Re: [PATCH v3 4/6] util/osdep: Introduce qemu_close_range()

[Markus Armbruster <armbru@redhat.com>]

Bin Meng <bmeng@tinylab.org> writes:

> This introduces a new QEMU API qemu_close_range() that closes all
> open file descriptors from first to last (included).
>
> This API will try a more efficient call to close_range(), or walk
> through of /proc/self/fd whenever these are possible, otherwise it
> falls back to a plain close loop.
>
> Co-developed-by: Zhangjin Wu <falcon@tinylab.org>
> Signed-off-by: Bin Meng <bmeng@tinylab.org>
>
> ---
>
> Changes in v3:
> - fix win32 build failure
>
> Changes in v2:
> - new patch: "util/osdep: Introduce qemu_close_range()"
>
>  include/qemu/osdep.h |  1 +
>  util/osdep.c         | 48 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
>
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index cc61b00ba9..e22434ce10 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -560,6 +560,7 @@ int qemu_open_old(const char *name, int flags, ...);
>  int qemu_open(const char *name, int flags, Error **errp);
>  int qemu_create(const char *name, int flags, mode_t mode, Error **errp);
>  int qemu_close(int fd);
> +int qemu_close_range(unsigned int first, unsigned int last);
>  int qemu_unlink(const char *name);
>  #ifndef _WIN32
>  int qemu_dup_flags(int fd, int flags);
> diff --git a/util/osdep.c b/util/osdep.c
> index e996c4744a..91275e70f8 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -30,6 +30,7 @@
>  #include "qemu/mprotect.h"
>  #include "qemu/hw-version.h"
>  #include "monitor/monitor.h"
> +#include <dirent.h>
>  
>  static const char *hw_version = QEMU_HW_VERSION;
>  
> @@ -411,6 +412,53 @@ int qemu_close(int fd)
>      return close(fd);
>  }
>  
> +int qemu_close_range(unsigned int first, unsigned int last)
> +{
> +    DIR *dir = NULL;
> +
> +#ifdef CONFIG_CLOSE_RANGE
> +    int r = close_range(first, last, 0);

close_range(2) explains flag

       CLOSE_RANGE_UNSHARE
              Unshare  the specified file descriptors from any other processes
              before closing them, avoiding races with other  threads  sharing
              the file descriptor table.

Can anybody explain the races this avoids?

> +    if (!r) {
> +        /* Success, no need to try other ways. */
> +        return 0;
> +    }

What are the failure modes of close_range() where the other ways are
worth trying?

> +#endif
> +
> +#ifdef __linux__
> +    dir = opendir("/proc/self/fd");
> +#endif
> +    if (!dir) {
> +        /*
> +         * If /proc is not mounted or /proc/self/fd is not supported,
> +         * try close() from first to last.
> +         */
> +        for (int i = first; i <= last; i++) {
> +            close(i);
> +        }
> +
> +        return 0;
> +    }
> +
> +#ifndef _WIN32
> +    /* Avoid closing the directory */
> +    int dfd = dirfd(dir);

This directory contains "." "..", "0", "1", ...

> +
> +    for (struct dirent *de = readdir(dir); de; de = readdir(dir)) {
> +        int fd = atoi(de->d_name);

Maps "." and ".." to 0.  Unclean.

Please use qemu_strtoi(de->d_name, NULL, 10, &fd), and skip entries
where it fails.

> +        if (fd < first || fd > last) {
> +            /* Exclude the fds outside the target range */
> +            continue;
> +        }
> +        if (fd != dfd) {
> +            close(fd);
> +        }
> +    }
> +    closedir(dir);
> +#endif /* _WIN32 */
> +
> +    return 0;
> +}

I'd prefer to order this from most to least preferred:

    close_range()
    iterate over /proc/self/fd
    iterate from first to last

Unlike close_range(), qemu_close_range() returns 0 when last < first.
If we want to deviate from close_range(), we better document the
differences.

This is a generalized version of async-teardown.c's close_all_open_fd().
I'd mention this in the commit message.  Suggestion, not demand.

> +
>  /*
>   * Delete a file from the filesystem, unless the filename is /dev/fdset/...
>   *