Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter
From: Alexei Starovoitov <hidden>
Date: 2021-01-05 16:28:12
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bpf
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:quoted
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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: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.
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.