Re: [RFC PATCH] virtfs: 9p: local: add default uid and gid options

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Monday, 1 December 2025 19:00:53 CET Andrey Erokhin wrote:
> I was trying to boot from a directory tree owned by an ordinary user,
> and some daemons weren't happy about non-root ownership of some files
> 
> Example use:
> -virtfs local,path=rootfs,mount_tag=root,security_model=mapped,uid=0,gid=0
> 
> Works with any security_model

First I thought do we really want to open that rabbit hole and add permission 
management to the CLI options? However I get why this might be useful for 
mapped[-*] security models.

But for passthrough it is not of any use, is it? Just saying, because you 
write it "Works with any security_model".

Also while it is very handy to have a short option name like "uid" and "gid", 
for the sake of long term progression and clarity an option name like 
"default-uid" would be more appropriate.

The patch is also missing the required documentation changes for these new 
options BTW.

/Christian

> Signed-off-by: Andrey Erokhin <language.lawyer@gmail.com>
> ---
>  fsdev/file-op-9p.h      |  5 +++++
>  fsdev/qemu-fsdev-opts.c | 12 ++++++++++++
>  fsdev/qemu-fsdev.c      |  2 ++
>  hw/9pfs/9p-local.c      | 15 +++++++++++++++
>  hw/9pfs/9p.c            |  2 ++
>  system/vl.c             |  9 +++++++++
>  6 files changed, 45 insertions(+)
> 
> diff --git a/fsdev/file-op-9p.h b/fsdev/file-op-9p.h
> index b9dae8c84c..46fb88001e 100644
> --- a/fsdev/file-op-9p.h
> +++ b/fsdev/file-op-9p.h
> @@ -15,6 +15,7 @@
>  #define FILE_OP_9P_H
> 
>  #include <dirent.h>
> +#include <sys/types.h>
>  #include <utime.h>
>  #include "qemu-fsdev-throttle.h"
>  #include "p9array.h"
> @@ -94,6 +95,8 @@ typedef struct FsDriverEntry {
>      FsThrottle fst;
>      mode_t fmode;
>      mode_t dmode;
> +    uid_t dflt_uid;
> +    gid_t dflt_gid;
>  } FsDriverEntry;
> 
>  struct FsContext {
> @@ -107,6 +110,8 @@ struct FsContext {
>      void *private;
>      mode_t fmode;
>      mode_t dmode;
> +    uid_t dflt_uid;
> +    gid_t dflt_gid;
>  };
> 
>  struct V9fsPath {
> diff --git a/fsdev/qemu-fsdev-opts.c b/fsdev/qemu-fsdev-opts.c
> index 07a18c6e48..c99abb3de6 100644
> --- a/fsdev/qemu-fsdev-opts.c
> +++ b/fsdev/qemu-fsdev-opts.c
> @@ -46,6 +46,12 @@ static QemuOptsList qemu_fsdev_opts = {
>          }, {
>              .name = "dmode",
>              .type = QEMU_OPT_NUMBER,
> +        }, {
> +            .name = "uid",
> +            .type = QEMU_OPT_NUMBER,
> +        }, {
> +            .name = "gid",
> +            .type = QEMU_OPT_NUMBER,
>          },
> 
>          THROTTLE_OPTS,
> @@ -92,6 +98,12 @@ static QemuOptsList qemu_virtfs_opts = {
>          }, {
>              .name = "dmode",
>              .type = QEMU_OPT_NUMBER,
> +        }, {
> +            .name = "uid",
> +            .type = QEMU_OPT_NUMBER,
> +        }, {
> +            .name = "gid",
> +            .type = QEMU_OPT_NUMBER,
>          },
> 
>          { /*End of list */ }
> diff --git a/fsdev/qemu-fsdev.c b/fsdev/qemu-fsdev.c
> index 57877dad0a..faa84dc033 100644
> --- a/fsdev/qemu-fsdev.c
> +++ b/fsdev/qemu-fsdev.c
> @@ -58,6 +58,8 @@ static FsDriverTable FsDrivers[] = {
>              "writeout",
>              "fmode",
>              "dmode",
> +            "uid",
> +            "gid",
>              "multidevs",
>              "throttling.bps-total",
>              "throttling.bps-read",
> diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
> index 5ce97b76a6..cecf4aa50c 100644
> --- a/hw/9pfs/9p-local.c
> +++ b/hw/9pfs/9p-local.c
> @@ -198,6 +198,12 @@ static int local_lstat(FsContext *fs_ctx, V9fsPath
> *fs_path, struct stat *stbuf) if (err) {
>          goto err_out;
>      }
> +    if (fs_ctx->dflt_uid != -1) {
> +        stbuf->st_uid = fs_ctx->dflt_uid;
> +    }
> +    if (fs_ctx->dflt_gid != -1) {
> +        stbuf->st_gid = fs_ctx->dflt_gid;
> +    }
>      if (fs_ctx->export_flags & V9FS_SM_MAPPED) {
>          /* Actual credentials are part of extended attrs */
>          uid_t tmp_uid;
> @@ -788,6 +794,12 @@ static int local_fstat(FsContext *fs_ctx, int fid_type,
> if (err) {
>          return err;
>      }
> +    if (fs_ctx->dflt_uid != -1) {
> +        stbuf->st_uid = fs_ctx->dflt_uid;
> +    }
> +    if (fs_ctx->dflt_gid != -1) {
> +        stbuf->st_gid = fs_ctx->dflt_gid;
> +    }
>      if (fs_ctx->export_flags & V9FS_SM_MAPPED) {
>          /* Actual credentials are part of extended attrs */
>          uid_t tmp_uid;
> @@ -1570,6 +1582,9 @@ static int local_parse_opts(QemuOpts *opts,
> FsDriverEntry *fse, Error **errp) return -1;
>      }
> 
> +    fse->dflt_uid = qemu_opt_get_number(opts, "uid", -1);
> +    fse->dflt_gid = qemu_opt_get_number(opts, "gid", -1);
> +
>      if (fse->export_flags & V9FS_SM_MAPPED ||
>          fse->export_flags & V9FS_SM_MAPPED_FILE) {
>          fse->fmode =
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index acfa7db4e1..492379d361 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -4317,6 +4317,8 @@ int v9fs_device_realize_common(V9fsState *s, const
> V9fsTransport *t,
> 
>      s->ctx.fmode = fse->fmode;
>      s->ctx.dmode = fse->dmode;
> +    s->ctx.dflt_uid = fse->dflt_uid;
> +    s->ctx.dflt_gid = fse->dflt_gid;
> 
>      s->fids = g_hash_table_new(NULL, NULL);
>      qemu_co_rwlock_init(&s->rename_lock);
> diff --git a/system/vl.c b/system/vl.c
> index 3b7057e6c6..d363b046a6 100644
> --- a/system/vl.c
> +++ b/system/vl.c
> @@ -3253,6 +3253,7 @@ void qemu_init(int argc, char **argv)
>                  QemuOpts *fsdev;
>                  QemuOpts *device;
>                  const char *writeout, *sock_fd, *socket, *path,
> *security_model, +                           *uid, *gid,
>                             *multidevs;
> 
>                  olist = qemu_find_opts("virtfs");
> @@ -3301,6 +3302,14 @@ void qemu_init(int argc, char **argv)
>                      qemu_opt_set(fsdev, "security_model", security_model,
>                                   &error_abort);
>                  }
> +                uid = qemu_opt_get(opts, "uid");
> +                if (uid) {
> +                    qemu_opt_set(fsdev, "uid", uid, &error_abort);
> +                }
> +                gid = qemu_opt_get(opts, "gid");
> +                if (gid) {
> +                    qemu_opt_set(fsdev, "gid", gid, &error_abort);
> +                }
>                  socket = qemu_opt_get(opts, "socket");
>                  if (socket) {
>                      qemu_opt_set(fsdev, "socket", socket, &error_abort);