Re: [PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework

[Björn Töpel]

Pu Lehui <pulehui@xxxxxxxxxx> writes:

> On 2023/7/19 4:06, Björn Töpel wrote:
>> Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:
>> 
>>> From: Pu Lehui <pulehui@xxxxxxxxxx>
>>>
>>> Commit 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to
>>> half") optimizes the detour code size of kernel functions to half with
>>> T0 register and the upcoming DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv
>>> is based on this optimization, we need to adapt riscv bpf trampoline
>>> based on this. One thing to do is to reduce detour code size of bpf
>>> programs, and the second is to deal with the return address after the
>>> execution of bpf trampoline. Meanwhile, add more comments and rename
>>> some variables to make more sense. The related tests have passed.
>>>
>>> This adaptation needs to be merged before the upcoming
>>> DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv, otherwise it will crash due
>>> to a mismatch in the return address. So we target this modification to
>>> bpf tree and add fixes tag for locating.
>> 
>> Thank you for working on this!
>> 
>>> Fixes: 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half")
>> 
>> This is not a fix. Nothing is broken. Only that this patch much come
>> before or as part of the ftrace series.
>
> Yep, it's really not a fix. I have no idea whether this patch target to 
> bpf-next tree can be ahead of the ftrace series of riscv tree?

For this patch, I'd say it's easier to take it via the RISC-V tree, IFF
the ftrace series is in for-next.

[...]

>>> +#define DETOUR_NINSNS	2
>> 
>> Better name? Maybe call this patchable function entry something? Also,
>
> How about RV_FENTRY_NINSNS?

Sure. And more importantly that it's actually used in the places where
nops/skips are done.

>> to catch future breaks like this -- would it make sense to have a
>> static_assert() combined with something tied to
>> -fpatchable-function-entry= from arch/riscv/Makefile?
>
> It is very necessary, but it doesn't seem to be easy. I try to find GCC 
> related functions, something like __builtin_xxx, but I can't find it so 
> far. Also try to make it as a CONFIG_PATCHABLE_FUNCTION_ENTRY=4 in 
> Makefile and then static_assert, but obviously it shouldn't be done. 
> Maybe we can deal with this later when we have a solution?

Ok!

[...]

>>> @@ -787,20 +762,19 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>>>   	int i, ret, offset;
>>>   	int *branches_off = NULL;
>>>   	int stack_size = 0, nregs = m->nr_args;
>>> -	int retaddr_off, fp_off, retval_off, args_off;
>>> -	int nregs_off, ip_off, run_ctx_off, sreg_off;
>>> +	int fp_off, retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off;
>>>   	struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
>>>   	struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
>>>   	struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
>>>   	void *orig_call = func_addr;
>>> -	bool save_ret;
>>> +	bool save_retval, traced_ret;
>>>   	u32 insn;
>>>   
>>>   	/* Generated trampoline stack layout:
>>>   	 *
>>>   	 * FP - 8	    [ RA of parent func	] return address of parent
>>>   	 *					  function
>>> -	 * FP - retaddr_off [ RA of traced func	] return address of traced
>>> +	 * FP - 16	    [ RA of traced func	] return address of
>>>   	traced
>> 
>> BPF code uses frame pointers. Shouldn't the trampoline frame look like a
>> regular frame [1], i.e. start with return address followed by previous
>> frame pointer?
>> 
>
> oops, will fix it. Also we need to consider two types of trampoline 
> stack layout, that is:
>
> * 1. trampoline called from function entry
> * --------------------------------------
> * FP + 8           [ RA of parent func ] return address of parent
> *                                        function
> * FP + 0           [ FP                ]
> *
> * FP - 8           [ RA of traced func ] return address of traced
> *                                        function
> * FP - 16          [ FP                ]
> * --------------------------------------
> *
> * 2. trampoline called directly
> * --------------------------------------
> * FP - 8           [ RA of caller func ] return address of caller
> *                                        function
> * FP - 16          [ FP                ]
> * --------------------------------------

Hmm, could you expand a bit on this? The stack frame top 16B (8+8)
should follow what the psabi suggests, regardless of the call site?

Maybe it's me that's not following -- please explain a bit more!


Björn