Messages in this thread | | | Date | Sun, 5 May 2019 15:52:23 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC] bpf: Add support for reading user pointers |
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On Sun, May 05, 2019 at 03:46:08PM +0100, Qais Yousef wrote: > On 05/05/19 13:29, Joel Fernandes wrote: > > On Sun, May 05, 2019 at 12:04:24PM +0100, Qais Yousef wrote: > > > On 05/03/19 09:49, Joel Fernandes wrote: > > > > On Fri, May 03, 2019 at 01:12:34PM +0100, Qais Yousef wrote: > > > > > Hi Joel > > > > > > > > > > On 05/02/19 16:49, Joel Fernandes (Google) wrote: > > > > > > The eBPF based opensnoop tool fails to read the file path string passed > > > > > > to the do_sys_open function. This is because it is a pointer to > > > > > > userspace address and causes an -EFAULT when read with > > > > > > probe_kernel_read. This is not an issue when running the tool on x86 but > > > > > > is an issue on arm64. This patch adds a new bpf function call based > > > > > > > > > > I just did an experiment and if I use Android 4.9 kernel I indeed fail to see > > > > > PATH info when running opensnoop. But if I run on 5.1-rc7 opensnoop behaves > > > > > correctly on arm64. > > > > > > > > > > My guess either a limitation that was fixed on later kernel versions or Android > > > > > kernel has some strict option/modifications that make this fail? > > > > > > > > Thanks a lot for checking, yes I was testing 4.9 kernel with this patch (pixel 3). > > > > > > > > I am not sure what has changed since then, but I still think it is a good > > > > idea to make the code more robust against such future issues anyway. In > > > > particular, we learnt with extensive discussions that user/kernel pointers > > > > are not necessarily distinguishable purely based on their address. > > > > > > Yes I wasn't arguing against that. But the commit message is misleading or > > > needs more explanation at least. I tried 4.9.y stable and arm64 worked on that > > > too. Why do you think it's an arm64 problem? > > > > Well it is broken on at least on at least one arm64 device and the patch I > > sent fixes it. We know that the bpf is using wrong kernel API so why not fix > > it? Are you saying we should not fix it like in this patch? Or do you have > > another fix in mind? > > Again I have no issue with the new API. But the claim that it's a fix for > a broken arm64 is a big stretch. AFAICT you don't understand the root cause of > why copy_to_user_inatomic() fails in your case. Given that Android 4.9 has > its own patches on top of 4.9 stable, it might be something that was introduced > in one of these patches that breaks opensnoop, and by making it use the new API > you might be simply working around the problem. All I can see is that vanilla > 4.9 stable works on arm64.
Agreed that commit message could be improved. I believe issue is something to do with differences in 4.9 PAN emulation backports. AIUI PAN was introduced in upstream only in 4.10 so 4.9 needed backports.
I did not root cause this completely because "doing the right thing" fixed the issue. I will look more closely once I am home.
Thank you.
> So I am happy about introducing the new API but not happy with the commit > message or the explanation given in it. Unless you can investigate the root > cause and relate how this fixes it (and not workaround a problem you're > specifically having) I think it's better to introduce this patch as a generic > new API that is more robust to handle reading __user data in BPF and drop > reference to opensnoop failures. They raise more questions and the real > intention of this patch anyway is to provide the new correct way for BPF > programs to read __user data regardless opensnoop fails or not AFAIU. > > Cheers > > -- > Qais Yousef
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