Re: [Qemu-trivial] [PATCH] 9pfs-local: simplify/optimize local_mapped_attr_path()

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Michael Tokarev <mjt@tls.msk.ru> writes:

> Omit one unnecessary memory allocation for components of the path
> and create the resulting path directly given lengths of the components.
>
> This uses (char*) cast because basename() accepts a char* without const,
> for unknown reason.  Maybe it is better to use strrchr(), but I'm not
> sure for various forms of directory component delimiter.

basename(3) says:
Both dirname() and basename() may modify the contents of path, so it may
be desirable to pass a copy when calling one of these functions


>
> And according to basename(3) manpage, it might return different strings
> for various corner cases, for example for empty argument it returns a
> string "." which is obviously at some other address, so both the old
> code and new code will do a bad thing here.  So maybe it is actually
> better to use strrchr() after all.
>
> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> ---
>  hw/9pfs/virtio-9p-local.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/hw/9pfs/virtio-9p-local.c b/hw/9pfs/virtio-9p-local.c
> index d05c917..fddc242 100644
> --- a/hw/9pfs/virtio-9p-local.c
> +++ b/hw/9pfs/virtio-9p-local.c
> @@ -45,19 +45,10 @@
>  
>  static char *local_mapped_attr_path(FsContext *ctx, const char *path)
>  {
> -    char *dir_name;
> -    char *tmp_path = g_strdup(path);
> -    char *base_name = basename(tmp_path);
> -    char *buffer;
> -
> -    /* NULL terminate the directory */
> -    dir_name = tmp_path;
> -    *(base_name - 1) = '\0';
> -
> -    buffer = g_strdup_printf("%s/%s/%s/%s",
> -             ctx->fs_root, dir_name, VIRTFS_META_DIR, base_name);
> -    g_free(tmp_path);
> -    return buffer;
> +    const char *name = basename((char*)path);

as per the man page basename could end up modifying 'path' and we don't
want to modify the input argument of the function.


> +    int dirlen = name - path - 1;
> +    return g_strdup_printf("%s/%.*s/%s/%s", ctx->fs_root,
> +                           dirlen, path, VIRTFS_META_DIR, name);
>  }
>  
>  static FILE *local_fopen(const char *path, const char *mode)

I am not sure whether we really need all these cleanups without really
fixing anyi specific issue.

-aneesh