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

[Luis Henriques]

Amir Goldstein <amir73il@xxxxxxxxx> writes:

> 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.

Sure, next iteration will have this test as a stand-alone c program.

Cheers,
-- 
Luís