Re: [PATCH 16/31] perf tools: Add prologue for BPF programs for fetching arguments

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/10/15 13:26, Namhyung Kim wrote:
> On Wed, Oct 14, 2015 at 12:41:27PM +0000, Wang Nan wrote:
>> From: He Kuang <hekuang@huawei.com>
>>
>> This patch generates prologue for a BPF program which fetch arguments
>> for it. With this patch, the program can have arguments as follow:
>>
>>   SEC("lock_page=__lock_page page->flags")
>>   int lock_page(struct pt_regs *ctx, int err, unsigned long flags)
>>   {
>> 	 return 1;
>>   }
>>
>> This patch passes at most 3 arguments from r3, r4 and r5. r1 is still
>> the ctx pointer. r2 is used to indicate the successfulness of
>> dereferencing.
>>
>> This patch uses r6 to hold ctx (struct pt_regs) and r7 to hold stack
>> pointer for result. Result of each arguments first store on stack:
>>
>>   low address
>>   BPF_REG_FP - 24  ARG3
>>   BPF_REG_FP - 16  ARG2
>>   BPF_REG_FP - 8   ARG1
>>   BPF_REG_FP
>>   high address
>>
>> Then loaded into r3, r4 and r5.
>>
>> The output prologue for offn(...off2(off1(reg)))) should be:
>>
>>       r6 <- r1			// save ctx into a callee saved register
>>       r7 <- fp
>>       r7 <- r7 - stack_offset	// pointer to result slot
>>       /* load r3 with the offset in pt_regs of 'reg' */
>>       (r7) <- r3			// make slot valid
>>       r3 <- r3 + off1		// prepare to read unsafe pointer
>>       r2 <- 8
>>       r1 <- r7			// result put onto stack
>>       call probe_read		// read unsafe pointer
>>       jnei r0, 0, err		// error checking
>>       r3 <- (r7)			// read result
>>       r3 <- r3 + off2		// prepare to read unsafe pointer
>>       r2 <- 8
>>       r1 <- r7
>>       call probe_read
>>       jnei r0, 0, err
>>       ...
>>       /* load r2, r3, r4 from stack */
>>       goto success
>> err:
>>       r2 <- 1
>>       /* load r3, r4, r5 with 0 */
>>       goto usercode
>> success:
>>       r2 <- 0
>> usercode:
>>       r1 <- r6	// restore ctx
>>       // original user code
>>
>> If all of arguments reside in register (dereferencing is not
>> required), gen_prologue_fastpath() will be used to create
>> fast prologue:
>>
>>       r3 <- (r1 + offset of reg1)
>>       r4 <- (r1 + offset of reg2)
>>       r5 <- (r1 + offset of reg3)
>>       r2 <- 0
>>
>> P.S.
>>
>> eBPF calling convention is defined as:
>>
>> * r0		- return value from in-kernel function, and exit value
>>                    for eBPF program
>> * r1 - r5	- arguments from eBPF program to in-kernel function
>> * r6 - r9	- callee saved registers that in-kernel function will
>>                    preserve
>> * r10		- read-only frame pointer to access stack
>>
>> Signed-off-by: He Kuang <hekuang@huawei.com>
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>> Cc: Alexei Starovoitov <ast@plumgrid.com>
>> Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Cc: David Ahern <dsahern@gmail.com>
>> Cc: He Kuang <hekuang@huawei.com>
>> Cc: Jiri Olsa <jolsa@kernel.org>
>> Cc: Kaixu Xia <xiakaixu@huawei.com>
>> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
>> Cc: Zefan Li <lizefan@huawei.com>
>> Cc: pi3orama@163.com
>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
>> Link: http://lkml.kernel.org/n/ebpf-6yw9eg0ej3l4jnqhinngkw86@git.kernel.org
>> ---
> [SNIP]
>> +int bpf__gen_prologue(struct probe_trace_arg *args, int nargs,
>> +		      struct bpf_insn *new_prog, size_t *new_cnt,
>> +		      size_t cnt_space)
>> +{
>> +	struct bpf_insn *success_code = NULL;
>> +	struct bpf_insn *error_code = NULL;
>> +	struct bpf_insn *user_code = NULL;
>> +	struct bpf_insn_pos pos;
>> +	bool fastpath = true;
>> +	int i;
>> +
>> +	if (!new_prog || !new_cnt)
>> +		return -EINVAL;
>> +
>> +	pos.begin = new_prog;
>> +	pos.end = new_prog + cnt_space;
>> +	pos.pos = new_prog;
>> +
>> +	if (!nargs) {
>> +		ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0),
>> +		    &pos);
>> +
>> +		if (check_pos(&pos))
>> +			goto errout;
>> +
>> +		*new_cnt = pos_get_cnt(&pos);
>> +		return 0;
>> +	}
>> +
>> +	if (nargs > BPF_PROLOGUE_MAX_ARGS)
>> +		nargs = BPF_PROLOGUE_MAX_ARGS;
> Wouldn't it be better to inform user if it ignored some arguments?

Correct. I'd like to add a notification in next version:

diff --git a/tools/perf/util/bpf-prologue.c b/tools/perf/util/bpf-prologue.c
index e4adb18..36093d9 100644
--- a/tools/perf/util/bpf-prologue.c
+++ b/tools/perf/util/bpf-prologue.c
@@ -337,8 +337,10 @@ int bpf__gen_prologue(struct probe_trace_arg *args, 
int nargs,
                 return 0;
         }

-       if (nargs > BPF_PROLOGUE_MAX_ARGS)
+       if (nargs > BPF_PROLOGUE_MAX_ARGS) {
+               pr_warning("bpf: prologue: too many arguments\n");
                 nargs = BPF_PROLOGUE_MAX_ARGS;
+       }
         if (cnt_space > BPF_MAXINSNS)
                 cnt_space = BPF_MAXINSNS;


Thank you.


> Thanks,
> Namhyung