linux-kernel - Re: [PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the stack of trampoline

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Message-ID: <20230607200348.dprmfvpzdvk5ldpp@macbook-pro-8.dhcp.thefacebook.com>
Date:   Wed, 7 Jun 2023 13:03:48 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     menglong8.dong@...il.com
Cc:     davem@...emloft.net, dsahern@...nel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, x86@...nel.org,
        imagedong@...cent.com, benbjiang@...cent.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 2/3] bpf, x86: clean garbage value in the
 stack of trampoline

On Wed, Jun 07, 2023 at 08:59:10PM +0800, menglong8.dong@...il.com wrote:
> From: Menglong Dong <imagedong@...cent.com>
> 
> There are garbage values in upper bytes when we store the arguments
> into stack in save_regs() if the size of the argument less then 8.
> 
> As we already reserve 8 byte for the arguments in regs and stack,
> it is ok to store/restore the regs in BPF_DW size. Then, the garbage
> values in upper bytes will be cleaned.
> 
> Reviewed-by: Jiang Biao <benbjiang@...cent.com>
> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 413b986b5afd..e9bc0b50656b 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1878,20 +1878,16 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
>  
>  		if (i <= 5) {
>  			/* copy function arguments from regs into stack */
> -			emit_stx(prog, bytes_to_bpf_size(arg_size),
> -				 BPF_REG_FP,
> +			emit_stx(prog, BPF_DW, BPF_REG_FP,
>  				 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
>  				 -(stack_size - i * 8));

This is ok,

>  		} else {
>  			/* copy function arguments from origin stack frame
>  			 * into current stack frame.
>  			 */
> -			emit_ldx(prog, bytes_to_bpf_size(arg_size),
> -				 BPF_REG_0, BPF_REG_FP,
> +			emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
>  				 (i - 6) * 8 + 0x18);
> -			emit_stx(prog, bytes_to_bpf_size(arg_size),
> -				 BPF_REG_FP,
> -				 BPF_REG_0,
> +			emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
>  				 -(stack_size - i * 8));

But this is not.
See https://godbolt.org/z/qW17f6cYe
mov dword ptr [rsp], 6

the compiler will store 32-bit only. The upper 32-bit are still garbage.

>  		}
>  
> @@ -1918,7 +1914,7 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
>  			next_same_struct = !next_same_struct;
>  		}
>  
> -		emit_ldx(prog, bytes_to_bpf_size(arg_size),
> +		emit_ldx(prog, BPF_DW,
>  			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
>  			 BPF_REG_FP,
>  			 -(stack_size - i * 8));
> @@ -1949,12 +1945,9 @@ static void prepare_origin_stack(const struct btf_func_model *m, u8 **prog,
>  		}
>  
>  		if (i > 5) {
> -			emit_ldx(prog, bytes_to_bpf_size(arg_size),
> -				 BPF_REG_0, BPF_REG_FP,
> +			emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
>  				 (i - 6) * 8 + 0x18);
> -			emit_stx(prog, bytes_to_bpf_size(arg_size),
> -				 BPF_REG_FP,
> -				 BPF_REG_0,
> +			emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
>  				 -(stack_size - (i - 6) * 8));
>  		}
>  
> -- 
> 2.40.1
>