From: Yonghong Song <yhs@meta.com>
To: menglong8.dong@gmail.com, alexei.starovoitov@gmail.com
Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
haoluo@google.com, jolsa@kernel.org, benbjiang@tencent.com,
bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
Menglong Dong <imagedong@tencent.com>
Subject: Re: [PATCH bpf-next v5 1/3] bpf, x86: clean garbage values when store args from regs into stack
Date: Sun, 18 Jun 2023 15:52:11 -0700 [thread overview]
Message-ID: <12ff965a-12d7-e7f4-a577-5e645d4ff6c0@meta.com> (raw)
In-Reply-To: <20230613025226.3167956-2-imagedong@tencent.com>
On 6/12/23 7:52 PM, menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.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.
Please make it clear that there are no bugs in the existing code
since for each argument, a type case will happen like
(parameter_type)ctx[stack_slot]
where ctx[] is an u64 type array. The compiler will generate
correct code to do type casting so garbage value will not impact
the final result. But indeed, uniformly
do save/restore with BPF_DW size can simplify code.
>
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
Acked-by: Yonghong Song <yhs@fb.com>
> ---
> arch/x86/net/bpf_jit_comp.c | 35 ++++++-----------------------------
> 1 file changed, 6 insertions(+), 29 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 1056bbf55b17..a407fbbffecd 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1860,57 +1860,34 @@ st: if (is_imm8(insn->off))
> static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> int stack_size)
> {
> - int i, j, arg_size;
> - bool next_same_struct = false;
> + int i;
>
> /* Store function arguments to stack.
> * For a function that accepts two pointers the sequence will be:
> * mov QWORD PTR [rbp-0x10],rdi
> * mov QWORD PTR [rbp-0x8],rsi
> */
> - for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> - /* The arg_size is at most 16 bytes, enforced by the verifier. */
> - arg_size = m->arg_size[j];
> - if (arg_size > 8) {
> - arg_size = 8;
> - next_same_struct = !next_same_struct;
> - }
> -
> - emit_stx(prog, bytes_to_bpf_size(arg_size),
> - BPF_REG_FP,
> + for (i = 0; i < min(nr_regs, 6); i++)
> + emit_stx(prog, BPF_DW, BPF_REG_FP,
> i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> -(stack_size - i * 8));
> -
> - j = next_same_struct ? j : j + 1;
> - }
> }
>
[...]
next prev parent reply other threads:[~2023-06-18 22:53 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-13 2:52 [PATCH bpf-next v5 0/3] bpf, x86: allow function arguments up to 12 for TRACING menglong8.dong
2023-06-13 2:52 ` [PATCH bpf-next v5 1/3] bpf, x86: clean garbage values when store args from regs into stack menglong8.dong
2023-06-18 22:52 ` Yonghong Song [this message]
2023-06-19 2:17 ` Menglong Dong
2023-06-13 2:52 ` [PATCH bpf-next v5 2/3] bpf, x86: allow function arguments up to 12 for TRACING menglong8.dong
2023-06-15 4:00 ` Menglong Dong
2023-06-18 23:10 ` Yonghong Song
2023-06-19 2:31 ` Menglong Dong
2023-06-22 9:06 ` David Laight
2023-06-22 13:05 ` Menglong Dong
2023-06-22 14:18 ` David Laight
2023-06-23 13:08 ` Menglong Dong
2023-06-22 16:26 ` Yonghong Song
2023-06-13 2:52 ` [PATCH bpf-next v5 3/3] selftests/bpf: add testcase for TRACING with 6+ arguments menglong8.dong
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