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

[Wangnan (F)]

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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> > Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> > Cc: Brendan Gregg <brendan.d.gregg@xxxxxxxxx>
> > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Cc: David Ahern <dsahern@xxxxxxxxx>
> > Cc: He Kuang <hekuang@xxxxxxxxxx>
> > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Kaixu Xia <xiakaixu@xxxxxxxxxx>
> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Zefan Li <lizefan@xxxxxxxxxx>
> > Cc: pi3orama@xxxxxxx
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Link: http://lkml.kernel.org/n/ebpf-6yw9eg0ej3l4jnqhinngkw86@xxxxxxxxxxxxxx
> > ---
> [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


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