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SubjectRe: [PATCH bpf-next] bpf, x86: Simplify the parsing logic of structure parameters
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On 1/2/23 5:31 PM, Pu Lehui wrote:
> From: Pu Lehui <pulehui@huawei.com>
>
> Extra_nregs of structure parameters and nr_args can be
> added directly at the beginning, and using a flip flag
> to identifiy structure parameters. Meantime, renaming
> some variables to make them more sense.
>
> Signed-off-by: Pu Lehui <pulehui@huawei.com>

Thanks for refactoring. Using nr_regs instead of nr_args indeed
making things easier to understand. Ack with a few nits below.

Acked-by: Yonghong Song <yhs@fb.com>

> ---
> arch/x86/net/bpf_jit_comp.c | 99 +++++++++++++++++--------------------
> 1 file changed, 46 insertions(+), 53 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index e3e2b57e4e13..e7b72299f5a4 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1839,62 +1839,57 @@ st: if (is_imm8(insn->off))
> return proglen;
> }
>
> -static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
> +static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> int stack_size)
> {
> - int i, j, arg_size, nr_regs;
> + int i, j, arg_size;
> + bool is_struct = false;
> +
> /* 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_args, 6); i++) {
> - if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
> - nr_regs = (m->arg_size[i] + 7) / 8;
> + for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
> + arg_size = m->arg_size[j];
> + if (arg_size > 8) {
> arg_size = 8;
> - } else {
> - nr_regs = 1;
> - arg_size = m->arg_size[i];
> + is_struct ^= 1;
> }
>
> - while (nr_regs) {
> - emit_stx(prog, bytes_to_bpf_size(arg_size),
> - BPF_REG_FP,
> - j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
> - -(stack_size - j * 8));
> - nr_regs--;
> - j++;
> - }
> + emit_stx(prog, bytes_to_bpf_size(arg_size),
> + BPF_REG_FP,
> + i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> + -(stack_size - i * 8));
> +
> + j = is_struct ? j : j + 1;
> }
> }
>
> -static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
> +static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
> int stack_size)
> {
> - int i, j, arg_size, nr_regs;
> + int i, j, arg_size;
> + bool is_struct = false;

Maybe
bool next_same_struct = false
to better characterize what it means?

>
> /* Restore function arguments from stack.
> * For a function that accepts two pointers the sequence will be:
> * EMIT4(0x48, 0x8B, 0x7D, 0xF0); mov rdi,QWORD PTR [rbp-0x10]
> * EMIT4(0x48, 0x8B, 0x75, 0xF8); mov rsi,QWORD PTR [rbp-0x8]
> */
> - for (i = 0, j = 0; i < min(nr_args, 6); i++) {
> - if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
> - nr_regs = (m->arg_size[i] + 7) / 8;
> + for (i = 0, j = 0; i < min(nr_regs, 6); i++) {

Let us put a comment here so the later users can understand the logic
behind 'is_struct ^= 1'.

/* 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;
> - } else {
> - nr_regs = 1;
> - arg_size = m->arg_size[i];
> + is_struct ^= 1;

next_same_struct = !next_same_struct;

The same for above save_regs().

> }
>
> - while (nr_regs) {
> - emit_ldx(prog, bytes_to_bpf_size(arg_size),
> - j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
> - BPF_REG_FP,
> - -(stack_size - j * 8));
> - nr_regs--;
> - j++;
> - }
> + emit_ldx(prog, bytes_to_bpf_size(arg_size),
> + i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
> + BPF_REG_FP,
> + -(stack_size - i * 8));
> +
> + j = is_struct ? j : j + 1;
> }
> }
>
[...]

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