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SubjectRe: [PATCH] nextfd(2)
Alexey Dobriyan wrote:
> Currently there is no reliable way to close all opened file descriptors
> (which daemons need and like to do):
>
> * dumb close(fd) loop is slow, upper bound is unknown and
> can be arbitrary large,
>
> * /proc/self/fd is unreliable:
> proc may be unconfigured or not mounted at expected place.
> Looking at /proc/self/fd requires opening directory
> which may not be available due to malicious rlimit drop or ENOMEM situations.
> Not opening directory is equivalent to dumb close(2) loop except slower.
>
> BSD added closefrom(fd) which is OK for this exact purpose but suboptimal
> on the bigger scale. closefrom(2) does only close(2) (obviously :-)
> closefrom(2) siletly ignores errors from close(2) which in theory is not OK
> for userspace.
>
> So, don't add closefrom(2), add nextfd(2).
>
> int nextfd(int fd)

Can we add "pid" argument to be able to search next fd in other task?
Together with sys_kcmp() this will be very useful for checkpoint/restore.

>
> returns next opened file descriptor which is>= than fd or -1/ESRCH
> if there aren't any descriptors>= than fd.
>
> Thus closefrom(3) can be rewritten through it in userspace:
>
> void closefrom(int fd)
> {
> while (1) {
> fd = nextfd(fd);
> if (fd == -1&& errno == ESRCH)
> break;
> (void)close(fd);
> fd++;
> }
> }
>
> Maybe it will grow other smart uses.
>
> nextfd(2) doesn't change kernel state and thus can't fail
> which is why it should go in. Other means may fail or
> may not be available or require linear time with only guessed
> upper boundaries (1024, getrlimit(RLIM_NOFILE), sysconf(_SC_OPEN_MAX).
>
> Signed-off-by: Alexey Dobriyan<adobriyan@gmail.com>
> ---
>
> arch/x86/syscalls/syscall_32.tbl | 1 +
> arch/x86/syscalls/syscall_64.tbl | 1 +
> fs/Makefile | 1 +
> fs/nextfd.c | 27 +++++++++++++++++++++++++++
> include/linux/syscalls.h | 1 +
> 5 files changed, 31 insertions(+)
>
> --- a/arch/x86/syscalls/syscall_32.tbl
> +++ b/arch/x86/syscalls/syscall_32.tbl
> @@ -355,3 +355,4 @@
> 346 i386 setns sys_setns
> 347 i386 process_vm_readv sys_process_vm_readv compat_sys_process_vm_readv
> 348 i386 process_vm_writev sys_process_vm_writev compat_sys_process_vm_writev
> +349 i386 nextfd sys_nextfd
> --- a/arch/x86/syscalls/syscall_64.tbl
> +++ b/arch/x86/syscalls/syscall_64.tbl
> @@ -318,6 +318,7 @@
> 309 common getcpu sys_getcpu
> 310 64 process_vm_readv sys_process_vm_readv
> 311 64 process_vm_writev sys_process_vm_writev
> +312 64 nextfd sys_nextfd
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> # for native 64-bit operation.
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -12,6 +12,7 @@ obj-y := open.o read_write.o file_table.o super.o \
> seq_file.o xattr.o libfs.o fs-writeback.o \
> pnode.o drop_caches.o splice.o sync.o utimes.o \
> stack.o fs_struct.o statfs.o
> +obj-y += nextfd.o
>
> ifeq ($(CONFIG_BLOCK),y)
> obj-y += buffer.o bio.o block_dev.o direct-io.o mpage.o ioprio.o
> --- /dev/null
> +++ b/fs/nextfd.c
> @@ -0,0 +1,27 @@
> +#include<linux/errno.h>
> +#include<linux/fdtable.h>
> +#include<linux/rcupdate.h>
> +#include<linux/sched.h>
> +#include<linux/syscalls.h>
> +
> +/* Return first opened file descriptor which is>= than the argument. */
> +SYSCALL_DEFINE1(nextfd, unsigned int, fd)
> +{
> + struct files_struct *files = current->files;
> + struct fdtable *fdt;
> +
> + rcu_read_lock();
> + fdt = files_fdtable(files);
> + while (fd< fdt->max_fds) {
> + struct file *file;
> +
> + file = rcu_dereference_check_fdtable(files, fdt->fd[fd]);
> + if (file) {
> + rcu_read_unlock();
> + return fd;
> + }
> + fd++;
> + }
> + rcu_read_unlock();
> + return -ESRCH;
> +}
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -857,5 +857,6 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
> const struct iovec __user *rvec,
> unsigned long riovcnt,
> unsigned long flags);
> +asmlinkage long sys_nextfd(unsigned int fd);
>
> #endif
> --
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