On Tue, Jan 5, 2021 at 9:11 AM Song Liu [off-list ref] wrote:

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On Jan 5, 2021, at 8:27 AM, Alexei Starovoitov [off-list ref] wrote:

On Mon, Jan 4, 2021 at 9:47 PM Song Liu [off-list ref] wrote:

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On Jan 4, 2021, at 5:46 PM, Alexei Starovoitov [off-list ref] wrote:

On Fri, Dec 18, 2020 at 05:23:25PM +0000, Song Liu wrote:

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On Dec 18, 2020, at 8:38 AM, Yonghong Song [off-list ref] wrote:



On 12/17/20 9:23 PM, Alexei Starovoitov wrote:

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On Thu, Dec 17, 2020 at 8:33 PM Song Liu [off-list ref] wrote:

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ahh. I missed that. Makes sense.
vm_file needs to be accurate, but vm_area_struct should be accessed as ptr_to_btf_id.

Passing pointer of vm_area_struct into BPF will be tricky. For example, shall we
allow the user to access vma->vm_file? IIUC, with ptr_to_btf_id the verifier will
allow access of vma->vm_file as a valid pointer to struct file. However, since the
vma might be freed, vma->vm_file could point to random data.

I don't think so. The proposed patch will do get_file() on it.
There is actually no need to assign it into a different variable.
Accessing it via vma->vm_file is safe and cleaner.

I did not check the code but do you have scenarios where vma is freed but old vma->vm_file is not freed due to reference counting, but
freed vma area is reused so vma->vm_file could be garbage?

AFAIK, once we unlock mmap_sem, the vma could be freed and reused. I guess ptr_to_btf_id
or probe_read would not help with this?

Theoretically we can hack the verifier to treat some ptr_to_btf_id as "less
valid" than the other ptr_to_btf_id, but the user experience will not be great.
Reading such bpf prog will not be obvious. I think it's better to run bpf prog
in mmap_lock then and let it access vma->vm_file. After prog finishes the iter
bit can do if (mmap_lock_is_contended()) before iterating. That will deliver
better performance too. Instead of task_vma_seq_get_next() doing
mmap_lock/unlock at every vma. No need for get_file() either. And no
__vm_area_struct exposure.

I think there might be concern calling BPF program with mmap_lock, especially that
the program is sleepable (for bpf_d_path). It shouldn't be a problem for common
cases, but I am not 100% sure for corner cases (many instructions in BPF + sleep).
Current version is designed to be very safe for the workload, which might be too
conservative.

I know and I agree with all that, but how do you propose to fix the
vm_file concern
without holding the lock while prog is running? I couldn't come up with a way.

I guess the gap here is that I don't see why __vm_area_struct exposure is an issue.
It is similar to __sk_buff, and simpler (though we had more reasons to introduce
__sk_buff back when there wasn't BTF).

If we can accept __vm_area_struct, current version should work, as it doesn't have
problem with vm_file

True. The problem with __vm_area_struct is that it is a hard coded
uapi with little to none
extensibility. In this form vma iterator is not really useful beyond
the example in selftest.