@ static int loadvm_process_command(QEMUFile *f)
> >   */
> >  static bool check_section_footer(QEMUFile *f, SaveStateEntry *se)
> >  {
> > +    int ret;
> >      uint8_t read_mark;
> >      uint32_t read_section_id;
> >  
> > @@ -1795,6 +1801,13 @@ static bool check_section_footer(QEMUFile *f, 
> > SaveStateEntry *se)
> >  
> >      read_mark = qemu_get_byte(f);
> >  
> > +    ret = qemu_file_get_error(f);
> > +    if (ret) {
> > +        error_report("%s: Read section footer failed: %d",
> > +                     __func__, ret);
> > +        return false;
> > +    }
> > +
> >      if (read_mark != QEMU_VM_SECTION_FOOTER) {
> >          error_report("Missing section footer for %s", se->idstr);
> >          return false;
> > @@ -1830,6 +1843,13 @@ qemu_loadvm_section_start_full(QEMUFile *f, 
> > MigrationIncomingState *mis)
> >      instance_id = qemu_get_be32(f);
> >      version_id = qemu_get_be32(f);
> >  
> > +    ret = qemu_file_get_error(f);
> > +    if (ret) {
> > +        error_report("%s: Failed to read instance/version ID: %d",
> > +                     __func__, ret);
> > +        return ret;
> > +    }
> > +
> >      trace_qemu_loadvm_state_section_startfull(section_id, idstr,
> >              instance_id, version_id);
> >      /* Find savevm section */
> > @@ -1877,6 +1897,13 @@ qemu_loadvm_section_part_end(QEMUFile *f, 
> > MigrationIncomingState *mis)
> >  
> >      section_id = qemu_get_be32(f);
> >  
> > +    ret = qemu_file_get_error(f);
> > +    if (ret) {
> > +        error_report("%s: Failed to read section ID: %d",
> > +                     __func__, ret);
> > +        return ret;
> > +    }
> > +
> >      trace_qemu_loadvm_state_section_partend(section_id);
> >      QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> >          if (se->load_section_id == section_id) {
> > @@ -1944,8 +1971,14 @@ static int qemu_loadvm_state_main(QEMUFile *f, 
> > MigrationIncomingState *mis)
> >      uint8_t section_type;
> >      int ret = 0;
> >  
> > -    while ((section_type = qemu_get_byte(f)) != QEMU_VM_EOF) {
> > -        ret = 0;
> > +    while (true) {
> > +        section_type = qemu_get_byte(f);
> > +
> > +        if (qemu_file_get_error(f)) {
> > +            ret = qemu_file_get_error(f);
> > +            break;
> > +        }
> > +
> >          trace_qemu_loadvm_state_section(section_type);
> >          switch (section_type) {
> >          case QEMU_VM_SECTION_START:
> > @@ -1969,6 +2002,10 @@ static int qemu_loadvm_state_main(QEMUFile *f, 
> > MigrationIncomingState *mis)
> >                  goto out;
> >              }
> >              break;
> > +        case QEMU_VM_EOF:
> > +            /* This is the end of migration */
> > +            goto out;
> > +            break;
> 
> You don't need the goto and the break (although it does no harm).

I still need the goto to break the loop? (I think a single break will
only break the select, but not the loop)

If I remove both "goto" and "break", it'll fall through to default, I
suppose that's not what we want?

> 
> Dave
> 
> >          default:
> >              error_report("Unknown savevm section type %d", section_type);
> >              ret = -EINVAL;
> > -- 
> > 2.7.4
> > 
> > 
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK

-- 
Peter Xu