On Wed, Dec 06, 2023 at 01:39:43PM -0800, Alexei Starovoitov wrote: > All is ok until kCFI comes into picture. > Here we probably need to teach arch_prepare_bpf_trampoline() to emit > different __kcfi_typeid depending on kernel function proto, > so that caller hash checking logic won't be tripped. > I suspect that requires to reverse engineer an algorithm of computing kcfi from clang. > other ideas? I was going to try and extend bpf_struct_ops with a pointer, this pointer will point to a struct of the right type with all ops filled out as stubs. Then I was going to have bpf_struct_ops_map_update_elem() pass a pointer to the stub op (using moff) into bpf_struct_ops_prepare_trampoline() and eventually arch_prepare_bpf_trampoline(). Additionally I was going to add BPF_TRAMP_F_INDIRECT. Then when F_INDIRECT is set, have it generate the CFI preamble based on the stub passed -- which will have the correct preamble for that method. At least, that's what I'm thinking now, I've yet to try and implement it. > > > The other case: > In the case of bpf_for_each_map_elem() the 'bloom_callback' is a subprog > of bpf_callback_t type. > So the kernel is doing: > ret = callback_fn((u64)(long)map, (u64)(long)&key, > (u64)(long)val, (u64)(long)callback_ctx, 0); > and that works on all archs including 32-bit. > The kernel is doing conversion from native calling convention to bpf calling convention > and for lucky archs like x86-64 the conversion is a true nop. > It's a plain indirect call to JITed bpf prog. > Note there is no interpreter support here. This works on archs with JITs only. > No ftrace and no trampoline. > > This case is easier to make work with kCFI. > The JIT will use: > cfi_bpf_hash: > .long __kcfi_typeid___bpf_prog_runX > like your patch already does. > And will use > extern u64 __bpf_callback_fn(u64, u64, u64, u64, u64); > cfi_bpf_subprog_hash: > .long __kcfi_typeid___bpf_callback_fn > to JIT all subprogs. See bpf_is_subprog(). Aaah!, yes it should be trivial to use another hash value when is_subprog in emit_prologue(). > btw there are two patchsets in progress that will touch core bits of JITs. > This one: > https://patchwork.kernel.org/project/netdevbpf/cover/20231201190654.1233153-1-song@xxxxxxxxxx/ > and this one: > https://patchwork.kernel.org/project/netdevbpf/cover/20231011152725.95895-1-hffilwlqm@xxxxxxxxx/ > > so do you mind resending your current set with get_cfi_offset() change and > I can land it into bpf-next, so we can fix one bug at a time, > build on top, and avoid conflicts? I can do. > The more we dig the more it looks like that the follow up you planned to do > on top of this set isn't going to happen soon. > So should be ok going through bpf-next and then you can follow up with x86 things > after merge window? Yes, we can do that. Plans have changed on my side too -- I'm taking a 6 week break soon, so I'll do whatever I can before I'm out, and then continue from whatever state I find when I get back. Thanks for the details!
