Re: [PATCH v3 02/10] virtiofsd: Add TempFd structure
On 06.08.21 16:41, Vivek Goyal wrote:
On Fri, Jul 30, 2021 at 05:01:26PM +0200, Max Reitz wrote:
We are planning to add file handles to lo_inode objects as an
alternative to lo_inode.fd. That means that everywhere where we
currently reference lo_inode.fd, we will have to open a temporary file
descriptor that needs to be closed after use.
So instead of directly accessing lo_inode.fd, there will be a helper
function (lo_inode_fd()) that either returns lo_inode.fd, or opens a new
file descriptor with open_by_handle_at(). It encapsulates this result
in a TempFd structure to let the caller know whether the FD needs to be
closed after use (opened from the handle) or not (copied from
lo_inode.fd).
I am wondering why this notion of "owned". Why not have this requirement
of always closing "fd". If we copied it from lo_inode.fd, then we will
need to dup() it. Otherwise we opened it from file handle and we will
need to close it anyway.
I guess you are trying to avoid having to call dup() and that's why
this notion of "owned" fd.
Yes, I don’t want to dup() it. One reason is that I’d rather just not.
It’s something that we can avoid, and dup-ing every time wouldn’t make
the code that much simpler (I think, without having tried).
One other is because this affects the current behavior (with O_PATH
FDs), which I don’t want to alter.
Well, and finally, as a pragmatic reason, virtiofsd-rs uses the same
structure and I don’t really want C virtiofsd and virtiofsd-rs to differ
too much.
By using g_auto(TempFd) to store this result, callers will not even have
to care about closing a temporary FD after use. It will be done
automatically once the object goes out of scope.
Signed-off-by: Max Reitz
Reviewed-by: Connor Kuehl
---
tools/virtiofsd/passthrough_ll.c | 49
1 file changed, 49 insertions(+)
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 1f27eeabc5..fb5e073e6a 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -178,6 +178,28 @@ struct lo_data {
int user_posix_acl, posix_acl;
};
+/**
+ * Represents a file descriptor that may either be owned by this
+ * TempFd, or only referenced (i.e. the ownership belongs to some
+ * other object, and the value has just been copied into this TempFd).
+ *
+ * The purpose of this encapsulation is to be used as g_auto(TempFd)
+ * to automatically clean up owned file descriptors when this object
+ * goes out of scope.
+ *
+ * Use temp_fd_steal() to get an owned file descriptor that will not
+ * be closed when the TempFd goes out of scope.
+ */
+typedef struct {
+int fd;
+bool owned; /* fd owned by this object? */
+} TempFd;
+
+#define TEMP_FD_INIT ((TempFd) { .fd = -1, .owned = false })
+
+static void temp_fd_clear(TempFd *temp_fd);
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(TempFd, temp_fd_clear);
+
static const struct fuse_opt lo_opts[] = {
{ "sandbox=namespace",
offsetof(struct lo_data, sandbox),
@@ -255,6 +277,33 @@ static struct lo_data *lo_data(fuse_req_t req)
return (struct lo_data *)fuse_req_userdata(req);
}
+/**
+ * Clean-up function for TempFds
+ */
+static void temp_fd_clear(TempFd *temp_fd)
+{
+if (temp_fd->owned) {
+close(temp_fd->fd);
+*temp_fd = TEMP_FD_INIT;
+}
+}
+
+/**
+ * Return an owned fd from *temp_fd that will not be closed when
+ * *temp_fd goes out of scope.
+ *
+ * (TODO: Remove __attribute__ once this is used.)
+ */
+static __attribute__((unused)) int temp_fd_steal(TempFd *temp_fd)
+{
+if (temp_fd->owned) {
+temp_fd->owned = false;
+return temp_fd->fd;
+} else {
+return dup(temp_fd->fd);
+}
+}
This also will be simpler if we always called dup() and every caller
will close() fd.
I think only downside is having to call dup()/close(). Not sure if this
is an expensive operation or not.
Vivek
Re: [PATCH v3 02/10] virtiofsd: Add TempFd structure
On Fri, Jul 30, 2021 at 05:01:26PM +0200, Max Reitz wrote:
> We are planning to add file handles to lo_inode objects as an
> alternative to lo_inode.fd. That means that everywhere where we
> currently reference lo_inode.fd, we will have to open a temporary file
> descriptor that needs to be closed after use.
>
> So instead of directly accessing lo_inode.fd, there will be a helper
> function (lo_inode_fd()) that either returns lo_inode.fd, or opens a new
> file descriptor with open_by_handle_at(). It encapsulates this result
> in a TempFd structure to let the caller know whether the FD needs to be
> closed after use (opened from the handle) or not (copied from
> lo_inode.fd).
I am wondering why this notion of "owned". Why not have this requirement
of always closing "fd". If we copied it from lo_inode.fd, then we will
need to dup() it. Otherwise we opened it from file handle and we will
need to close it anyway.
I guess you are trying to avoid having to call dup() and that's why
this notion of "owned" fd.
>
> By using g_auto(TempFd) to store this result, callers will not even have
> to care about closing a temporary FD after use. It will be done
> automatically once the object goes out of scope.
>
> Signed-off-by: Max Reitz
> Reviewed-by: Connor Kuehl
> ---
> tools/virtiofsd/passthrough_ll.c | 49
> 1 file changed, 49 insertions(+)
>
> diff --git a/tools/virtiofsd/passthrough_ll.c
> b/tools/virtiofsd/passthrough_ll.c
> index 1f27eeabc5..fb5e073e6a 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -178,6 +178,28 @@ struct lo_data {
> int user_posix_acl, posix_acl;
> };
>
> +/**
> + * Represents a file descriptor that may either be owned by this
> + * TempFd, or only referenced (i.e. the ownership belongs to some
> + * other object, and the value has just been copied into this TempFd).
> + *
> + * The purpose of this encapsulation is to be used as g_auto(TempFd)
> + * to automatically clean up owned file descriptors when this object
> + * goes out of scope.
> + *
> + * Use temp_fd_steal() to get an owned file descriptor that will not
> + * be closed when the TempFd goes out of scope.
> + */
> +typedef struct {
> +int fd;
> +bool owned; /* fd owned by this object? */
> +} TempFd;
> +
> +#define TEMP_FD_INIT ((TempFd) { .fd = -1, .owned = false })
> +
> +static void temp_fd_clear(TempFd *temp_fd);
> +G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(TempFd, temp_fd_clear);
> +
> static const struct fuse_opt lo_opts[] = {
> { "sandbox=namespace",
>offsetof(struct lo_data, sandbox),
> @@ -255,6 +277,33 @@ static struct lo_data *lo_data(fuse_req_t req)
> return (struct lo_data *)fuse_req_userdata(req);
> }
>
> +/**
> + * Clean-up function for TempFds
> + */
> +static void temp_fd_clear(TempFd *temp_fd)
> +{
> +if (temp_fd->owned) {
> +close(temp_fd->fd);
> +*temp_fd = TEMP_FD_INIT;
> +}
> +}
> +
> +/**
> + * Return an owned fd from *temp_fd that will not be closed when
> + * *temp_fd goes out of scope.
> + *
> + * (TODO: Remove __attribute__ once this is used.)
> + */
> +static __attribute__((unused)) int temp_fd_steal(TempFd *temp_fd)
> +{
> +if (temp_fd->owned) {
> +temp_fd->owned = false;
> +return temp_fd->fd;
> +} else {
> +return dup(temp_fd->fd);
> +}
> +}
This also will be simpler if we always called dup() and every caller
will close() fd.
I think only downside is having to call dup()/close(). Not sure if this
is an expensive operation or not.
Vivek
[PATCH v3 02/10] virtiofsd: Add TempFd structure
We are planning to add file handles to lo_inode objects as an
alternative to lo_inode.fd. That means that everywhere where we
currently reference lo_inode.fd, we will have to open a temporary file
descriptor that needs to be closed after use.
So instead of directly accessing lo_inode.fd, there will be a helper
function (lo_inode_fd()) that either returns lo_inode.fd, or opens a new
file descriptor with open_by_handle_at(). It encapsulates this result
in a TempFd structure to let the caller know whether the FD needs to be
closed after use (opened from the handle) or not (copied from
lo_inode.fd).
By using g_auto(TempFd) to store this result, callers will not even have
to care about closing a temporary FD after use. It will be done
automatically once the object goes out of scope.
Signed-off-by: Max Reitz
Reviewed-by: Connor Kuehl
---
tools/virtiofsd/passthrough_ll.c | 49
1 file changed, 49 insertions(+)
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 1f27eeabc5..fb5e073e6a 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -178,6 +178,28 @@ struct lo_data {
int user_posix_acl, posix_acl;
};
+/**
+ * Represents a file descriptor that may either be owned by this
+ * TempFd, or only referenced (i.e. the ownership belongs to some
+ * other object, and the value has just been copied into this TempFd).
+ *
+ * The purpose of this encapsulation is to be used as g_auto(TempFd)
+ * to automatically clean up owned file descriptors when this object
+ * goes out of scope.
+ *
+ * Use temp_fd_steal() to get an owned file descriptor that will not
+ * be closed when the TempFd goes out of scope.
+ */
+typedef struct {
+int fd;
+bool owned; /* fd owned by this object? */
+} TempFd;
+
+#define TEMP_FD_INIT ((TempFd) { .fd = -1, .owned = false })
+
+static void temp_fd_clear(TempFd *temp_fd);
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(TempFd, temp_fd_clear);
+
static const struct fuse_opt lo_opts[] = {
{ "sandbox=namespace",
offsetof(struct lo_data, sandbox),
@@ -255,6 +277,33 @@ static struct lo_data *lo_data(fuse_req_t req)
return (struct lo_data *)fuse_req_userdata(req);
}
+/**
+ * Clean-up function for TempFds
+ */
+static void temp_fd_clear(TempFd *temp_fd)
+{
+if (temp_fd->owned) {
+close(temp_fd->fd);
+*temp_fd = TEMP_FD_INIT;
+}
+}
+
+/**
+ * Return an owned fd from *temp_fd that will not be closed when
+ * *temp_fd goes out of scope.
+ *
+ * (TODO: Remove __attribute__ once this is used.)
+ */
+static __attribute__((unused)) int temp_fd_steal(TempFd *temp_fd)
+{
+if (temp_fd->owned) {
+temp_fd->owned = false;
+return temp_fd->fd;
+} else {
+return dup(temp_fd->fd);
+}
+}
+
/*
* Load capng's state from our saved state if the current thread
* hadn't previously been loaded.
--
2.31.1
