Re: [RFC PATCH bpf-next 03/16] bpf: provide a way for targets to register themselves

[Yonghong Song <yhs@fb.com>]

On 4/10/20 3:18 PM, Andrii Nakryiko wrote:
> On Wed, Apr 8, 2020 at 4:26 PM Yonghong Song <yhs@fb.com> wrote:
>>
>> Here, the target refers to a particular data structure
>> inside the kernel we want to dump. For example, it
>> can be all task_structs in the current pid namespace,
>> or it could be all open files for all task_structs
>> in the current pid namespace.
>>
>> Each target is identified with the following information:
>>     target_rel_path   <=== relative path to /sys/kernel/bpfdump
>>     target_proto      <=== kernel func proto which represents
>>                            bpf program signature for this target
>>     seq_ops           <=== seq_ops for seq_file operations
>>     seq_priv_size     <=== seq_file private data size
>>     target_feature    <=== target specific feature which needs
>>                            handling outside seq_ops.
> 
> It's not clear what "feature" stands for here... Is this just a sort
> of private_data passed through to dumper?

This is described later. It is some kind of target passed to the dumper.

> 
>>
>> The target relative path is a relative directory to /sys/kernel/bpfdump/.
>> For example, it could be:
>>     task                  <=== all tasks
>>     task/file             <=== all open files under all tasks
>>     ipv6_route            <=== all ipv6_routes
>>     tcp6/sk_local_storage <=== all tcp6 socket local storages
>>     foo/bar/tar           <=== all tar's in bar in foo
> 
> ^^ this seems useful, but I don't think code as is supports more than 2 levels?

Currently implement should support it.
You need
  - first register 'foo'. target name 'foo'.
  - then register 'foo/bar'. 'foo' will be the parent of 'bar'. target 
name 'foo/bar'.
  - then 'foo/bar/tar'. 'foo/bar' will be the parent of 'tar'. target 
name 'foo/bar/tar'.

> 
>>
>> The "target_feature" is mostly used for reusing existing seq_ops.
>> For example, for /proc/net/<> stats, the "net" namespace is often
>> stored in file private data. The target_feature enables bpf based
>> dumper to set "net" properly for itself before calling shared
>> seq_ops.
>>
>> bpf_dump_reg_target() is implemented so targets
>> can register themselves. Currently, module is not
>> supported, so there is no bpf_dump_unreg_target().
>> The main reason is that BTF is not available for modules
>> yet.
>>
>> Since target might call bpf_dump_reg_target() before
>> bpfdump mount point is created, __bpfdump_init()
>> may be called in bpf_dump_reg_target() as well.
>>
>> The file-based dumpers will be regular files under
>> the specific target directory. For example,
>>     task/my1      <=== dumper "my1" iterates through all tasks
>>     task/file/my2 <=== dumper "my2" iterates through all open files
>>                        under all tasks
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/bpf.h |   4 +
>>   kernel/bpf/dump.c   | 190 +++++++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 193 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index fd2b2322412d..53914bec7590 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1109,6 +1109,10 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd);
>>   int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>>   int bpf_obj_get_user(const char __user *pathname, int flags);
>>
>> +int bpf_dump_reg_target(const char *target, const char *target_proto,
>> +                       const struct seq_operations *seq_ops,
>> +                       u32 seq_priv_size, u32 target_feature);
>> +
>>   int bpf_percpu_hash_copy(struct bpf_map *map, void *key, void *value);
>>   int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value);
>>   int bpf_percpu_hash_update(struct bpf_map *map, void *key, void *value,
>> diff --git a/kernel/bpf/dump.c b/kernel/bpf/dump.c
>> index e0c33486e0e7..45528846557f 100644
>> --- a/kernel/bpf/dump.c
>> +++ b/kernel/bpf/dump.c
>> @@ -12,6 +12,173 @@
>>   #include <linux/filter.h>
>>   #include <linux/bpf.h>
>>
>> +struct bpfdump_target_info {
>> +       struct list_head list;
>> +       const char *target;
>> +       const char *target_proto;
>> +       struct dentry *dir_dentry;
>> +       const struct seq_operations *seq_ops;
>> +       u32 seq_priv_size;
>> +       u32 target_feature;
>> +};
>> +
>> +struct bpfdump_targets {
>> +       struct list_head dumpers;
>> +       struct mutex dumper_mutex;
> 
> nit: would be a bit simpler if these were static variables with static
> initialization, similar to how bpfdump_dentry is separate?

yes, we could do that. not 100% sure whether it will be simpler or not.
the structure is to glue them together.

> 
>> +};
>> +
>> +/* registered dump targets */
>> +static struct bpfdump_targets dump_targets;
>> +
>> +static struct dentry *bpfdump_dentry;
>> +
>> +static struct dentry *bpfdump_add_dir(const char *name, struct dentry *parent,
>> +                                     const struct inode_operations *i_ops,
>> +                                     void *data);
>> +static int __bpfdump_init(void);
>> +
>> +static int dumper_unlink(struct inode *dir, struct dentry *dentry)
>> +{
>> +       kfree(d_inode(dentry)->i_private);
>> +       return simple_unlink(dir, dentry);
>> +}
>> +
>> +static const struct inode_operations bpf_dir_iops = {
>> +       .lookup         = simple_lookup,
>> +       .unlink         = dumper_unlink,
>> +};
>> +
>> +int bpf_dump_reg_target(const char *target,
>> +                       const char *target_proto,
>> +                       const struct seq_operations *seq_ops,
>> +                       u32 seq_priv_size, u32 target_feature)
>> +{
>> +       struct bpfdump_target_info *tinfo, *ptinfo;
>> +       struct dentry *dentry, *parent;
>> +       const char *lastslash;
>> +       bool existed = false;
>> +       int err, parent_len;
>> +
>> +       if (!bpfdump_dentry) {
>> +               err = __bpfdump_init();
> 
> This will be called (again) if bpfdump_init() fails? Not sure why? In
> rare cases, some dumper will fail to initialize, but then some might
> succeed, which is going to be even more confusing, no?

I can have a static variable to say bpfdump_init has been attempted to
avoid such situation to avoid any second try.

> 
>> +               if (err)
>> +                       return err;
>> +       }
>> +
>> +       tinfo = kmalloc(sizeof(*tinfo), GFP_KERNEL);
>> +       if (!tinfo)
>> +               return -ENOMEM;
>> +
>> +       tinfo->target = target;
>> +       tinfo->target_proto = target_proto;
>> +       tinfo->seq_ops = seq_ops;
>> +       tinfo->seq_priv_size = seq_priv_size;
>> +       tinfo->target_feature = target_feature;
>> +       INIT_LIST_HEAD(&tinfo->list);
>> +
>> +       lastslash = strrchr(target, '/');
>> +       if (!lastslash) {
>> +               parent = bpfdump_dentry;
> 
> Two nits here. First, it supports only one and two levels. But it
> seems like it wouldn't be hard to support multiple? Instead of
> reverse-searching for /, you can forward search and keep track of
> "current parent".
> 
> nit2:
> 
> parent = bpfdump_dentry;
> if (lastslash) {
> 
>      parent = ptinfo->dir_dentry;
> }
> 
> seems a bit cleaner (and generalizes to multi-level a bit better).
> 
>> +       } else {
>> +               parent_len = (unsigned long)lastslash - (unsigned long)target;
>> +
>> +               mutex_lock(&dump_targets.dumper_mutex);
>> +               list_for_each_entry(ptinfo, &dump_targets.dumpers, list) {
>> +                       if (strlen(ptinfo->target) == parent_len &&
>> +                           strncmp(ptinfo->target, target, parent_len) == 0) {
>> +                               existed = true;
>> +                               break;
>> +                       }
>> +               }
>> +               mutex_unlock(&dump_targets.dumper_mutex);
>> +               if (existed == false) {
>> +                       err = -ENOENT;
>> +                       goto free_tinfo;
>> +               }
>> +
>> +               parent = ptinfo->dir_dentry;
>> +               target = lastslash + 1;
>> +       }
>> +       dentry = bpfdump_add_dir(target, parent, &bpf_dir_iops, tinfo);
>> +       if (IS_ERR(dentry)) {
>> +               err = PTR_ERR(dentry);
>> +               goto free_tinfo;
>> +       }
>> +
>> +       tinfo->dir_dentry = dentry;
>> +
>> +       mutex_lock(&dump_targets.dumper_mutex);
>> +       list_add(&tinfo->list, &dump_targets.dumpers);
>> +       mutex_unlock(&dump_targets.dumper_mutex);
>> +       return 0;
>> +
>> +free_tinfo:
>> +       kfree(tinfo);
>> +       return err;
>> +}
>> +
> 
> [...]
> 
>> +       if (S_ISDIR(mode)) {
>> +               inode->i_op = i_ops;
>> +               inode->i_fop = f_ops;
>> +               inc_nlink(inode);
>> +               inc_nlink(dir);
>> +       } else {
>> +               inode->i_fop = f_ops;
>> +       }
>> +
>> +       d_instantiate(dentry, inode);
>> +       dget(dentry);
> 
> lookup_one_len already bumped refcount, why the second time here?

good question. this is what security/inode.c is doing and seems working.
do not really know the science behind this. will check more.

> 
>> +       inode_unlock(dir);
>> +       return dentry;
>> +
>> +dentry_put:
>> +       dput(dentry);
>> +       dentry = ERR_PTR(err);
>> +unlock:
>> +       inode_unlock(dir);
>> +       return dentry;
>> +}
>> +
> 
> [...]
>