Re: [PATCH bpf v2] riscv, bpf: fix incorrect usage of BPF_TRAMP_F_ORIG_STACK

[Pu Lehui <pulehui@huawei.com>]

On 2025/12/19 22:29, Menglong Dong wrote:
> The usage of BPF_TRAMP_F_ORIG_STACK in __arch_prepare_bpf_trampoline() is
> wrong, and it should be BPF_TRAMP_F_CALL_ORIG, which caused crash as
> Andreas reported:
> 
>    Insufficient stack space to handle exception!
>    Task stack:     [0xff20000000010000..0xff20000000014000]
>    Overflow stack: [0xff600000ffdad070..0xff600000ffdae070]
>    CPU: 1 UID: 0 PID: 1 Comm: systemd Not tainted 6.18.0-rc5+ #15 PREEMPT(voluntary)
>    Hardware name: riscv-virtio qemu/qemu, BIOS 2025.10 10/01/2025
>    epc : copy_from_kernel_nofault+0xa/0x198
>     ra : bpf_probe_read_kernel+0x20/0x60
>    epc : ffffffff802b732a ra : ffffffff801e6070 sp : ff2000000000ffe0
>     gp : ffffffff82262ed0 tp : 0000000000000000 t0 : ffffffff80022320
>     t1 : ffffffff801e6056 t2 : 0000000000000000 s0 : ff20000000010040
>     s1 : 0000000000000008 a0 : ff20000000010050 a1 : ff60000083b3d320
>     a2 : 0000000000000008 a3 : 0000000000000097 a4 : 0000000000000000
>     a5 : 0000000000000000 a6 : 0000000000000021 a7 : 0000000000000003
>     s2 : ff20000000010050 s3 : ff6000008459fc18 s4 : ff60000083b3d340
>     s5 : ff20000000010060 s6 : 0000000000000000 s7 : ff20000000013aa8
>     s8 : 0000000000000000 s9 : 0000000000008000 s10: 000000000058dcb0
>     s11: 000000000058dca7 t3 : 000000006925116d t4 : ff6000008090f026
>     t5 : 00007fff9b0cbaa8 t6 : 0000000000000016
>    status: 0000000200000120 badaddr: 0000000000000000 cause: 8000000000000005
>    Kernel panic - not syncing: Kernel stack overflow
>    CPU: 1 UID: 0 PID: 1 Comm: systemd Not tainted 6.18.0-rc5+ #15 PREEMPT(voluntary)
>    Hardware name: riscv-virtio qemu/qemu, BIOS 2025.10 10/01/2025
>    Call Trace:
>    [<ffffffff8001a1f8>] dump_backtrace+0x28/0x38
>    [<ffffffff80002502>] show_stack+0x3a/0x50
>    [<ffffffff800122be>] dump_stack_lvl+0x56/0x80
>    [<ffffffff80012300>] dump_stack+0x18/0x22
>    [<ffffffff80002abe>] vpanic+0xf6/0x328
>    [<ffffffff80002d2e>] panic+0x3e/0x40
>    [<ffffffff80019ef0>] handle_bad_stack+0x98/0xa0
>    [<ffffffff801e6070>] bpf_probe_read_kernel+0x20/0x60
> 
> Just fix it.
> 
> Fixes: 47c9214dcbea ("bpf: fix the usage of BPF_TRAMP_F_SKIP_FRAME")
> Reported-by: Andreas Schwab <schwab@linux-m68k.org>
> Closes: https://lore.kernel.org/bpf/874ipnkfvt.fsf@igel.home/
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
> v2:
> - merge the code
> ---
>   arch/riscv/net/bpf_jit_comp64.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 5f9457e910e8..37888abee70c 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1133,10 +1133,6 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>   
>   	store_args(nr_arg_slots, args_off, ctx);
>   
> -	/* skip to actual body of traced function */
> -	if (flags & BPF_TRAMP_F_ORIG_STACK)

Oh, how did this weird flags get in here...

> -		orig_call += RV_FENTRY_NINSNS * 4;
> -
>   	if (flags & BPF_TRAMP_F_CALL_ORIG) {
>   		emit_imm(RV_REG_A0, ctx->insns ? (const s64)im : RV_MAX_COUNT_IMM, ctx);
>   		ret = emit_call((const u64)__bpf_tramp_enter, true, ctx);
> @@ -1171,6 +1167,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
>   	}
>   
>   	if (flags & BPF_TRAMP_F_CALL_ORIG) {
> +		/* skip to actual body of traced function */
> +		orig_call += RV_FENTRY_NINSNS * 4;


LGTM, let's revert it.

Reviewed-by: Pu Lehui <pulehui@huawei.com>

>   		restore_args(min_t(int, nr_arg_slots, RV_MAX_REG_ARGS), args_off, ctx);
>   		restore_stack_args(nr_arg_slots - RV_MAX_REG_ARGS, args_off, stk_arg_off, ctx);
>   		ret = emit_call((const u64)orig_call, true, ctx);