netdev - Re: [PATCH 5/5] bpf: Allow to resolve bpf trampoline in unwind

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Date:   Tue, 7 Jan 2020 09:30:12 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        David Miller <davem@...hat.com>, bjorn.topel@...el.com
Subject: Re: [PATCH 5/5] bpf: Allow to resolve bpf trampoline in unwind

On 1/7/20 12:46 AM, Alexei Starovoitov wrote:
> On Sun, Dec 29, 2019 at 03:37:40PM +0100, Jiri Olsa wrote:
>> When unwinding the stack we need to identify each
>> address to successfully continue. Adding latch tree
>> to keep trampolines for quick lookup during the
>> unwind.
>>
>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ...
>> +bool is_bpf_trampoline(void *addr)
>> +{
>> +	return latch_tree_find(addr, &tree, &tree_ops) != NULL;
>> +}
>> +
>>   struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>>   {
>>   	struct bpf_trampoline *tr;
>> @@ -65,6 +98,7 @@ struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>>   	for (i = 0; i < BPF_TRAMP_MAX; i++)
>>   		INIT_HLIST_HEAD(&tr->progs_hlist[i]);
>>   	tr->image = image;
>> +	latch_tree_insert(&tr->tnode, &tree, &tree_ops);
> 
> Thanks for the fix. I was thinking to apply it, but then realized that bpf
> dispatcher logic has the same issue.
> Could you generalize the fix for both?
> May be bpf_jit_alloc_exec_page() can do latch_tree_insert() ?
> and new version of bpf_jit_free_exec() is needed that will do latch_tree_erase().
> Wdyt?

Also this patch is buggy since your latch lookup happens under RCU, but
I don't see anything that waits a grace period once you remove from the
tree. Instead you free the trampoline right away.

On a different question, given we have all the kallsym infrastructure
for BPF already in place, did you look into whether it's feasible to
make it a bit more generic to also cover JITed buffers from trampolines?