Subject: Re: [PATCH] lib/fs: fix issue when {name, open}_to_handle_at() is not implemented

From: Petr Vorel <[email-masked]>
Date: Thu, 29 Apr 2021 21:19:24 +0200

Hi,

> With commit d5e6ee0dac64b64e the usage of functions name_to_handle_at() and
> open_by_handle_at() are introduced. But these function are not available
> e.g. in uclibc-ng < 1.0.35. To have a backward compatibility check for the
> availability in the configure script and in case of absence do a direct
> syscall.

LGTM, thanks for implementing it.
I'd consider to check only one of the functions (very unlikely only one will be
supported).

> Signed-off-by: Heiko Thiery <[email-masked]>
> ---
>  configure | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  lib/fs.c  | 25 ++++++++++++++++++++++++
>  2 files changed, 83 insertions(+)

> diff --git a/configure b/configure
> index 2c363d3b..f1be9977 100755
> --- a/configure
> +++ b/configure
> @@ -202,6 +202,58 @@ EOF
>      rm -f $TMPDIR/setnstest.c $TMPDIR/setnstest
>  }

> +check_name_to_handle_at()
> +{
> +
nit: empty line.
> +    cat >$TMPDIR/name_to_handle_at_test.c <<EOF
It's interesting it does not require _GNU_SOURCE ? (IMHO this is normally
required for all glibc, musl, uclibc and binder):

#define _GNU_SOURCE
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <fcntl.h>
> +int main(int argc, char **argv)
> +{
> +	struct file_handle *fhp;
> +	int mount_id, flags, dirfd;
> +	char *pathname;
> +	name_to_handle_at(dirfd, pathname, fhp, &mount_id, flags);
> +	return 0;
> +}
> +EOF
> +
> +    if $CC -I$INCLUDE -o $TMPDIR/name_to_handle_at_test $TMPDIR/name_to_handle_at_test.c >/dev/null 2>&1; then
> +	echo "IP_CONFIG_NAME_TO_HANDLE_AT:=y" >>$CONFIG
> +	echo "yes"
> +	echo "CFLAGS += -DHAVE_NAME_TO_HANDLE_AT" >>$CONFIG
> +    else
> +	echo "no"
> +    fi
> +    rm -f $TMPDIR/name_to_handle_at_test.c $TMPDIR/name_to_handle_at_test
> +}
> +
> +check_open_by_handle_at()
> +{
> +
nit: empty line.
> +    cat >$TMPDIR/open_by_handle_at_test.c <<EOF
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <fcntl.h>
> +int main(int argc, char **argv)
> +{
> +	struct file_handle *fhp;
> +	int mount_fd;
> +	open_by_handle_at(mount_fd, fhp, O_RDONLY);
> +	return 0;
> +}
> +EOF
> +    if $CC -I$INCLUDE -o $TMPDIR/open_by_handle_at_test $TMPDIR/open_by_handle_at_test.c >/dev/null 2>&1; then
> +	echo "IP_CONFIG_OPEN_BY_HANDLE_AT:=y" >>$CONFIG
> +	echo "yes"
> +	echo "CFLAGS += -DHAVE_OPEN_BY_HANDLE_AT" >>$CONFIG
> +    else
> +	echo "no"
> +    fi
> +    rm -f $TMPDIR/open_by_handle_at_test.c $TMPDIR/open_by_handle_at_test
> +}
> +
>  check_ipset()
>  {
>      cat >$TMPDIR/ipsettest.c <<EOF
> @@ -492,6 +544,12 @@ fi
>  echo -n "libc has setns: "
>  check_setns

> +echo -n "libc has name_to_handle_at: "
> +check_name_to_handle_at
> +
> +echo -n "libc has open_by_handle_at: "
> +check_open_by_handle_at
> +
>  echo -n "SELinux support: "
>  check_selinux

> diff --git a/lib/fs.c b/lib/fs.c
> index ee0b130b..c561d85b 100644
> --- a/lib/fs.c
> +++ b/lib/fs.c
> @@ -30,6 +30,31 @@
>  /* if not already mounted cgroup2 is mounted here for iproute2's use */
>  #define MNT_CGRP2_PATH  "/var/run/cgroup2"

> +
> +#if (!defined HAVE_NAME_TO_HANDLE_AT && !defined HAVE_OPEN_BY_HANDLE_AT)
> +struct file_handle {
> +	unsigned handle_bytes;
> +	int handle_type;
> +	unsigned char f_handle[];
> +};
> +#endif
> +
> +#ifndef HAVE_NAME_TO_HANDLE_AT
> +int name_to_handle_at(int dirfd, const char *pathname,
> +	struct file_handle *handle, int *mount_id, int flags)
> +{
> +	return syscall(name_to_handle_at, 5, dirfd, pathname, handle,
> +	               mount_id, flags);
> +}
> +#endif
> +
> +#ifndef HAVE_OPEN_BY_HANDLE_AT
> +int open_by_handle_at(int mount_fd, struct file_handle *handle, int flags)
> +{
> +	return syscall(open_by_handle_at, 3, mount_fd, handle, flags);
> +}
> +#endif
> +
>  /* return mount path of first occurrence of given fstype */
>  static char *find_fs_mount(const char *fs_to_find)
>  {