Re: [PATCH v2] 9pfs: fix 'total_open_fd' decrementation

[Greg Kurz <groug@kaod.org>]

On Thu, 20 Mar 2025 13:16:20 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> According to 'man 2 close' errors returned by close() should only be used
> for either diagnostic purposes or for catching data loss due to a previous
> write error, as an error result of close() usually indicates a deferred
> error of a previous write operation.
> 
> Therefore not decrementing 'total_open_fd' on a close() error is wrong
> and would yield in a higher open file descriptor count than actually the
> case, leading to 9p server reclaiming open file descriptors too soon.
> 
> Based-on: <20250312152933.383967-7-groug@kaod.org>
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  V2: log a warning message on unexpected close() -> EBADF case
> 
>  hw/9pfs/9p.c     | 10 +++++++++-
>  hw/9pfs/codir.c  |  7 ++++++-
>  hw/9pfs/cofile.c |  7 ++++++-
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index b22df3aa2b..8b001b9112 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -510,7 +510,15 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
>              err = (f->fid_type == P9_FID_DIR) ?
>                  s->ops->closedir(&s->ctx, &f->fs_reclaim) :
>                  s->ops->close(&s->ctx, &f->fs_reclaim);
> -            if (!err) {
> +
> +            /* 'man 2 close' suggests to ignore close() errors except of EBADF */
> +            if (unlikely(err && errno == EBADF)) {
> +                /*
> +                 * unexpected case as FIDs were picked above by having a valid
> +                 * file descriptor
> +                 */
> +                error_report("9pfs: v9fs_reclaim_fd() WARNING: close() failed with EBADF");

Reviewed-by: Greg Kurz <groug@kaod.org>

I just hope there isn't a way to reach this 100% or we'll end up
saturating the logs. ;-)

> +            } else {
>                  /* total_open_fd must only be mutated on main thread */
>                  nclosed++;
>              }
> diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c
> index 2068a4779d..bce7dd96e9 100644
> --- a/hw/9pfs/codir.c
> +++ b/hw/9pfs/codir.c
> @@ -20,6 +20,7 @@
>  #include "fsdev/qemu-fsdev.h"
>  #include "qemu/thread.h"
>  #include "qemu/main-loop.h"
> +#include "qemu/error-report.h"
>  #include "coth.h"
>  #include "9p-xattr.h"
>  #include "9p-util.h"
> @@ -353,7 +354,11 @@ int coroutine_fn v9fs_co_closedir(V9fsPDU *pdu, V9fsFidOpenState *fs)
>                  err = -errno;
>              }
>          });
> -    if (!err) {
> +    /* 'man 2 close' suggests to ignore close() errors except of EBADF */
> +    if (unlikely(err && errno == EBADF)) {
> +        /* unexpected case as we should have checked for a valid file handle */
> +        error_report("9pfs: WARNING: v9fs_co_closedir() failed with EBADF");
> +    } else {
>          total_open_fd--;
>      }
>      return err;
> diff --git a/hw/9pfs/cofile.c b/hw/9pfs/cofile.c
> index 71174c3e4a..6e775c8e41 100644
> --- a/hw/9pfs/cofile.c
> +++ b/hw/9pfs/cofile.c
> @@ -20,6 +20,7 @@
>  #include "fsdev/qemu-fsdev.h"
>  #include "qemu/thread.h"
>  #include "qemu/main-loop.h"
> +#include "qemu/error-report.h"
>  #include "coth.h"
>  
>  int coroutine_fn v9fs_co_st_gen(V9fsPDU *pdu, V9fsPath *path, mode_t st_mode,
> @@ -197,7 +198,11 @@ int coroutine_fn v9fs_co_close(V9fsPDU *pdu, V9fsFidOpenState *fs)
>                  err = -errno;
>              }
>          });
> -    if (!err) {
> +    /* 'man 2 close' suggests to ignore close() errors except of EBADF */
> +    if (unlikely(err && errno == EBADF)) {
> +        /* unexpected case as we should have checked for a valid file handle */
> +        error_report("9pfs: WARNING: v9fs_co_close() failed with EBADF");
> +    } else {
>          total_open_fd--;
>      }
>      return err;



-- 
Greg