Re: [PATCH v4 3/6] util: add Error object for qemu_open_internal error reporting

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Tue, Aug 25, 2020 at 05:14:21PM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > Instead of relying on the limited information from errno, we can now
>> > also provide detailed error messages.
>> 
>> The more detailed error messages are currently always ignored, but the
>> next patches will fix that.
>> 
>> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
>> > ---
>> >  util/osdep.c | 21 +++++++++++++++++++--
>> >  1 file changed, 19 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/util/osdep.c b/util/osdep.c
>> > index 9ff92551e7..9c7118d3cb 100644
>> > --- a/util/osdep.c
>> > +++ b/util/osdep.c
>> > @@ -284,7 +284,7 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive)
>> >   * Opens a file with FD_CLOEXEC set
>> >   */
>> >  static int
>> > -qemu_open_internal(const char *name, int flags, mode_t mode)
>> > +qemu_open_internal(const char *name, int flags, mode_t mode, Error **errp)
>> >  {
>> >      int ret;
>> >  
>> > @@ -298,24 +298,31 @@ qemu_open_internal(const char *name, int flags, mode_t mode)
>> >  
>> >          fdset_id = qemu_parse_fdset(fdset_id_str);
>> >          if (fdset_id == -1) {
>> > +            error_setg(errp, "Could not parse fdset %s", name);
>> >              errno = EINVAL;
>> >              return -1;
>> >          }
>> >  
>> >          fd = monitor_fdset_get_fd(fdset_id, flags);
>> >          if (fd < 0) {
>> > +            error_setg_errno(errp, -fd, "Could not acquire FD for %s flags %x",
>> > +                             name, flags);
>> >              errno = -fd;
>> >              return -1;
>> >          }
>> >  
>> >          dupfd = qemu_dup_flags(fd, flags);
>> >          if (dupfd == -1) {
>> > +            error_setg_errno(errp, errno, "Could not dup FD for %s flags %x",
>> > +                             name, flags);
>> >              return -1;
>> >          }
>> >  
>> >          ret = monitor_fdset_dup_fd_add(fdset_id, dupfd);
>> >          if (ret == -1) {
>> >              close(dupfd);
>> > +            error_setg(errp, "Could not save FD for %s flags %x",
>> > +                       name, flags);
>> 
>> Can this happen?
>
> Well there's code in monitor_fdset_dup_fd_add that can return -1.

It fails when

* @fdset_id contains @dupfd

  @dupfd is a fresh file descriptor.  If @fdset_id already contains it,
  it's stale there.  That would be a programming error.  Recommend to
  assert.

* @fdset_id is not in @mon_fdsets

  monitor_fdset_get_fd() fails the same way.  monitor_fdset_dup_fd_add()
  can fail that way after monitor_fdset_get_fd() succeed only if the fd
  set went away between the two.  Could that happen?  Would it be safe?

  This is the only user of monitor_fdset_dup_fd_add().  Why not remove
  the awkward failure mode by making monitor_fdset_dup_fd_add() dup the
  fd and add?

>> >              errno = EINVAL;
>> >              return -1;
>> >          }
>> > @@ -336,6 +343,16 @@ qemu_open_internal(const char *name, int flags, mode_t mode)
>> >      }
>> >  #endif /* ! O_CLOEXEC */
>> >  
>> > +    if (ret == -1) {
>> > +        const char *action = "open";
>> > +        if (flags & O_CREAT) {
>> > +            action = "create";
>> > +        }
>> > +        error_setg_errno(errp, errno, "Could not %s '%s' flags 0x%x",
>> > +                         action, name, flags);
>> 
>> Not a good user experience:
>> 
>>     Could not open '/etc/shadow' flags 0x0: Permission denied
>> 
>> Better:
>> 
>>     Could not open '/etc/shadow' for reading: Permission denied
>> 
>> Are you sure flags other than the access mode (O_RDONLY, O_WRONLY,
>> O_RDWR) must be included in the error message?
>
> It was the flags other than access mode that I was thinking were
> more important to log. I'm ambivalent htough, so can drop the
> flags if it is thought to be overkill.

Hexadecimal flags are borderline useless even for developers: to make
sense of them, you have to grep -R /usr/include/.  For mere mortals,
they are confusing in addition to useless.

>> If you must report flags in hexadecimal, then please reporting them more
>> consistently.  Right now you have
>> 
>>     for %s flags 0x%x
>>     '%s' flags %x
>> 
>> Perhaps '%s' with flags 0x%x
>> 
>> > +    }
>> > +
>> > +
>> >      return ret;
>> >  }
>> >  
>> > @@ -352,7 +369,7 @@ int qemu_open_old(const char *name, int flags, ...)
>> >      }
>> >      va_end(ap);
>> >  
>> > -    ret = qemu_open_internal(name, flags, mode);
>> > +    ret = qemu_open_internal(name, flags, mode, NULL);
>> >  
>> >  #ifdef O_DIRECT
>> >      if (ret == -1 && errno == EINVAL && (flags & O_DIRECT)) {
>> 
>> 
>
> Regards,
> Daniel