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:

quoted

quoted

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



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

quoted

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

quoted

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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. 

Thanks,
Song