Re: [RFC PATCH v1 4/5] selftests/filesystems: add fuse symlink caching test

[Amir Goldstein]

On Wed, Jul 8, 2026 at 3:11 PM Luis Henriques <luis@xxxxxxxxxx> wrote:
>
> This patch adds a simple test that allows to verify that, when resolving a
> symlink, user-space is called only the first time when caching is enabled
> or, if caching is disabled, every time the symlink resolution is requested.
>
> Signed-off-by: Luis Henriques <luis@xxxxxxxxxx>
> ---
>  .../selftests/filesystems/fuse/.gitignore     |   1 +
>  .../selftests/filesystems/fuse/Makefile       |   6 +-
>  .../selftests/filesystems/fuse/symlink_fs.c   | 115 ++++++++++++++++++
>  .../filesystems/fuse/symlink_test.sh          |  52 ++++++++
>  4 files changed, 173 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/filesystems/fuse/symlink_fs.c
>  create mode 100755 tools/testing/selftests/filesystems/fuse/symlink_test.sh
>
> diff --git a/tools/testing/selftests/filesystems/fuse/.gitignore b/tools/testing/selftests/filesystems/fuse/.gitignore
> index 3e72e742d08e..cfdc5ca3ded4 100644
> --- a/tools/testing/selftests/filesystems/fuse/.gitignore
> +++ b/tools/testing/selftests/filesystems/fuse/.gitignore
> @@ -1,3 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  fuse_mnt
>  fusectl_test
> +symlink_fs
> diff --git a/tools/testing/selftests/filesystems/fuse/Makefile b/tools/testing/selftests/filesystems/fuse/Makefile
> index 422cd1b1688d..342da0878006 100644
> --- a/tools/testing/selftests/filesystems/fuse/Makefile
> +++ b/tools/testing/selftests/filesystems/fuse/Makefile
> @@ -3,7 +3,8 @@
>  CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES)
>
>  TEST_GEN_PROGS := fusectl_test
> -TEST_GEN_FILES := fuse_mnt
> +TEST_PROGS := symlink_test.sh
> +TEST_GEN_FILES := fuse_mnt symlink_fs
>
>  include ../../lib.mk
>
> @@ -19,3 +20,6 @@ endif
>
>  $(OUTPUT)/fuse_mnt: CFLAGS += $(VAR_CFLAGS)
>  $(OUTPUT)/fuse_mnt: LDLIBS += $(VAR_LDLIBS)
> +
> +$(OUTPUT)/symlink_fs: CFLAGS += $(VAR_CFLAGS)
> +$(OUTPUT)/symlink_fs: LDLIBS += $(VAR_LDLIBS)
> diff --git a/tools/testing/selftests/filesystems/fuse/symlink_fs.c b/tools/testing/selftests/filesystems/fuse/symlink_fs.c
> new file mode 100644
> index 000000000000..1aee26c91b3f
> --- /dev/null
> +++ b/tools/testing/selftests/filesystems/fuse/symlink_fs.c
> @@ -0,0 +1,115 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Simple filesystem to test FUSE symlink cache
> + *
> + * This is a simple FUSE filesystem that contains two objects: a file named
> + * 'file' and a symlink to that file named 'link'.  Whenever the ->readlink() is
> + * executed to resolve 'link' a counter will be incremented.  A ->read() to any
> + * filesystem object will return the value in this counter.
> + *
> + * A '--cache' argument will allow to enable symlink caching (disabled by
> + * default).  This means that, if caching is enabled, resolving a symlink will
> + * only call into user-space the first time.
> + */
> +
> +#define FUSE_USE_VERSION 31
> +
> +#include <fuse.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <stddef.h>
> +#include <assert.h>
> +
> +#define FILE   "file"
> +#define LINK   "link"
> +
> +static struct options {
> +       int cache_symlinks;
> +} options;
> +
> +static const struct fuse_opt option_spec[] = {
> +       { "--cache", offsetof(struct options, cache_symlinks), 1 },
> +       FUSE_OPT_END
> +};
> +
> +static int readlink_counter = 0;
> +
> +static void *symlink_init(struct fuse_conn_info *conn, struct fuse_config *cfg)
> +{
> +       if (options.cache_symlinks)
> +               fuse_set_feature_flag(conn, FUSE_CAP_CACHE_SYMLINKS);
> +
> +       return NULL;
> +}
> +
> +static int symlink_getattr(const char *path, struct stat *stbuf,
> +                          struct fuse_file_info *fi)
> +{
> +       int res = 0;
> +
> +       memset(stbuf, 0, sizeof(struct stat));
> +       if (strcmp(path, "/") == 0) {
> +               stbuf->st_mode = S_IFDIR | 0755;
> +               stbuf->st_nlink = 2;
> +       } else if (strcmp(path + 1, FILE) == 0) {
> +               char data[64];
> +
> +               stbuf->st_mode = S_IFREG | 0444;
> +               stbuf->st_nlink = 1;
> +               stbuf->st_size = sprintf(data, "%d\n", readlink_counter);
> +       } else if (strcmp(path + 1, LINK) == 0) {
> +               stbuf->st_mode = S_IFLNK | 0444;
> +               stbuf->st_nlink = 1;
> +               stbuf->st_size = strlen(FILE);
> +       } else
> +               res = -ENOENT;
> +
> +       return res;
> +}
> +
> +static int symlink_readlink(const char *path, char *buf, size_t size)
> +{
> +       if (strcmp(path + 1, LINK) != 0)
> +               return -ENOENT;
> +
> +       memcpy(buf, FILE, strlen(FILE));
> +       readlink_counter++;
> +
> +       return 0;
> +}
> +
> +static int symlink_read(const char *path, char *buf, size_t sz, off_t off,
> +                       struct fuse_file_info *fi)
> +{
> +       char data[64];
> +       int len;
> +
> +       len = sprintf(data, "%d\n", readlink_counter);
> +       memcpy(buf, data, len);
> +
> +       return len;
> +}
> +
> +static const struct fuse_operations symlink_oper = {
> +       .init           = symlink_init,
> +       .getattr        = symlink_getattr,
> +       .readlink       = symlink_readlink,
> +       .read           = symlink_read,
> +};
> +
> +int main(int argc, char *argv[])
> +{
> +       int ret;
> +       struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
> +
> +       options.cache_symlinks = 0;
> +       if (fuse_opt_parse(&args, &options, option_spec, NULL) == -1)
> +               return 1;
> +
> +       ret = fuse_main(args.argc, args.argv, &symlink_oper, NULL);
> +       fuse_opt_free_args(&args);
> +
> +       return ret;
> +}
> diff --git a/tools/testing/selftests/filesystems/fuse/symlink_test.sh b/tools/testing/selftests/filesystems/fuse/symlink_test.sh
> new file mode 100755
> index 000000000000..546541c1920e
> --- /dev/null
> +++ b/tools/testing/selftests/filesystems/fuse/symlink_test.sh
> @@ -0,0 +1,52 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +exit_cleanup()
> +{
> +       fusermount -u ./mnt
> +       rmdir ./mnt
> +}
> +
> +assert_value ()
> +{
> +       if [ $1 -ne $2 ]; then
> +               echo "FAILED"
> +               echo $3
> +               exit 1
> +       fi
> +}
> +
> +set -e
> +
> +trap exit_cleanup EXIT
> +
> +mkdir -p mnt
> +
> +./symlink_fs ./mnt
> +
> +echo -n "Testing symlink without cached: "
> +
> +# When symlink caching is disabled every access to a symlink is expected to
> +# result in a call to user-space
> +for i in $(seq 1 10); do
> +       readlink ./mnt/link > /dev/null
> +done
> +
> +res=$(cat ./mnt/file)
> +assert_value $res $i "Got $res, expected $i"
> +echo "PASSED"
> +
> +fusermount -u ./mnt
> +
> +./symlink_fs --cache ./mnt
> +
> +echo -n "Testing symlink with cache: "
> +
> +# With caching enabled, there will only be a single call into user-space
> +for i in $(seq 0 10); do
> +       readlink ./mnt/link > /dev/null
> +done
> +
> +res=$(cat ./mnt/file)
> +assert_value 1 $res "Got $ res, expected 1"
> +echo "PASSED"
>

As we discussed, consider doing this as a single c test program,
but not a must as you wish.

Thanks,
Amir.