Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter

[Song Liu <songliubraving@fb.com>]

> On Dec 16, 2020, at 4:34 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Tue, Dec 15, 2020 at 3:37 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> Introduce task_vma bpf_iter to print memory information of a process. It
>> can be used to print customized information similar to /proc/<pid>/maps.
>> 
>> task_vma iterator releases mmap_lock before calling the BPF program.
>> Therefore, we cannot pass vm_area_struct directly to the BPF program. A
>> new __vm_area_struct is introduced to keep key information of a vma. On
>> each iteration, task_vma gathers information in __vm_area_struct and
>> passes it to the BPF program.
>> 
>> If the vma maps to a file, task_vma also holds a reference to the file
>> while calling the BPF program.
>> 
>> Signed-off-by: Song Liu <songliubraving@fb.com>
>> ---
>> include/linux/bpf.h    |   2 +-
>> kernel/bpf/task_iter.c | 205 ++++++++++++++++++++++++++++++++++++++++-
>> 2 files changed, 205 insertions(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 07cb5d15e7439..49dd1e29c8118 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1325,7 +1325,7 @@ enum bpf_iter_feature {
>>        BPF_ITER_RESCHED        = BIT(0),
>> };
>> 
>> -#define BPF_ITER_CTX_ARG_MAX 2
>> +#define BPF_ITER_CTX_ARG_MAX 3
>> struct bpf_iter_reg {
>>        const char *target;
>>        bpf_iter_attach_target_t attach_target;
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 0458a40edf10a..15a066b442f75 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -304,9 +304,183 @@ static const struct seq_operations task_file_seq_ops = {
>>        .show   = task_file_seq_show,
>> };
>> 
>> +/*
>> + * Key information from vm_area_struct. We need this because we cannot
>> + * assume the vm_area_struct is still valid after each iteration.
>> + */
>> +struct __vm_area_struct {
>> +       __u64 start;
>> +       __u64 end;
>> +       __u64 flags;
>> +       __u64 pgoff;
> 
> I'd keep the original names of the fields (vm_start, vm_end, etc).

I thought about the names. Unlike the kernel fs/mm code, where there
are many different start/end/offset/flags, the prefix doesn't seem to 
be helpful in the BPF programs. In fact, it is probably easier for 
the developers to differentiate __vm_area_struct->start and 
vm_area_struct->vm_start.

Also, we have bpf_iter__task_vma->file, which is the same as 
vm_area_struct->vm_file. If we prefix __vm_area_struct members, it 
will be a little weird to name it either "vm_file" or "file".

Given these reasons, I decided to not have vm_ prefix. Does this make
sense? 

> But
> there are some more fields which seem useful, like vm_page_prot,
> vm_mm, etc.

vm_page_prot doesn't really provide extra information than vm_flags. 
Please refer to mm/mmap.c vm_get_page_prot(). 

We have the vm_mm in task->mm, so no need to add it to __vm_area_struct.

> 
> It's quite unfortunate, actually, that bpf_iter program doesn't get
> access to the real vm_area_struct, because it won't be able to do much
> beyond using fields that we pre-defined here. E.g., there could be
> interesting things to do with vm_mm, but unfortunately it won't be
> possible.
> 
> Is there any way to still provide access to the original
> vm_area_struct and let BPF programs use BTF magic to follow all those
> pointers (like vm_mm) safely?

We hold a reference to task, and the task holds a reference to task->mm,
so we can safely probe_read information in mm_struct, like the page 
table. 

Thanks,
Song