Re: [PATCH v5 06/11] 9p: darwin: Move XATTR_SIZE_MAX->P9_XATTR_SIZE_MAX

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 7. Februar 2022 23:40:19 CET Will Cohen wrote:
> From: Keno Fischer <keno@juliacomputing.com>
> 
> Signed-off-by: Keno Fischer <keno@juliacomputing.com>
> Signed-off-by: Michael Roitzsch <reactorcontrol@icloud.com>
> 
> Because XATTR_SIZE_MAX is not defined on Darwin,
> create a cross-platform P9_XATTR_SIZE_MAX instead.
> 
> [Will Cohen: - Adjust coding style
>              - Lower XATTR_SIZE_MAX to 64k
>              - Add explanatory context related to XATTR_SIZE_MAX]
> [Fabian Franz: - Move XATTR_SIZE_MAX reference from 9p.c to
>                  P9_XATTR_SIZE_MAX in 9p.h]
> Signed-off-by: Will Cohen <wwcohen@gmail.com>
> Signed-off-by: Fabian Franz <fabianfranz.oss@gmail.com>
> ---
>  hw/9pfs/9p.c |  2 +-
>  hw/9pfs/9p.h | 11 +++++++++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 14e84c3bcf..7405352c37 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -3949,7 +3949,7 @@ static void coroutine_fn v9fs_xattrcreate(void
> *opaque) rflags |= XATTR_REPLACE;
>      }
> 
> -    if (size > XATTR_SIZE_MAX) {
> +    if (size > P9_XATTR_SIZE_MAX) {
>          err = -E2BIG;
>          goto out_nofid;
>      }
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index 1567b67841..6a1856b4dc 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -479,4 +479,15 @@ struct V9fsTransport {
>      void        (*push_and_notify)(V9fsPDU *pdu);
>  };
> 
> +/*
> + * Darwin doesn't seem to define a maximum xattr size in its user
> + * space header, so manually configure it across platforms as 64k.
> + *
> + * Having no limit at all can lead to QEMU crashing during large g_malloc()
> + * calls. Because QEMU does not currently support macOS guests, the below
> + * preliminary solution only works due to its being a reflection of the limit of
> + * Linux guests.
> + */
> +#define P9_XATTR_SIZE_MAX 65536

It would be cleaner in a way like this:

#if defined(XATTR_SIZE_MAX)
/* Linux */
#define P9_XATTR_SIZE_MAX XATTR_SIZE_MAX
#elif defined(CONFIG_DARWIN)
/* darwin comment goes here */
#define P9_XATTR_SIZE_MAX 65536
#else
#error Missing definition for P9_XATTR_SIZE_MAX for this host system
#endif

Sorry, I haven't noticed that before. You actually had that wrapped into some
ifdefs in v2 before:

#if defined(CONFIG_DARWIN) && !defined(XATTR_SIZE_MAX)
...
#endif

> +
>  #endif