Re: [PATCH iproute2-next v3] lib/fs: fix issue when {name,open}_to_handle_at() is not implemented

[David Ahern <dsahern@gmail.com>]

On 5/8/21 12:49 AM, Heiko Thiery wrote:
> With commit d5e6ee0dac64 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.
> 
> Fixes: d5e6ee0dac64 ("ss: introduce cgroup2 cache and helper functions")
> Cc: Dmitry Yakunin <zeil@yandex-team.ru>
> Cc: Petr Vorel <petr.vorel@gmail.com>
> Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
> ---
> v3:
>  - use correct syscall number (thanks to Petr Vorel)
>  - add #include <sys/syscall.h> (thanks to Petr Vorel)
>  - remove bogus parameters (thanks to Petr Vorel)
>  - fix #ifdef (thanks to Petr Vorel)
>  - added Fixes tag (thanks to David Ahern)
>  - build test with buildroot 2020.08.3 using uclibc 1.0.34
> 
> v2:
>  - small correction to subject
>  - removed IP_CONFIG_HANDLE_AT:=y option since it is not required
>  - fix indentation in check function
>  - removed empty lines (thanks to Petr Vorel)
>  - add #define _GNU_SOURCE in check (thanks to Petr Vorel)
>  - check only for name_to_handle_at (thanks to Petr Vorel)
> 
>  configure | 28 ++++++++++++++++++++++++++++
>  lib/fs.c  | 25 +++++++++++++++++++++++++
>  2 files changed, 53 insertions(+)
> 
> diff --git a/configure b/configure
> index 2c363d3b..179eae08 100755
> --- a/configure
> +++ b/configure
> @@ -202,6 +202,31 @@ EOF
>      rm -f $TMPDIR/setnstest.c $TMPDIR/setnstest
>  }
>  
> +check_name_to_handle_at()
> +{
> +    cat >$TMPDIR/name_to_handle_at_test.c <<EOF
> +#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 $TMPDIR/name_to_handle_at_test.c >/dev/null 2>&1; then
> +        echo "yes"
> +        echo "CFLAGS += -DHAVE_HANDLE_AT" >>$CONFIG
> +    else
> +        echo "no"
> +    fi
> +    rm -f $TMPDIR/name_to_handle_at_test.c $TMPDIR/name_to_handle_at_test
> +}
> +
>  check_ipset()
>  {
>      cat >$TMPDIR/ipsettest.c <<EOF
> @@ -492,6 +517,9 @@ fi
>  echo -n "libc has setns: "
>  check_setns
>  
> +echo -n "libc has name_to_handle_at: "
> +check_name_to_handle_at
> +
>  echo -n "SELinux support: "
>  check_selinux
>  
> diff --git a/lib/fs.c b/lib/fs.c
> index f161d888..05697a7e 100644
> --- a/lib/fs.c
> +++ b/lib/fs.c
> @@ -25,11 +25,36 @@
>  
>  #include "utils.h"
>  
> +#ifndef HAVE_HANDLE_AT
> +# include <sys/syscall.h>
> +#endif
> +
>  #define CGROUP2_FS_NAME "cgroup2"
>  
>  /* if not already mounted cgroup2 is mounted here for iproute2's use */
>  #define MNT_CGRP2_PATH  "/var/run/cgroup2"
>  
> +
> +#ifndef HAVE_HANDLE_AT
> +struct file_handle {
> +	unsigned handle_bytes;
> +	int handle_type;
> +	unsigned char f_handle[];
> +};
> +
> +static int name_to_handle_at(int dirfd, const char *pathname,
> +	struct file_handle *handle, int *mount_id, int flags)
> +{
> +	return syscall(__NR_name_to_handle_at, dirfd, pathname, handle,
> +	               mount_id, flags);
> +}
> +
> +static int open_by_handle_at(int mount_fd, struct file_handle *handle, int flags)
> +{
> +	return syscall(__NR_open_by_handle_at, mount_fd, handle, flags);
> +}
> +#endif
> +
>  /* return mount path of first occurrence of given fstype */
>  static char *find_fs_mount(const char *fs_to_find)
>  {
> 

This causes compile failures if anyone is reusing a tree. It would be
good to require config.mk to be updated if configure is newer.