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SubjectRe: [PATCH 1/2] fs, close_range: add flag CLOSE_RANGE_CLOEXEC
On Tue, Oct 13, 2020 at 04:06:08PM +0200, Giuseppe Scrivano wrote:

Hey Guiseppe,

Thanks for the patch!

> When the flag CLOSE_RANGE_CLOEXEC is set, close_range doesn't
> immediately close the files but it sets the close-on-exec bit.

Hm, please expand on the use-cases a little here so people know where
and how this is useful. Keeping the rationale for a change in the commit
log is really important.

>
> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
> ---

> fs/file.c | 56 ++++++++++++++++++++++----------
> include/uapi/linux/close_range.h | 3 ++
> 2 files changed, 42 insertions(+), 17 deletions(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index 21c0893f2f1d..ad4ebee41e09 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -672,6 +672,17 @@ int __close_fd(struct files_struct *files, unsigned fd)
> }
> EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
>
> +static unsigned int __get_max_fds(struct files_struct *cur_fds)
> +{
> + unsigned int max_fds;
> +
> + rcu_read_lock();
> + /* cap to last valid index into fdtable */
> + max_fds = files_fdtable(cur_fds)->max_fds;
> + rcu_read_unlock();
> + return max_fds;
> +}
> +
> /**
> * __close_range() - Close all file descriptors in a given range.
> *
> @@ -683,27 +694,23 @@ EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
> */
> int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
> {
> - unsigned int cur_max;
> + unsigned int cur_max = UINT_MAX;
> struct task_struct *me = current;
> struct files_struct *cur_fds = me->files, *fds = NULL;
>
> - if (flags & ~CLOSE_RANGE_UNSHARE)
> + if (flags & ~(CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC))
> return -EINVAL;
>
> if (fd > max_fd)
> return -EINVAL;
>
> - rcu_read_lock();
> - cur_max = files_fdtable(cur_fds)->max_fds;
> - rcu_read_unlock();
> -
> - /* cap to last valid index into fdtable */
> - cur_max--;
> -
> if (flags & CLOSE_RANGE_UNSHARE) {
> int ret;
> unsigned int max_unshare_fds = NR_OPEN_MAX;
>
> + /* cap to last valid index into fdtable */
> + cur_max = __get_max_fds(cur_fds) - 1;
> +
> /*
> * If the requested range is greater than the current maximum,
> * we're closing everything so only copy all file descriptors
> @@ -724,16 +731,31 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
> swap(cur_fds, fds);
> }
>
> - max_fd = min(max_fd, cur_max);
> - while (fd <= max_fd) {
> - struct file *file;
> + if (flags & CLOSE_RANGE_CLOEXEC) {
> + struct fdtable *fdt;
>
> - file = pick_file(cur_fds, fd++);
> - if (!file)
> - continue;
> + spin_lock(&cur_fds->file_lock);
> + fdt = files_fdtable(cur_fds);
> + cur_max = fdt->max_fds - 1;
> + max_fd = min(max_fd, cur_max);
> + while (fd <= max_fd)
> + __set_close_on_exec(fd++, fdt);
> + spin_unlock(&cur_fds->file_lock);
> + } else {
> + /* Initialize cur_max if needed. */
> + if (cur_max == UINT_MAX)
> + cur_max = __get_max_fds(cur_fds) - 1;

The separation between how cur_fd is retrieved in the two branches makes
the code more difficult to follow imho. Unless there's a clear reason
why you've done it that way I would think that something like the patch
I appended below might be a little clearer and easier to maintain(?).

> + max_fd = min(max_fd, cur_max);
> + while (fd <= max_fd) {
> + struct file *file;
>
> - filp_close(file, cur_fds);
> - cond_resched();
> + file = pick_file(cur_fds, fd++);
> + if (!file)
> + continue;
> +
> + filp_close(file, cur_fds);
> + cond_resched();
> + }
> }

I think I don't have quarrels with this patch in principle but I wonder
if something like the following wouldn't be easier to follow:

diff --git a/fs/file.c b/fs/file.c
index 21c0893f2f1d..872a4098c3be 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -672,6 +672,32 @@ int __close_fd(struct files_struct *files, unsigned fd)
}
EXPORT_SYMBOL(__close_fd); /* for ksys_close() */

+static inline void __range_cloexec(struct files_struct *cur_fds,
+ unsigned int fd, unsigned max_fd)
+{
+ struct fdtable *fdt;
+ spin_lock(&cur_fds->file_lock);
+ fdt = files_fdtable(cur_fds);
+ while (fd <= max_fd)
+ __set_close_on_exec(fd++, fdt);
+ spin_unlock(&cur_fds->file_lock);
+}
+
+static inline void __range_close(struct files_struct *cur_fds, unsigned int fd,
+ unsigned max_fd)
+{
+ while (fd <= max_fd) {
+ struct file *file;
+
+ file = pick_file(cur_fds, fd++);
+ if (!file)
+ continue;
+
+ filp_close(file, cur_fds);
+ cond_resched();
+ }
+}
+
/**
* __close_range() - Close all file descriptors in a given range.
*
@@ -687,7 +713,7 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
struct task_struct *me = current;
struct files_struct *cur_fds = me->files, *fds = NULL;

- if (flags & ~CLOSE_RANGE_UNSHARE)
+ if (flags & ~(CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC))
return -EINVAL;

if (fd > max_fd)
@@ -725,16 +751,10 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
}

max_fd = min(max_fd, cur_max);
- while (fd <= max_fd) {
- struct file *file;
-
- file = pick_file(cur_fds, fd++);
- if (!file)
- continue;
-
- filp_close(file, cur_fds);
- cond_resched();
- }
+ if (flags & CLOSE_RANGE_CLOEXEC)
+ __range_cloexec(cur_fds, fd, max_fd);
+ else
+ __range_close(cur_fds, fd, max_fd);

if (fds) {
/*
diff --git a/include/uapi/linux/close_range.h b/include/uapi/linux/close_range.h
index 6928a9fdee3c..2d804281554c 100644
--- a/include/uapi/linux/close_range.h
+++ b/include/uapi/linux/close_range.h
@@ -5,5 +5,8 @@
/* Unshare the file descriptor table before closing file descriptors. */
#define CLOSE_RANGE_UNSHARE (1U << 1)

+/* Set the FD_CLOEXEC bit instead of closing the file descriptor. */
+#define CLOSE_RANGE_CLOEXEC (1U << 2)
+
#endif /* _UAPI_LINUX_CLOSE_RANGE_H */

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