On Fri, Nov 8, 2019 at 5:42 AM Alexei Starovoitov <ast@xxxxxx> wrote: > > On 11/8/19 1:36 AM, Peter Zijlstra wrote: > > On Fri, Nov 08, 2019 at 10:11:56AM +0100, Peter Zijlstra wrote: > >> On Thu, Nov 07, 2019 at 10:40:23PM -0800, Alexei Starovoitov wrote: > >>> Add bpf_arch_text_poke() helper that is used by BPF trampoline logic to patch > >>> nops/calls in kernel text into calls into BPF trampoline and to patch > >>> calls/nops inside BPF programs too. > >> > >> This thing assumes the text is unused, right? That isn't spelled out > >> anywhere. The implementation is very much unsafe vs concurrent execution > >> of the text. > > > > Also, what NOP/CALL instructions will you be hijacking? If you're > > planning on using the fentry nops, then what ensures this and ftrace > > don't trample on one another? Similar for kprobes. > > > > In general, what ensures every instruction only has a single modifier? > > Looks like you didn't bother reading cover letter and missed a month > of discussions between my and Steven regarding exactly this topic > though you were directly cc-ed in all threads :( > tldr for kernel fentry nops it will be converted to use > register_ftrace_direct() whenever it's available. > For all other nops, calls, jumps that are inside BPF programs BPF infra > will continue modifying them through this helper. > Daniel's upcoming bpf_tail_call() optimization will use text_poke as well. > > > I'm very uncomfortable letting random bpf proglets poke around in the > kernel text. > > 1. There is no such thing as 'proglet'. Please don't invent meaningless > names. > 2. BPF programs have no ability to modify kernel text. > 3. BPF infra taking all necessary measures to make sure that poking > kernel's and BPF generated text is safe. > If you see specific issue please say so. We'll be happy to address > all issues. Being 'uncomfortable' is not constructive. > I was thinking more about this. Peter, do you mind we apply your first patch: https://lore.kernel.org/lkml/20191007081944.88332264.2@xxxxxxxxxxxxx/ to both tip and bpf-next trees? Then I can use text_poke_bp() as-is without any additional ugliness on my side that would need to be removed in few weeks. Do you have it in tip already? I can cherry-pick from there to make sure it's exactly the same commit log then there will be no merge issues during merge window.
