| Message ID | 20210730150134.216126-3-mreitz@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | virtiofsd: Allow using file handles instead of O_PATH FDs | expand |
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 <mreitz@redhat.com> > Reviewed-by: Connor Kuehl <ckuehl@redhat.com> > --- > 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
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 <mreitz@redhat.com> >> Reviewed-by: Connor Kuehl <ckuehl@redhat.com> >> --- >> 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 >
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.