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Re: [Qemu-devel] [PATCH] qemu-ga: use key-value store to avoid recycling


From: Luiz Capitulino
Subject: Re: [Qemu-devel] [PATCH] qemu-ga: use key-value store to avoid recycling fd handles after restart
Date: Wed, 20 Mar 2013 10:54:51 -0400

On Fri,  1 Mar 2013 11:40:27 -0600
Michael Roth <address@hidden> wrote:

> Hosts hold on to handles provided by guest-file-open for periods that can
> span beyond the life of the qemu-ga process that issued them. Since these
> are issued starting from 0 on every restart, we run the risk of issuing
> duplicate handles after restarts/reboots.
> 
> As a result, users with a stale copy of these handles may end up
> reading/writing corrupted data due to their existing handles effectively
> being re-assigned to an unexpected file or offset.
> 
> We unfortunately do not issue handles as strings, but as integers, so a
> solution such as using UUIDs can't be implemented without introducing a
> new interface.
> 
> As a workaround, we fix this by implementing a persistent key-value store
> that will be used to track the value of the last handle that was issued
> across restarts/reboots to avoid issuing duplicates.
> 
> The store is automatically written to the same directory we currently
> set via --statedir to track fsfreeze state, and so should be applicable
> for stable releases where this flag is supported.

Did you consider that statedir is normally set to /var/run which is
cleared by some distros on system reboot? Is this OK?

Also, I find it unfortunate that fd_counter never goes down...

One more comment below.

> 
> A follow-up can use this same store for handling fsfreeze state, but
> that change is cosmetic and left out for now.
> 
> Signed-off-by: Michael Roth <address@hidden>
> Cc: address@hidden
> ---
>  qga/commands-posix.c   |   25 +++++--
>  qga/guest-agent-core.h |    1 +
>  qga/main.c             |  184 
> ++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 204 insertions(+), 6 deletions(-)
> 
> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> index 7a0202e..5d12716 100644
> --- a/qga/commands-posix.c
> +++ b/qga/commands-posix.c
> @@ -129,14 +129,22 @@ static struct {
>      QTAILQ_HEAD(, GuestFileHandle) filehandles;
>  } guest_file_state;
>  
> -static void guest_file_handle_add(FILE *fh)
> +static uint64_t guest_file_handle_add(FILE *fh, Error **errp)
>  {
>      GuestFileHandle *gfh;
> +    uint64_t handle;
> +
> +    handle = ga_get_fd_handle(ga_state, errp);
> +    if (error_is_set(errp)) {
> +        return 0;
> +    }
>  
>      gfh = g_malloc0(sizeof(GuestFileHandle));
> -    gfh->id = fileno(fh);
> +    gfh->id = handle;
>      gfh->fh = fh;
>      QTAILQ_INSERT_TAIL(&guest_file_state.filehandles, gfh, next);
> +
> +    return handle;
>  }
>  
>  static GuestFileHandle *guest_file_handle_find(int64_t id, Error **err)
> @@ -158,7 +166,7 @@ int64_t qmp_guest_file_open(const char *path, bool 
> has_mode, const char *mode, E
>  {
>      FILE *fh;
>      int fd;
> -    int64_t ret = -1;
> +    int64_t ret = -1, handle;
>  
>      if (!has_mode) {
>          mode = "r";
> @@ -184,9 +192,14 @@ int64_t qmp_guest_file_open(const char *path, bool 
> has_mode, const char *mode, E
>          return -1;
>      }
>  
> -    guest_file_handle_add(fh);
> -    slog("guest-file-open, handle: %d", fd);
> -    return fd;
> +    handle = guest_file_handle_add(fh, err);
> +    if (error_is_set(err)) {
> +        fclose(fh);
> +        return -1;
> +    }
> +
> +    slog("guest-file-open, handle: %d", handle);
> +    return handle;
>  }
>  
>  void qmp_guest_file_close(int64_t handle, Error **err)
> diff --git a/qga/guest-agent-core.h b/qga/guest-agent-core.h
> index 3354598..624a559 100644
> --- a/qga/guest-agent-core.h
> +++ b/qga/guest-agent-core.h
> @@ -35,6 +35,7 @@ bool ga_is_frozen(GAState *s);
>  void ga_set_frozen(GAState *s);
>  void ga_unset_frozen(GAState *s);
>  const char *ga_fsfreeze_hook(GAState *s);
> +int64_t ga_get_fd_handle(GAState *s, Error **errp);
>  
>  #ifndef _WIN32
>  void reopen_fd_to_null(int fd);
> diff --git a/qga/main.c b/qga/main.c
> index db281a5..3635430 100644
> --- a/qga/main.c
> +++ b/qga/main.c
> @@ -15,6 +15,7 @@
>  #include <stdbool.h>
>  #include <glib.h>
>  #include <getopt.h>
> +#include <glib/gstdio.h>
>  #ifndef _WIN32
>  #include <syslog.h>
>  #include <sys/wait.h>
> @@ -30,6 +31,7 @@
>  #include "qapi/qmp/qerror.h"
>  #include "qapi/qmp/dispatch.h"
>  #include "qga/channel.h"
> +#include "qemu/bswap.h"
>  #ifdef _WIN32
>  #include "qga/service-win32.h"
>  #include <windows.h>
> @@ -53,6 +55,11 @@
>  #endif
>  #define QGA_SENTINEL_BYTE 0xFF
>  
> +typedef struct GAPersistentState {
> +#define QGA_PSTATE_DEFAULT_FD_COUNTER 1000
> +    int64_t fd_counter;
> +} GAPersistentState;
> +
>  struct GAState {
>      JSONMessageParser parser;
>      GMainLoop *main_loop;
> @@ -76,6 +83,8 @@ struct GAState {
>  #ifdef CONFIG_FSFREEZE
>      const char *fsfreeze_hook;
>  #endif
> +    const gchar *pstate_filepath;
> +    GAPersistentState pstate;
>  };
>  
>  struct GAState *ga_state;
> @@ -724,6 +733,171 @@ VOID WINAPI service_main(DWORD argc, TCHAR *argv[])
>  }
>  #endif
>  
> +static void set_persistent_state_defaults(GAPersistentState *pstate)
> +{
> +    g_assert(pstate);
> +    pstate->fd_counter = QGA_PSTATE_DEFAULT_FD_COUNTER;
> +}
> +
> +static void persistent_state_from_keyfile(GAPersistentState *pstate,
> +                                          GKeyFile *keyfile)
> +{
> +    g_assert(pstate);
> +    g_assert(keyfile);
> +    /* if any fields are missing, either because the file was tampered with
> +     * by agents of chaos, or because the field wasn't present at the time 
> the
> +     * file was created, the best we can ever do is start over with the 
> default
> +     * values. so load them now, and ignore any errors in accessing key-value
> +     * pairs
> +     */
> +    set_persistent_state_defaults(pstate);
> +
> +    if (g_key_file_has_key(keyfile, "global", "fd_counter", NULL)) {
> +        pstate->fd_counter =
> +            g_key_file_get_int64(keyfile, "global", "fd_counter", NULL);
> +    }
> +}
> +
> +static void persistent_state_to_keyfile(const GAPersistentState *pstate,
> +                                        GKeyFile *keyfile)
> +{
> +    g_assert(pstate);
> +    g_assert(keyfile);
> +
> +    g_key_file_set_int64(keyfile, "global", "fd_counter", 
> pstate->fd_counter);
> +}
> +
> +static gboolean write_persistent_state(const GAPersistentState *pstate,
> +                                       const gchar *path)
> +{
> +    GKeyFile *keyfile = g_key_file_new();
> +    GError *gerr = NULL;
> +    gboolean ret = true;
> +    gchar *data = NULL;
> +    gsize data_len;
> +
> +    g_assert(pstate);
> +
> +    persistent_state_to_keyfile(pstate, keyfile);
> +    data = g_key_file_to_data(keyfile, &data_len, &gerr);
> +    if (gerr) {
> +        g_critical("failed to convert persistent state to string: %s",
> +                   gerr->message);
> +        ret = false;
> +        goto out;
> +    }
> +
> +    g_file_set_contents(path, data, data_len, &gerr);
> +    if (gerr) {
> +        g_critical("failed to write persistent state to %s: %s",
> +                    path, gerr->message);
> +        ret = false;
> +        goto out;
> +    }
> +
> +out:
> +    if (gerr) {
> +        g_error_free(gerr);
> +    }
> +    if (keyfile) {
> +        g_key_file_free(keyfile);
> +    }
> +    g_free(data);
> +    return ret;
> +}
> +
> +static gboolean read_persistent_state(GAPersistentState *pstate,
> +                                      const gchar *path, gboolean frozen)
> +{
> +    GKeyFile *keyfile = NULL;
> +    GError *gerr = NULL;
> +    struct stat st;
> +    gboolean ret = true;
> +
> +    g_assert(pstate);
> +
> +    if (stat(path, &st) == -1) {
> +        /* it's okay if state file doesn't exist, but any other error
> +         * indicates a permissions issue or some other misconfiguration
> +         * that we likely won't be able to recover from.
> +         */
> +        if (errno != ENOENT) {
> +            g_critical("unable to access state file at path %s: %s",
> +                       path, strerror(errno));
> +            ret = false;
> +            goto out;
> +        }
> +
> +        /* file doesn't exist. initialize state to default values and
> +         * attempt to save now. (we could wait till later when we have
> +         * modified state we need to commit, but if there's a problem,
> +         * such as a missing parent directory, we want to catch it now)
> +         *
> +         * there is a potential scenario where someone either managed to
> +         * update the agent from a version that didn't use a key store
> +         * while qemu-ga thought the filesystem was frozen, or
> +         * deleted the key store prior to issuing a fsfreeze, prior
> +         * to restarting the agent. in this case we go ahead and defer
> +         * initial creation till we actually have modified state to
> +         * write, otherwise fail to recover from freeze.
> +         */
> +        set_persistent_state_defaults(pstate);
> +        if (!frozen) {
> +            ret = write_persistent_state(pstate, path);
> +            if (!ret) {
> +                g_critical("unable to create state file at path %s", path);
> +                ret = false;
> +                goto out;
> +            }
> +        }
> +        ret = true;
> +        goto out;
> +    }
> +
> +    keyfile = g_key_file_new();
> +    g_key_file_load_from_file(keyfile, path, 0, &gerr);
> +    if (gerr) {
> +        g_critical("error loading persistent state from path: %s, %s",
> +                   path, gerr->message);
> +        ret = false;
> +        goto out;
> +    }
> +
> +    persistent_state_from_keyfile(pstate, keyfile);
> +
> +out:
> +    if (keyfile) {
> +        g_key_file_free(keyfile);
> +    }
> +    if (gerr) {
> +        g_error_free(gerr);
> +    }
> +
> +    return ret;
> +}
> +
> +int64_t ga_get_fd_handle(GAState *s, Error **errp)
> +{
> +    int64_t handle;
> +
> +    g_assert(s->pstate_filepath);
> +    /* we blacklist commands and avoid operations that potentially require
> +     * writing to disk when we're in a frozen state. this includes opening
> +     * new files, so we should never get here in that situation
> +     */
> +    g_assert(!ga_is_frozen(s));
> +
> +    handle = s->pstate.fd_counter++;
> +    if (s->pstate.fd_counter < 0) {
> +        s->pstate.fd_counter = 0;
> +    }

Is this handling the overflow case? Can't fd 0 be in use already?

I'd check against fd_counter max value and return an error if that's reached.

> +    if (!write_persistent_state(&s->pstate, s->pstate_filepath)) {
> +        error_setg(errp, "failed to commit persistent state to disk");
> +    }
> +
> +    return handle;
> +}
> +
>  int main(int argc, char **argv)
>  {
>      const char *sopt = "hVvdm:p:l:f:F::b:s:t:";
> @@ -853,7 +1027,9 @@ int main(int argc, char **argv)
>      ga_enable_logging(s);
>      s->state_filepath_isfrozen = g_strdup_printf("%s/qga.state.isfrozen",
>                                                   state_dir);
> +    s->pstate_filepath = g_strdup_printf("%s/qga.state", state_dir);
>      s->frozen = false;
> +
>  #ifndef _WIN32
>      /* check if a previous instance of qemu-ga exited with filesystems' state
>       * marked as frozen. this could be a stale value (a non-qemu-ga process
> @@ -910,6 +1086,14 @@ int main(int argc, char **argv)
>          }
>      }
>  
> +    /* load persistent state from disk */
> +    if (!read_persistent_state(&s->pstate,
> +                               s->pstate_filepath,
> +                               ga_is_frozen(s))) {
> +        g_critical("failed to load persistent state");
> +        goto out_bad;
> +    }
> +
>      if (blacklist) {
>          s->blacklist = blacklist;
>          do {




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