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

From: Pu Lehui
Date: Fri Jul 21 2023 - 05:11:47 EST

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In reply to: Björn Töpel: "Re: [PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework"
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On 2023/7/21 16:53, Björn Töpel wrote:

Pu Lehui <pulehui@xxxxxxxxxx> writes:

On 2023/7/19 23:18, Björn Töpel wrote:

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.

alright, so let's make it target to riscv-tree to avoid that cracsh.

[...]

+#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.

the new one is suited up.

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 I've missed something important! Or maybe I'm misunderstanding
what you mean. But anyway there is something to show. In my perspective,
we should construct a complete stack frame, otherwise one layer of stack
will be lost in calltrace when enable CONFIG_FRAME_POINTER.

We can verify it by `echo 1 >
/sys/kernel/debug/tracing/options/stacktrace`, and the results as show
below:

1. complete stack frame
* --------------------------------------
* 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 ]
* --------------------------------------
the stacktrace is:

=> trace_event_raw_event_bpf_trace_printk
=> bpf_trace_printk
=> bpf_prog_ad7f62a5e7675635_bpf_prog
=> bpf_trampoline_6442536643
=> do_empty
=> meminfo_proc_show
=> seq_read_iter
=> proc_reg_read_iter
=> copy_splice_read
=> vfs_splice_read
=> splice_direct_to_actor
=> do_splice_direct
=> do_sendfile
=> sys_sendfile64
=> do_trap_ecall_u
=> ret_from_exception

2. omit one FP
* --------------------------------------
* FP + 0 [ RA of parent func ] return address of parent
* function
* FP - 8 [ RA of traced func ] return address of traced
* function
* FP - 16 [ FP ]
* --------------------------------------
the stacktrace is:

=> trace_event_raw_event_bpf_trace_printk
=> bpf_trace_printk
=> bpf_prog_ad7f62a5e7675635_bpf_prog
=> bpf_trampoline_6442491529
=> do_empty
=> seq_read_iter
=> proc_reg_read_iter
=> copy_splice_read
=> vfs_splice_read
=> splice_direct_to_actor
=> do_splice_direct
=> do_sendfile
=> sys_sendfile64
=> do_trap_ecall_u
=> ret_from_exception

it lost the layer of 'meminfo_proc_show'.

(Lehui was friendly enough to explain the details for me offlist.)

Aha, now I get what you mean! When we're getting into the trampoline
from the fentry-side, an additional stack frame needs to be
created. Otherwise, the unwinding will be incorrect.

So (for the rest of the readers ;-)), the BPF trampoline can be called
from:

A. A tracing point of view; Here, we're calling into the trampoline via
the fentry/patchable entry. In this scenario, an additional stack
frame needs to be constructed for proper unwinding.

B. For kfuncs. Here, the call into the trampoline is just a "regular
call", and no additional stack frame is needed.

@Guo @Song Is the RISC-V ftrace code creating an additional stack frame,
or is the stack unwinding incorrect when the fentry is patched?

Thanks for clearing it up for me, Lehui!

It's my honor, will keep push riscv-bpf.

Björn

Next message: Andy Shevchenko: "Re: [PATCH v3] backlight: gpio_backlight: Drop output gpio direction check for initial power state"
Previous message: Kemeng Shi: "[PATCH 10/10] ext4: correct some stale comment of criteria"
In reply to: Björn Töpel: "Re: [PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework"
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